Kazuo Tsumuraya

Crystallization and glass formation processes in liquid sodium: A molecular dynamics study

Crystallization and glass formation processes are studied for the sodium system with 108 atoms by means of molecular dynamics simulation with constant volume. The equilibrium liquids, with four different initial conditions, are cooled with four different rates. The processes are analyzed by means of Voronoi polyhedra, diffusion coefficients, pair‐distribution functions, and mean‐square displacements. Embryo formation process, being accompanied by the abrupt slope increment of the displacement, appears with incubation period prior to nucleation and growth processes. The temperatures for the embryo formation and crystallization decrease with increasing cooling rate. The diffusion mechanism in supercooled liquid before the embryo formation is the same as that in equilibrium liquid. The critical cooling rate, which separates glass formation and crystallization processes, is greater than 8.00×1013 K s−1.

Icosahedral clustering in a supercooled liquid and glass

We study the icosahedral clustering process in a supercooled single‐component metal liquid by use of Voronoi polyhedron analysis. The number of the cluster increases with decreasing temperature from just below the melting temperature. The life of the cluster increases with decreasing temperature from slightly above the glass transition temperature Tg. The atoms with 0.56 fraction in the system become the central atoms of the cluster at least one time during the cooling process, although the total fraction of the clusters is 0.04 at temperatures far below the Tg. After atoms become the central atoms for the first time which we call fresh atoms, the atoms become central atoms of the other types of clusters, and after then revive as the central atoms of the icosahedral cluster. The life is longer for the revived atoms than for the fresh atoms in the low temperature region. The longer life is due to the energetic stability of the coordinate atoms around the revived atoms. We present atomistic transition proce...

Local structure and stability in a model glass

We study the relation between the potential energy of an atom and the shape of the Voronoi polyhedron around the atom in a sodium glass produced by the molecular dynamics method. The energy of an atom decreases with an increasing number of pentagon on the polyhedra, whereas it increases with increasing numbers of tetragon and hexagon. The central atom of the 13‐atom icosahedral cluster is found to have the lowest potential energy among the abundant top 13 clusters which appear in the glass. We define the local pair‐distribution function (PDF) around atoms with a specified shape of the polyhedron. The splitting of the second peak in the local PDF is the sharpest for the central atoms of the icosahedral cluster. The second peak splitting which appears in the average PDF is due to the existences of the icosahedral cluster and packing order of atoms around the cluster, i.e., the first peak comes from the first shell around the central atom of the icosahedral cluster, the second main peak from the second shell...

GPU based acceleration of first principles calculation

We present a Graphics Processing Unit (GPU) accelerated simulations of first principles electronic structure calculations. The FFT, which is the most time-consuming part, is about 10 times accelerated. As the result, the total computation time of a first principles calculation is reduced to 15 percent of that of the CPU.

Glass formation in continuous cooling processes: A molecular dynamic study of a monatomic metal system

We simulate the continuous cooling processes of a single‐component metallic model liquid with the molecular dynamics method in order to reexamine the claim [K. Shinjo, J. Chem. Phys. 90, 6627 (1989)] that the liquid–glass transition cannot be simulated with the method, i.e., the volume versus temperature relationship does not show a sharp or broad break but shows a meniscus curve in the continuous cooling processes. A sharp break means that the volume decreases linearly on both sides of the glass transition temperature. A broad break means that the volume decreases continuously in the intermediate temperature region although it changes linearly at both the sides. A meniscus curve means that the volume decreases continuously taking the shape of a bow. We analyze the structure of the continuously cooled state with the pair‐distribution function and Voronoi polyhedron analysis. We find that the glass state can be formed even in the continuously cooled processes and even when the potential energy versus tempe...

Liquid–glass transition: A molecular dynamics study of the sodium system

Glass formation process of the sodium system is studied by means of molecular dynamics simulation with constant volume. An equilibrium liquid with 864 atoms is cooled with the rate of 8.00×1013 K s−1 to form the glassy state. A second peak splitting of pair‐distribution function (PDF) disappears at high temperatures in the glassy state region and appears clearly with decreasing temperature. The disappearance is attributed to the thermal vibration effect of atoms in the glass. The harmonic vibration model is valid in the glassy state. The Wendt–Abraham parameter, the ratio of the first minimum to the first maximum in the PDF, is 0.08 at the glass transition temperature. The value is equal to the one for the soft‐core system, which is lower than the value 0.14 for the Lennard‐Jones and the Gaussian‐core system. The constant‐volume specific heat is found to be 3 Rg in the glassy state, where Rg is the gas constant. The difference of the specific heat between the supercooled liquid and the glassy state is Rg.

Dynamic Correlation between Superionic Coppers in α-CuI

We determine the instantaneous and simultaneous occupancy sites of the superionic coppers in α-CuI by means of the first principles molecular dynamics method. The coppers are located at incommensurate sites in the periodic lattice potential. The covalent bonds exist between the nearest coppers and between the copper and its nearest iodine. The formation of the dynamic dimers is the origin of the conduction. The ionicities of the atoms depend on their locations. The average ionicities are Cu +0.28 and I -0.28 . The incomplete electron transfer enables the atoms to form the covalent bonds between them. The superionic conductor α-CuI is an intermediate electronic state between the ionic sodium chloride crystal and the covalent bonded zinc blend crystal with respect to both the ionicities and the average locations of the coppers.

Ab initio molecular dynamics studies on volume stability of Voronoi polyhedra under pressures in a metal glass

We study volume stability of Voronoi polyhedra under hydrostatic pressures in a sodium glass. We optimize the atom positions under the pressures by an ab initio molecular dynamics method with an orbital‐free density functional. The number of atoms in the supercell is 128. The change of the volumes is insensitive to pressure for the Voronoi polyhedra with smaller volumes. We introduce a method to evaluate the average charge density around individual atom. The average electron charge density around individual atom increases with decreasing Voronoi volume. The volume change of Voronoi polyhedron with signature (0,0,12,0) is the most stable to the applied pressures. The density around the atoms with the signature (0,0,12,0) is the greatest among the signatures.

Electron Transfer from Hydrogen Molecule to Au(111) During Dissociative Adsorption: A First-Principles Study

We investigate the electron transfer from a dissociatively adsorbed H 2 molecule to a Au(111) surface using the first-principles methods. A fractional electron transfers from a molecule to a substrate, and potential energy increases during the process. The initial energy increase coincides with that of the isolated, separated, and positively charged H 2 molecule calculated by the real-space density functional method. The barrier formation is due to the destabilization of the molecule induced by the electron transfer. The electronegativity difference between the adsorbate and the substrate determines the direction of the electron transfer.

Indirect Attractions between Superionic Di-interstitial Protons on Mid-Oxygens in Water at Extreme Conditions: First-Principles Molecular Dynamics Study

The first principles molecular dynamics study enables us to elucidate attractions between the di-interstitial protons on mid-oxygens in superionic water at an extreme high pressure and the temperature. The present analyses elucidate the existence of inner and outer bonded interstitial protons around oxygens and reveal two types of attractions between the protons and oxygens in the water. One is a covalent attraction between them which induces a covalent bonded indirect attraction between the di-interstitial protons on the mid-oxygens. The other is a Coulomb attraction between them which induces a Coulomb type indirect attraction between the di-interstitial protons on the mid-oxygens. The indirect attractions decrease the impurity diffusion of the protons which reduces Haven’s ratios of the protons in the water. The negligible amount of the instantaneously formed proton dimers indicates a failure of the caterpillar diffusion mechanism or the Frenkel–Kontorova model for the water. The incomplete electron tr...