Satoshi Ohmura

A divide-conquer-recombine algorithmic paradigm for large spatiotemporal quantum molecular dynamics simulations.

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 × 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.

Molecular control of photoexcited charge transfer and recombination at a quaterthiophene/zinc oxide interface

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.

Charge and Nuclear Dynamics Induced by Deep Inner-Shell Multiphoton Ionization of CH3I Molecules by Intense X-ray Free-Electron Laser Pulses

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.

Large nonadiabatic quantum molecular dynamics simulations on parallel computers

Abstract 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.

Enhanced charge transfer by phenyl groups at a rubrene/C60 interface

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.

Dissociation dynamics of ethylene molecules on a Ni cluster using ab initio molecular dynamics simulations.

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.

Crystalline anisotropy of shock-induced phenomena: Omni-directional multiscale shock technique

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.

Structural Changes of Short- and Intermediate-Range Order in Liquid Arsenic under Pressure

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 As4 units exist at lower pressures and that the correlation between the As4 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.

Ab initio study of dissociation reaction of ethylene molecules on Ni cluster

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.

Dynamic asymmetry of self-diffusion in liquid ZnCl2 under pressure: An ab initio molecular-dynamics study

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.