Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

We studied the relationship between the lattice constant and magnetism of [Formula: see text]-ordered FePt under high pressure by means of first-principles calculations and synchrotron x-ray measurements. Based on our calculations, we found that the c/ a ratio shows a local maximum at ∼20 GPa and that the Pt magnetic moment first remains almost unchanged and is sharply suppressed at ∼60 GPa. As for the c/ a, we experimentally verified the local maximum at ∼20 GPa by powder x-ray diffraction. We also measured the x-ray magnetic circular dichroism at the Pt L edge up to ∼20 GPa. Any significant change in the Pt magnetic moment was not observed in agreement with the calculations. These results, thus, indicate that magnetic states, where the magnetization of Fe decreased and that of Pt did not change, can be created in [Formula: see text]-ordered FePt by lattice deformation under high pressure.

DOI： 10.1063/5.0139441

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.jpca.2c04075

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3389/feart.2022.873088

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Layer stacking of two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, is critical for controlling their physical and transport properties. By exploiting a specific stacking order, the electronic band structures of such 2D materials can be tuned, from which unusual and exotic properties may emerge. Graphene oxide (GO) undergoes layer stacking along with its photo- and thermal-induced reduction process; however, the underlying mechanism and dynamics during its layer stacking have not been revealed. In this study, we demonstrate time-resolved electron diffraction for monitoring the structural dynamics during the layer stacking of GO induced by ultraviolet photoexcitation. The experimental results accompanied by the density functional theory calculations reveal that AB stacking of graphitic domains of GO layers coincides within ∼40 ps with photoinduced removal of the epoxy-oxygen from the basal plane of GO via the strong interactions between the GO layers.

DOI： 10.1016/j.carbon.2021.07.058

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.apmt.2021.101167

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In situ femtosecond x-ray diffraction measurements and ab initio molecular dynamics simulations were performed to study the liquid structure of tantalum shock released from several hundred gigapascals (GPa) on the nanosecond timescale. The results show that the internal negative pressure applied to the liquid tantalum reached -5.6 (0.8) GPa, suggesting the existence of a liquid-gas mixing state due to cavitation. This is the first direct evidence to prove the classical nucleation theory which predicts that liquids with high surface tension can support GPa regime tensile stress.

DOI： 10.1103/PhysRevLett.126.175503

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Dissociation mechanisms are studied by ab initio molecular dynamics simulations based on density functional theory for the highly charged bromophenol (C H OHBr) (n ≤ 10) in the ground electronic state and in an electronic state which has a high electronic temperature Te characterized by Fermi–Dirac distribution. In the case of the ground state, the dissociation occurs through a sequential multi-stage process. At times shorter than 20 fs after the molecule is charged, hydrogens are dissociated from the molecule and, subsequently, the carbon ring breaks at about 150 fs In the case of an electronic state with high Te, the mechanism changes from a sequential dissociation process to a simultaneous process occurring at Te > 5 eV. To estimate the charge transfer time in a molecular bromide parent ion with +6 charge, which is generated through Auger cascades, we also performed nonadiabatic quantum-mechanical molecular dynamics (NAQMD) simulations that include the effects of nonadiabatic electronic transition with a surface-hopping approach. 6 4 n+

DOI： 10.1515/zpch-2020-1634

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

The effects of light elements such as H, C, O, Si, and S on the local structures of liquid Fe under high pressure are investigated by ab initio molecular dynamics (MD) simulations. The simulations clarify that H, C, and O are incorporated into liquid Fe interstitially while Si and S are “substitutional” type impurities. From the calculated partial pair distribution functions, it is found that an interaction between light elements exists even under high-pressure conditions exceeding 100 GPa. Additionally, the interaction depends on the type of element. The C-C interactions are stronger than those of other light elements. The S-S interactions depend on the S concentration, in the sense that the shape of pair distribution functions for the S-S correlation changes with increasing S concentration.

DOI： 10.1002/pssb.202000098

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Authorship：Lead author,　Corresponding author   Language：Japanese  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using Graphene oxide ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

DOI： 10.1021/acsnano.9b03060

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

QXMD is a scalable, parallel program for Quantum Molecular Dynamics simulations with various eXtensions. Its simulation engine is based on (time-dependent) density functional theory using pseudopotentials and a plane-wave basis set, while extensions include nonadiabatic electron-nuclei dynamics and multiscale shock technique. QXMD serves as a community-development platform for new methods and algorithms, a research platform on high-end parallel supercomputers, and an educational platform for hands-on training. (C) 2019 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.softx.2019.100307

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/1361-648x/ab0a35

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

The exothermic heterogeneous electrochemical energy conversion to the electric energy through interaction of the ZrO2 based nanopowder system with atmospheric moisture have been explored within this work. Electrical properties of the experimental samples were investigated during humidification at the conditions of molecular flux density gradient. The morphological features of the surface cross-section and aggregates of 3 mol% Y2O3 doped ZrO2 nanopowder systems were investigated. Initial conditions for molecular dynamics modelling of the adsorption processes were obtained. A novel approach for developing of chemo-electronic converters based on nanoscale processes and materials with dielectric conductivity type proposed.

DOI： 10.1088/1742-6596/848/1/012021

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Understanding x-ray radiation damage is a crucial issue for both medical applications of x rays and x-ray free-electron-laser (XFEL) science aimed at molecular imaging. Decrypting the charge and fragmentation dynamics of nucleobases, the smallest units of a macro-biomolecule, contributes to a bottom-up understanding of the damage via cascades of phenomena following x-ray exposure. We investigate experimentally and by numerical simulations the ultrafast radiation damage induced on a nucleobase analogue (5-iodouracil) by an ultrashort (10 fs) high-intensity radiation pulse generated by XFEL at SPring-8 Angstrom Compact free electron Laser (SACLA). The present study elucidates a plausible underlying radiosensitizing mechanism of 5-iodouracil. This mechanism is independent of the exact composition of 5-iodouracil and thus relevant to other such radiosensitizers. Furthermore, we found that despite a rapid increase of the net molecular charge in the presence of iodine, and of the ultrafast release of hydrogen, the other atoms are almost frozen within the 10-fs duration of the exposure. This validates single-shot molecular imaging as a consistent approach, provided the radiation pulse used is brief enough.

DOI： 10.1103/PhysRevX.6.021035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The atomistic mechanism of dissociative adsorption of ethylene molecules on a Ni cluster is investigated by ab initio molecular-dynamics simulations. The activation free energy to dehydrogenate an ethylene molecule on the Ni cluster and the corresponding reaction rate is estimated. A remarkable finding is that the adsorption energy of ethylene molecules on the Ni cluster is considerably larger than the activation free energy, which explains why the actual reaction rate is faster than the value estimated based on only the activation free energy. It is also found from the dynamic simulations that hydrogen molecules and an ethane molecule are formed from the dissociated hydrogen atoms, whereas some exist as single atoms on the surface or in the interior of the Ni cluster. On the other hand, the dissociation of the C-C bonds of ethylene molecules is not observed. On the basis of these simulation results, the nature of the initial stage of carbon nanotube growth is discussed.

DOI： 10.1088/0953-8984/28/14/145001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We propose an omni-directional multiscale shock technique (OD-MSST) to study the shock waves in an arbitrary direction of crystalline materials, atomistically based on the molecular dynamics simulation method. Using OD-MSST, we found transitions from elastic to shear-banding to plastic behaviors for a model covalent crystal. In addition to such a shock "phase diagram," a transition from inter-molecular to intra-molecular mechanochemical reaction pathways was found as a function of crystallographic orientation in an energetic van der Waals crystal. (C) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4942191

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Liquid chalcogen-halogen A(2)X(2) (A: S, Se, X: Cl, Br) is a racemic mixture of enantiomers between left-handed (L) and right-handed (D) chiral molecules. The lone-pair orbital of the chalcogen atom significantly affects the molecular conformation and intermolecular interaction. The latter depends on the size of the orbital and number density. High-energy X-ray diffraction measurements and reverse Monte Carlo (RMC) structural modelling were performed for liquid S2Cl2 in addition to the previous structural analysis for liquid Se2Br2. By comparing the structures of the RMC model and hard-sphere Monte Carlo (HSMC) model, the effect of refinement on the experimental structure factor can be analysed. In this paper, nearest-neighbour intermolecular pairs are classified in terms of enantiomer pairs such as like-pair (L-L and D-D) and unlike-pair (L-D). As a result, the effect of a strong intermolecular attractive interaction is detected in Se2Br2 as increasing the number of like-pairs with geometrical advantages from HSMC to RMC, whereas that of an intermolecular repulsive interaction is observed in S2Cl2 as elongation of the averaged intermolecular S-S distance from HSMC to RMC. These results are consistent with the results of the ab initio molecular dynamics simulation for Se2Cl2 and S2Cl2.
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DOI： 10.1080/00268976.2015.1100345

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Using ab-initio theoretical methods, we demonstrate possible enhancement of photo-conversion efficiency of an organic solar cell via intentional doping in molecular graphene-fullerene heterojunction [the hexabenzocoronene (HBC)-triethylene glycol (TEG)-C-60 molecule]. Photoabsorption analysis indicates oxygen substitution into HBC leads to an extension of the spectra up to an infrared regime. A quantum-mechanical molecular dynamics simulation incorporating nonadiabatic electronic transitions reveals that a dissociated charge state (D+ and A(-)) in the O-doped system is more stable than the pristine case due to the presence of an effective barrier by the TEG HOMO/LUMO level. We also find that oxygen doping in HBC enhances the intermolecular carrier mobility after charge separation. On the other hand, the pristine molecule undergoes rapid recombination between donor and acceptor charges at the interface. These analyses suggest that the graphene oxidation opens a new window in the application of organic super-molecules to solar cells. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

DOI： 10.1063/1.4939848

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

We studied the electronic and nuclear dynamics of I-containing organic molecules induced by intense hard X-ray pulses at the XFEL facility SACLA in Japan. The interaction with the intense XFEL pulse causes absorption of multiple X-ray photons by the iodine atom, which results in the creation of many electronic vacancies (positive charges) via the sequential electronic relaxation in the iodine, followed by intramolecular charge redistribution. In a previous study we investigated the subsequent fragmentation by Coulomb explosion of the simplest I-substituted hydrocarbon, iodomethane (CH3I). We carried out three-dimensional momentum correlation measurements of the atomic ions created via Coulomb explosion of the molecule and found that a classical Coulomb explosion model including charge evolution (CCE-CE model), which accounts for the concerted dynamics of nuclear motion and charge creation/charge redistribution, reproduces well the observed momentum correlation maps of fragment ions emitted after XFEL irradiation. Then we extended the study to 5-iodouracil (C4H3IN2O2, 5-IU), which is a more complex molecule of biological relevance, and confirmed that, in both CH3I and 5-IU, the charge build-up takes about 10 fs, while the charge is redistributed among atoms within only a few fs. We also adopted a self-consistent charge density-functional based tight-binding (SCC-DFTB) method to treat the fragmentations of highly charged 5-IU ions created by XFEL pulses. Our SCC-DFTB modeling reproduces well the experimental and CCE-CE results. We have also investigated the influence of the nuclear dynamics on the charge redistribution (charge transfer) using nonadiabatic quantum-mechanical molecular dynamics (NAQMD) simulation. The time scale of the charge transfer from the iodine atomic site to the uracil ring induced by nuclear motion turned out to be only similar to 5 fs, indicating that, besides the molecular Auger decay in which molecular orbitals delocalized over the iodine site and the uracil ring are involved, the nuclear dynamics also play a role for ultrafast charge redistribution. The present study illustrates that the CCE-CE model as well as the SCC-DFTB method can be used for reconstructing the positions of atoms in motion, in combination with the momentum correlation measurement of the atomic ions created via XFEL-induced Coulomb explosion of molecules.

DOI： 10.1039/c6fd00085a

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The pressure dependence of the structural properties of liquid arsenic (l-As) is studied in detail by ab initio molecular dynamics simulations and X-ray diffraction experiments. In this study, we have clarified that network structures consisting mainly of As-4 units exist at lower pressures and that the correlation between the As-4 units is the origin of an intermediate-range order, which is seen as a prepeak of the static structure factor. When the pressure increases, the intermediate-range order disappears and structural change occurs gradually. At approximately 7 GPa, the pair distribution function and the bond angle distribution show some similarities to those of the simple cubic structure as seen in the high-pressure phase of crystalline As.

DOI： 10.7566/JPSJ.84.094602

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

In recent years, free-electron lasers operating in the true X-ray regime have opened up access to the femtosecond-scale dynamics induced by deep inner-shell ionization. We have investigated charge creation and transfer dynamics in the context of molecular Coulomb explosion of a single molecule, exposed to sequential deep inner-shell ionization within an ultrashort (10 fs) X-ray pulse. The target molecule was CH3I, methane sensitized to X-rays by halogenization with a heavy element, iodine. Time-of-flight ion spectroscopy and coincident ion analysis was employed to investigate, via the properties of the atomic fragments, single-molecule charge states of up to +22. Experimental findings have been compared with a parametric model of simultaneous Coulomb explosion and charge transfer in the molecule. The study demonstrates that including realistic charge dynamics is imperative when molecular Coulomb explosion experiments using short-pulse facilities are performed.

DOI： 10.1021/acs.jpclett.5b01205

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

The dynamic properties of liquid B2O3 under pressure and highly-charged bromophenol molecule are studied by using molecular dynamics (MD) simulations based on density functional theory (DFT). Diffusion properties of covalent liquids under high pressure are very interesting in the sense that they show unexpected pressure dependence. It is found from our simulation that the magnitude relation of diffusion coefficients for boron and oxygen in liquid B2O3 shows the anomalous pressure dependence. The simulation clarified the microscopic origin of the anomalous diffusion properties. Our simulation also reveals the dissociation mechanism in the coulomb explosion of the highly-charged bromophenol molecule. When the charge state n is 6, hydrogen atom in the hydroxyl group dissociates at times shorter than 20 fs while all hydrogen atoms dissociate when n is 8. After the hydrogen dissociation, the carbon ring breaks at about 100 fs. There is also a difference on the mechanism of the ring breaking depending on charge states, in which the ring breaks with expanding (n = 6) or shrink (n = 8).

DOI： 10.1063/1.4928260

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DOI： 10.14723/tmrsj.40.215

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

DOI： 10.7566/JPSJ.83.105002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

It is hoped that a hydrogen-on-demand generator may one day start with just the turn of an ignition key, if the reaction kinetics is accelerated for the production of hydrogen gas from water. Our quantum molecular dynamics simulations have revealed the atomistic mechanism of rapid hydrogen production from water using aluminum nanoclusters, Al-n (n = 16, 17, and 18). We have found a low activation-barrier mechanism of hydrogen production, in which a pair of Lewis add and base sites on the nanocluster surface plays a crucial role. Hydrogen production is assisted by rapid proton transport in water via a chain of hydrogen-bond switching events similar to the Grotthuss mechanism. The solvation shell has been shown to greatly reduce the energy barrier. We have also found that the reaction rate does not depend strongly on the cluster size n, in contrast to the existence of magic numbers in gas-phase reaction. This work paves a way for a rational design of hydrogen-on-demand technologies. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.ssi.2013.12.025

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We introduce an extension of the divide-and-conquer (DC) algorithmic paradigm called divide-conquer-recombine (DCR) to perform large quantum molecular dynamics (QMD) simulations on massively parallel supercomputers, in which interatomic forces are computed quantum mechanically in the framework of density functional theory (DFT). In DCR, the DC phase constructs globally informed, overlapping local-domain solutions, which in the recombine phase are synthesized into a global solution encompassing large spatiotemporal scales. For the DC phase, we design a lean divide-and-conquer (LDC) DFT algorithm, which significantly reduces the prefactor of the O(N) computational cost for N electrons by applying a density-adaptive boundary condition at the peripheries of the DC domains. Our globally scalable and locally efficient solver is based on a hybrid real-reciprocal space approach that combines: (1) a highly scalable real-space multigrid to represent the global charge density; and (2) a numerically efficient plane-wave basis for local electronic wave functions and charge density within each domain. Hybrid space-band decomposition is used to implement the LDC-DFT algorithm on parallel computers. A benchmark test on an IBM Blue Gene/Q computer exhibits an isogranular parallel efficiency of 0.984 on 786 432 cores for a 50.3 x 10(6)-atom SiC system. As a test of production runs, LDC-DFT-based QMD simulation involving 16 661 atoms is performed on the Blue Gene/Q to study on-demand production of hydrogen gas from water using LiAl alloy particles. As an example of the recombine phase, LDC-DFT electronic structures are used as a basis set to describe global photoexcitation dynamics with nonadiabatic QMD (NAQMD) and kinetic Monte Carlo (KMC) methods. The NAQMD simulations are based on the linear response time-dependent density functional theory to describe electronic excited states and a surface-hopping approach to describe transitions between the excited states. A series of techniques are employed for efficiently calculating the long-range exact exchange correction and excited-state forces. The NAQMD trajectories are analyzed to extract the rates of various excitonic processes, which are then used in KMC simulation to study the dynamics of the global exciton flow network. This has allowed the study of large-scale photoexcitation dynamics in 6400-atom amorphous molecular solid, reaching the experimental time scales. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4869342

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Charge state dependence of dissociation mechanisms of the highly charged bromophenol (C6H5OHBr)(n+) (n <= 10) is studied by ab initio molecular dynamics simulations based on density functional theory. When the charge state n is 6 or 7, one or two hydrogen atoms dissociate in the first stage (at times shorter than 20 fs) while all hydrogen atoms dissociate when the charge state is over 7. After hydrogen dissociation, the carbon ring breaks at about 150 fs. There is also a difference on the mechanism of the ring breaking depending on charge states, in which the ring breaks with expanding (n = 6,7) or shrink (n >= 8).

DOI： 10.1088/1742-6596/510/1/012039

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The static and dynamic structures of a molecular liquid S0.5Cl0.5 consisting of Cl-S-S-Cl (S2Cl2) type molecules are studied by means of ab initio molecular dynamics simulations. Both the calculated static and dynamic structure factors are in good agreement with experimental results. The dynamic structures are discussed based on van-Hove distinct correlation functions, molecular translational mean-square displacements (TMSD) and rotational mean-square displacements (RMSD). In the TMSD and RMSD, there are ballistic and diffusive regimes in the sub-picosecond and picosecond time regions, respectively. These time scales are consistent with the decay time observed experimentally. The interaction between molecules in the liquid is also discussed in comparison with that in another liquid chalcogen-halogen system Se0.5Cl0.5.

DOI： 10.7566/JPSJ.82.074602

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The static and dynamic properties of liquid ZnCl2 under pressure are investigated by ab initio molecular-dynamics simulations. The pressure range covers ambient to approximately 80 GPa. The ZnCl4 tetrahedra, which are rather stable at ambient pressure, are shown to deform and collapse with increasing pressure while maintaining an almost constant nearest-neighbor distance between Zn and Cl atoms. The average coordination number of Cl atoms around Zn atoms increases monotonically with pressure, from four at ambient pressure to seven at approximately 80 GPa. Although the self-diffusion coefficients of Zn and Cl atoms, d(Zn) and d(Cl), are almost the same at ambient pressure, the difference between them increases with pressure. At around 10 GPa, d(Zn) is about two times larger than d(Cl). Under further compression, this dynamic asymmetry becomes smaller. The microscopic mechanism of the appearance of the dynamic asymmetry is discussed in relation to the pressure dependence of the local structure. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4798376]

DOI： 10.1063/1.4798376

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have implemented a quantum molecular dynamics simulation incorporating nonadiabatic electronic transitions on massively parallel computers to study photoexcitation dynamics of electrons and ions. The nonadiabatic quantum molecular dynamics (NAQMD) simulation is based on Casida's linear response time-dependent density functional theory to describe electronic excited states and Tully's fewest-switches surface hopping approach to describe nonadiabatic electron-ion dynamics. To enable large NAQMD simulations, a series of techniques are employed for efficiently calculating long-range exact exchange correction and excited-state forces. The simulation program is parallelized using hybrid spatial and band decomposition, and is tested for various materials. (C) 2012 Elsevier B.V. All rights reserved,

DOI： 10.1016/j.cpc.2012.08.001

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The dissociation reaction of ethylene molecules on the Ni cluster surface. is investigated by ab initio molecular dynamics simulations. We observe that hydrogen atoms are generated from ethylene molecules at a rate of about 20 ps (1). The activation energy for the dissociation of a hydrogen atom is estimated to be about 0.52 eV, which corresponds to a rate of only about 0.1 ps(-1). We find that the adsorption energy of an ethylene molecule on the Ni cluster is more than 1.5 eV, which is three times greater than the activation energy for the hydrogen dissociation. It is, therefore, suggested that the adsorption energy is responsible for the increase of the rate of the dissociation reaction. Based on these results, we discuss the microscopic process of the reaction of ethylene molecules on the Ni cluster in detail.

DOI： 10.1088/1742-6596/454/1/012022

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Nonadiabatic quantum molecular dynamics simulations are performed to study photoexcited charge transfer (CT) and charge recombination (CR) at an interface between a conjugated oligomer donor, quaterthiophene (QT), and an inorganic acceptor (ZnO). Simulations reveal a detrimental effect of static disorder in QT conformation on the efficiency of hybrid QT/ZnO solar cells due to increased CR. On the contrary, dynamic disorder (i.e., fluctuation of carbon-hydrogen bonds in QT) is essential for high efficiency by assisting CT. The separate controllability of CT and CR at the molecular level has impacts on molecular design for efficient solar cells and explains recent experimental observations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4719206]

DOI： 10.1063/1.4719206

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Exciton dynamics at an interface between an electron donor, rubrene, and a C-60 acceptor is studied by nonadiabatic quantum molecular dynamics simulation. Simulation results reveal an essential role of the phenyl groups in rubrene in increasing the charge-transfer rate by an order-of-magnitude. The atomistic mechanism of the enhanced charge transfer is found to be the amplification of aromatic breathing modes by the phenyl groups, which causes large fluctuations of electronic excitation energies. These findings provide insight into molecular structure design for efficient solar cells, while explaining recent experimental observations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4712616]

DOI： 10.1063/1.4712616

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The mechanism of void formation in crystalline silicon after laser irradiation is extensively studied by molecular-dynamics simulations. When the laser-irradiated region is melted due to a rapid temperature increase, small voids are generated in that region because of large density fluctuations. Because tensile stresses are generated in the melting region, the large empty hole is formed upon cooling. When the temperature drops below the melting point, recrystallization occurs around the large void. We find that the void persists even after the recrystallization process is stopped, and that a part of the laser-irradiated region is amorphous even at room temperature. The stress distribution around the void and the amorphous region in crystalline silicon is also discussed.

DOI： 10.1088/1742-6596/402/1/012044

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Computer simulation based on molecular dynamics is powerful approach to clarify the microscopic mechanism of atomic diffusion in liquid state. Here, the diffusion properties and the microscopic diffusion mechanisms in liquid B2O3 under pressure are studied by ab initio molecular dynamics simulations. It is found that diffusivities of liquid B2O3 show anomalous pressure dependence. Diffusion coefficients of liquid materials usually decrease with pressure. However, that of liquid B2O3 increases with increasing pressure up to about 10 GPa. Additionally, diffusivity of boron becomes about twice larger than that of oxygen under pressure above 20 GPa. These anomalous pressure dependences are strong related to the microscopic atomic diffusion in liquid B2O3.

DOI： 10.1088/1742-6596/402/1/012012

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the pressure dependence of the structural and electronic properties of liquid AsS by ab initio molecular dynamics simulations. We confirm that liquid AsS consists of As4S4 molecules at ambient pressure, as in the crystalline state. With increasing pressure, a structural transition from molecular to polymeric liquid occurs near 2 GPa, which is eventually followed by metallization. The pressure dependence of the density and diffusion coefficients changes qualitatively with this transition. We find that, during metallization in the polymeric phase at higher pressures, the remnants of covalent interactions between atoms play an important role in the dynamics, i.e., the As-S bond length becomes longer with increasing pressure and the diffusion coefficients have a local maximum near 5 GPa. When the pressure approaches about 15 GPa, the covalent nature of the liquid becomes quite weak. These results explain recent experiments on the pressure dependence of the viscosity.

DOI： 10.1103/PhysRevB.84.224202

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Reaction of aluminum clusters, Al-n (n = 16, 17 and 18), with liquid water is investigated using quantum molecular dynamics simulations, which show rapid production of hydrogen molecules assisted by proton transfer along a chain of hydrogen bonds (H-bonds) between water molecules, i.e. Grotthuss mechanism. The simulation results provide answers to two unsolved questions: (1) What is the role of a solvation shell formed by non-reactingH-bonds surrounding theH-bond chain; and (2) whether the high size-selectivity observed in gas-phase Al-n-water reaction persists in liquid phase? First, the solvation shell is found to play a crucial role in facilitating proton transfer and hence H-2 production. Namely, it greatly modifies the energy barrier, generally to much lower values (< 0.1 eV). Second, we find that H-2 production by Aln in liquid water does not depend strongly on the cluster size, in contrast to the existence of magic numbers in gas-phase reaction. This paper elucidates atomistic mechanisms underlying these observations. Copyright 2011 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [doi: 10.1063/1.3664751]

DOI： 10.1063/1.3664751

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The atomistic mechanism of rapid hydrogen production from water by an aluminum cluster is investigated by ab initio molecular dynamics simulations on a parallel computer. A low activation-barrier mechanism of hydrogen production is found, in which a pair of Lewis acid and base sites on the cluster surface plays a crucial role. Hydrogen production is assisted by rapid proton transport in water via a chain of hydrogen-bond switching events similar to the Grotthuss mechanism, where hydroxide ions are converted to water molecules at the Lewis-acid sites and hydrogen atoms are supplied at the Lewis-base sites. The activation free energy is estimated along various reaction paths associated with hydrogen production, and the corresponding reaction rates are discussed based on the transition state theory. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3602326]

DOI： 10.1063/1.3602326

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：EDP Sciences  

A linear-scaling algorithm based on a divide-and-conquer (DC) scheme is designed to perform large-scale molecular-dynamics simulations, in which interatomic forces are computed quantum mechanically in the framework of the density functional theory (DFT). This scheme is applied to the thermite reaction at an Al/Fe2O3 interface. It is found that mass diffusion and reaction rate at the interface are enhanced by a concerted metal-oxygen flip mechanism. Preliminary simulations are carried out for an aluminum particle in water based on the conventional DFT, as a target system for large-scale DC-DFT simulations. A pair of Lewis acid and base sites on the aluminum surface preferentially catalyzes hydrogen production in a low activation-barrier mechanism found in the simulations

DOI： 10.1051/epjconf/20111503005

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：EDP Sciences  

The structural and electronic properties of liquid MgSiO3 under pressure are investigated by ab initio molecular-dynamics simulations. At ambient pressure, most of Si atoms have the same coordination even in the liquid state as in the crystalline phase, i.e., each Si atom is bonded to two bridging oxygen (BO), i.e., O atoms twofold-coordinated to Si, and two nonbridging oxygen (NBO), i.e., O atoms onefold-coodinated to Si. It is, however, found that the structural defects, such as fivefold- or threefold-coordinated Si atoms, are always formed with the rearrangement of Si-O covalent bonds in the atomic diffusion processes. The population analysis clarifies that maximum of the diffusivity in the pressure dependence results from the increasing of the number of defects under compression.

DOI： 10.1051/epjconf/20111502004

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：EDP Sciences  

The microscopic mechanisms of atomic diffusion in liquid GeO2 and SrGeO3 are investigated by ab initio molecular-dynamics simulations. We clarify the differences of diffusion mechanism between liquid GeO2 and SrGeO3. In both liquids, non-bridging oxygen double bonded to only one germanate plays a key role in the atomic diffusion mechanism. It is found that, in liquid SrGeO3 which has non-bridging oxygen in the equilibrium state at ambient pressure, atomic diffusion is possible without generating overcoordinated atoms at ambient pressure, while the over coordinated atoms are always needed for the formation of non-bridging oxygens in liquid GeO2. When the pressure increases, only liquid GeO2 has a diffusion maximum, which is given by, the atomic diffusion with concerted reaction gives the dif fusion maximum.

DOI： 10.1051/epjconf/20111502003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

A linear-scaling algorithm based on a divide-and-conquer (DC) scheme is designed to perform large-scale molecular-dynamics simulations, in which interatomic forces are computed quantum mechanically in the framework of the density functional theory (DFT). This scheme is applied to the thermite reaction at an Al/Fe(2)O(3) interface. It is found that mass diffusion and reaction rate at the interface are enhanced by a concerted metal-oxygen flip mechanism. Preliminary simulations are carried out for an aluminum particle in water based on the conventional DFT, as a target system for large-scale DC-DFT simulations. A pair of Lewis acid and base sites on the aluminum surface preferentially catalyzes hydrogen production in a low activation-barrier mechanism found in the simulations.

DOI： 10.1140/epjst/e2011-01418-y

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

The microscopic mechanism of the metallization of liquid selenium under pressure is studied by ab initio molecular dynamics simulations. From the obtained electronic properties, such as the electronic density of states and the bond-overlap population, the pressure-induced metallization is found to be completed at a pressure of approximately 5 GPa, in agreement with recent experimental observations. When the pressure exceeds 5 GPa, the nearest-neighbor distance and the coordination number increase drastically with increasing pressure up to 10 GPa. An important finding is that, during this structural change, there is a microscopic competition between metalliclike and covalentlike interactions, which results in a peculiar atomic structure in this pressure range. The pressure dependence of the self-diffusion coefficient is in qualitative agreement with that of the experimental share viscosity. A comparison with the static structure of liquid tellurium under pressure is also discussed. © 2011 American Physical society.

DOI： https://doi.org/10.1103/PhysRevB.83.134206

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Synthetic supermolecules such as pi-conjugated light-harvesting dendrimers efficiently harvest energy from sunlight, which is of significant importance for the global energy problem. Key to their success is rapid transport of electronic excitation energy from peripheral antennas to photochemical reaction cores, the atomistic mechanisms of which remains elusive. Here, quantum-mechanical molecular dynamics simulation incorporating nonadiabatic electronic transitions reveals the key molecular motion that significantly accelerates the energy transport based on the Dexter mechanism. (C) 2011 American Institute of Physics. [doi:10.1063/1.3565962]

DOI： 10.1063/1.3565962

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Hydrogen production by metal particles in water could provide a renewable energy cycle, if its reaction kinetics is accelerated. Here, ab initio molecular dynamics simulation reveals rapid hydrogen production from water by a cluster (or superatom) consisting of a magic number of aluminum atoms, Al(n) (for instance, n = 12 or 17). We find a low activation-barrier mechanism, in which a pair of Lewis-acid and base sites on the Al(n) surface preferentially catalyzes hydrogen production. This reaction is immensely assisted by rapid proton transport in water via a chain of hydrogen-bond switching events similar to the Grotthuss mechanism, which converts hydroxide ions to water molecules at the Lewis-acid sites and supplies hydrogen atoms at the Lewis-base sites.

DOI： 10.1103/PhysRevLett.104.126102

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The structural, bonding, and dynamic properties of liquid boron oxide, B2O3, under pressure are studied by ab initio molecular-dynamics simulations. To investigate the pressure dependence of the static structure, we obtain the structure factors, the pair distribution functions, and the distribution of the coordination numbers as a function of pressure. Planar BO3 units are hardly deformed under pressures up to about 3 GPa, and the number of tetrahedral BO4 units increases gradually under further compression. The bond-overlap populations and the Mulliken charges as well as the electronic density of states show that the covalent character is well preserved in the liquid state up to at least 200 GPa, although bond weakening occurs due to the increase in the coordination number. When the temperature is relatively low, the self-diffusion coefficients of boron and oxygen have a maximum at about 10 GPa because the concerted reactions, which enhance the atomic diffusion, occur more frequently with the increase in pressure below 10 GPa and are suppressed at higher pressures. The maximum behavior of the diffusivity becomes weaker with increasing temperature. A remarkable feature of the dynamic properties is that, under higher pressures over 20 GPa, the diffusivity of oxygen becomes much smaller than that of boron, regardless of temperature, while the former is slightly larger than the latter at lower pressures. Detailed discussions on the microscopic origin of this anomalous pressure dependence of the diffusivity are given.

DOI： 10.1103/PhysRevB.81.014208

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The pressure dependence of the static and dynamic properties of liquid boron oxide, B2O3, is studied by ab initio molecular dynamics simulations. Planar BO3 units are found to be scarcely deformed under pressures up to about 3 GPa. Under further compression, the number of tetrahedral BO4 units increases gradually, and approximately 90% of boron atoms have fourfold coordination near 100 GPa. The self-diffusion coefficients of boron and oxygen have a maximum at a pressure of about 10 GPa. We find that under pressures above 20 GPa, the diffusivity of boron becomes about two times larger than that of oxygen, while the former is 10-20% smaller than the latter at lower pressures. We reveal the microscopic origin of this anomalous pressure dependence of diffusivity.

DOI： 10.1103/PhysRevB.80.020202

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The structural properties of a molecular liquid consisting of Cl-Se-Se-Cl molecules are studied by ab initio molecular-dynamics simulations. The calculated structure factor is shown to be in good agreement with the experimental results. It is seen that while neighboring molecules have only a small effect on intramolecular parameters such as bond lengths and bond angles, the intermolecular orientational correlations depend largely on the distance between molecules. The interaction energy between two molecules is investigated to explore the molecular arrangement in the liquid state. A weak attractive interaction is found to exist among molecules, which plays an important role for short-range intermolecular correlations.

DOI： 10.1143/JPSJ.78.074601

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The structural and bonding properties of liquid boron oxide are studied by ab initio molecular dynamics simulations. It is seen that, even in the liquid state, boron atoms are predominantly threefold coordinated to oxygen atoms, and most of oxygen atoms bridge two adjacent boron atoms. The neighboring atoms are connected to each other with single covalent bonds, and there exist no homopolar bonds, as in the crystalline and vitreous states. We find that a nonbridging oxygen double bonded to a twofold-coordinated boron is always involved with atomic diffusion accompanied by the rearrangement of the covalent bonds. We discuss the formation mechanism of such bond defects in detail by the population analysis.

DOI： 10.1103/PhysRevB.78.224206

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Publisher：Models in Bioscience and Materials Research: Molecular Dynamics and Related Techniques  

The energy transport in light harvesting dendrimer and the coulomb explosion of bromophenol are studied by ab initio molecular-dynamics simulations. The thermal molecular motion plays an important role in electron transport in the light harvesting dendrimer. The simulation reveals that tetrahydrofuran solvent suppresses the molecular motion and consequently the electron transfer. The dissociation mechanism of a charged bromophenol is also investigated. It is clarified that the hydrogen atomof hydroxyl group dissociates fromthemolecule before the aromatic ring is broken. © 2013 by Nova Science Publishers, Inc. All rights reserved.

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Publisher：Molecular Dynamics of Nanobiostructures  

The atomic dynamics of liquid B2O3 and the energy-transfer mechanism in light harvesting dendrimers are studied by ab initio moleculardynamics simulations. In liquid B2O3, it is found that the diffusivity of boron becomes about two times larger than that of oxygen under pressure above 20 GPa while the former is 10-20% smaller than the latter at lower pressures. We reveal the microscopic origin of this anomalous pressure dependence of diffusivity. We take into account the nonadiabatic effects in molecular-dynamics simulations to describe the photo-exitation state of dendrimers. Our simulation reveals the key role of thermal molecular motion that significantly accelerates the energy transport based on the Dexter mechanism. © 2012 by Nova Science Publishers, Inc. All rights reserved.

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Publisher：Molecular Dynamics of Nanobiostructures  

© 2017 Nova Science Publishers, Inc. All rights reserved. The atomic dynamics of liquid B2O3and the energy-transfer mechanism in light harvesting dendrimers are studied by ab initio molecular-dynamics simulations. In liquid B2O3, it is found that the diffusivity of boron becomes about two times larger than that of oxygen under pressure above 20 GPa while the former is 10-20 % smaller than the latter at lower pressures. We reveal the microscopic origin of this anomalous pressure dependence of diffusivity. We take into account the nonadiabatic effects in molecular-dynamics simulations to describe the photo-exitation state of dendrimers. Our simulation reveals the key role of thermal molecular motion that significantly accelerates the energy transport based on the Dexter mechanism.

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Language：English   Publisher：Kumamoto University  

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Language：English   Publisher：Kumamoto University  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：English   Publisher：Kumamoto University  

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