Takahisa Kouno

Collaborative Simulation Grid: Multiscale Quantum-Mechanical/Classical Atomistic Simulations on Distributed PC Clusters in the US and Japan

A multidisciplinary,collaborative simulation has been performed on a Grid of geographically distributed PC clusters.The multiscale simulation approach seamlessly combines i) atomistic simulation based on the molecular dynamics (MD) method and ii) quantum mechanical (QM) calculation based on the density functional theory (DFT), so that accurate but less scalable computations are performed only where they are needed. The multiscale MD/QM simulation code has been Grid-enabled using i) a modular, additive hybridization scheme, ii) multiple QM clustering, and iii) computation/communication overlapping. The Gridified MD/QM simulation code has been used to study environmental effects of water molecules on fracture in silicon. A preliminary run of the code has achieved a parallel efficiency of 94% on 25 PCs distributed over 3 PC clusters in the US and Japan, and a larger test involving 154 processors on 5 distributed PC clusters is in progress.

Linear scaling algorithm of real-space density functional theory of electrons with correlated overlapping domains

Abstract The real-space grid based implementation of the Kohn–Sham density functional theory of electrons using the finite difference method for derivatives of variables, has attractive features of parallelizability and applicability to various boundary conditions in addition to universality in target materials. Following the divide-and-conquer strategy, we propose a linear scaling algorithm of it by advancing the algorithm in [F. Shimojo et al., Comput. Phys. Comm. 167 (2005) 151]. In the Kohn–Sham-type equation for a domain, we introduce (i) the density-template potential for density continuity with simple stepwise weight functions and (ii) the embedding potential to take into account all the quantum correlation effects with other overlapping domains in addition to the classical effects of ionic and electronic Coulomb potentials. We thereby realize reasonably high accuracies in atomic forces with relatively small numbers of buffer ions irrespective of the electronic characters of materials. The timing tests on parallel machines demonstrate the linear scaling of the code with little communication time between the domains.

Activation Energy for Oxygen Diffusion in Strained Silicon: A Hybrid Quantum-Classical Simulation Study with the Nudged Elastic Band Method

The activation energy for oxygen diffusion in strained silicon crystal is investigated using the hybrid quantum-classical simulation scheme in combination with the nudged elastic band method. The electronic density-functional theory is applied to a local region containing the oxygen atom, while the classical inter-atomic potential, to the rest of the system. The system is stretched to three mutually perpendicular directions at a wide range of degree between -2 and 9%. We thereby find that the activation energy changes by between -0.4 and 0.2 eV depending sensitively on both direction and degree of the stretch, and that the peripheral atoms located far from the oxygen atom in the system contribute little to the change. Microscopic mechanisms of the strain-dependence of the activation energy are elucidated through combined analyses about the atomic and electronic structures.

Enhanced Si–O Bond Breaking in Silica Glass by Water Dimer: A Hybrid Quantum–Classical Simulation Study

A hybrid quantum–classical simulation of a 4,608-atom silica glass is performed at a temperature of 400 K with either a water monomer or dimer inserted in a void. The quantum region that includes the water and the surrounding atoms is treated by the density-functional theory (DFT). During a simulation, the silica glass is gradually compressed or expanded. No Si–O bond breaking occurs with a water monomer until the silica glass collapses. With a water dimer, we find that Si–O bond breaking occurs through three steps in 3 out of 24 compression cases: (i) H-transfer as 2H2O → OH− + H3O+ accompanied by the adsorption of OH− at a strained Si to make it five-coordinated, (ii) breaking of a Si–O bond that originates from the five-coordinated Si, and (iii) H-transfer from H3O+ to the O of the broken Si–O bond. A separate DFT calculation confirms that the barrier energy of the bond breaking with a water dimer under compression is smaller than that with a water monomer and that the barrier energy decreases signific...

Hybrid Simulations for Desinging of Nano-Interfacial Structures

There is growing demand to perform dynamic, atomistic computer-simulation of nano-scaled interfaces. For dynamic simulation of interesting processes at the nano-interfaces, we have been developing the hybrid simulation schemes by concurrently coupling the quantum description as the electronic density-functional theory and the classical description as the classical molecular dynamics. A quantum (QM) region composed of a relatively small number of atoms, is embedded with the novel buffered-cluster method in a classical (CL) region of atoms interacting through an empirical inter-atomic potential. The hybrid QM-CL simulation scheme is applied to various kinds of nano-processes including implantation of oxygen atoms to a Si slab relating to SIMOX technology.