Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set

Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

/pdf/quantum-espresso-a-modular-and-open-source-software-project-4t6vlupbeh.pdf

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

While electrical compatibility constraints normally prevent head-to-head (HH) and tail-to-tail (TT) domain walls from forming in ferroelectric materials, we propose that such domain walls could be stabilized by intentional growth of atomic layers in which the cations are substituted from a neighboring column of the periodic table. In particular, we carry out predictive first-principles calculations of superlattices in which Sc, Nb, or other substitutional layers are inserted periodically into ${\mathrm{PbTiO}}_{3}$. We confirm that this gives rise to a domain structure with the longitudinal component of the polarization alternating from domain to domain, and with the substitutional layers serving as HH and TT domain walls. We also find that a substantial transverse component of the polarization can also be present.

/pdf/theory-of-hypothetical-ferroelectric-superlattices-2xclxcofa4.pdf

Theory of hypothetical ferroelectric superlattices incorporating head-to-head and tail-to-tail 180° domain walls

A wide variety of properties of materials are determined by processes which take place on an atomic scale. In recent years, theoretical developments and advances in computer power have made it possible to examine these properties and processes directly, using a number of different calculational and simulation techniques. The papers here include representatives of major theoretical approaches, from first-principles studies of electronic and structural properties to simulations of extended crystal defects using empirical potentials. Applications are made to metals, semiconductors and insulators, in the form of crystals, defects, glasses, and liquids. This book presents an introduction to this research area as well as a record of the present status.

Atomic scale calculations in materials science

We investigate the energetics of the single-period and double-period core reconstructions of the 90-degree partial dislocation in the homopolar semiconductors C, Si, and Ge The double-period geometry is found to be lower in energy in all three materials, and the energy difference between the two geometries is shown to follow the same trends as the energy gap and the stiffness Both structures are fully reconstructed, consisting entirely of fourfold coordinated atoms They differ primarily in the detail of the local strains introduced by the the two reconstructions in the core region The double-period structure is shown to introduce smaller average bond-length deviations, at the expense of slightly larger average bond-angle bending distortions, with respect to the single-period core The balance between these two strain components leads to the lower energy of the double-period reconstruction

Core reconstruction of the 90-degree partial dislocation in non-polar semiconductors

We extend our previous first-principles theory for perovskite ferroelectric phase transitions to treat also antiferrodistortive phase transitions. Our approach involves construction of a model Hamiltonian from a Taylor expansion, first-principles calculations to determine expansion parameters, and Monte Carlo simulations to study the resulting system. We apply this approach to three cubic perovskite compounds, SrTiO3, CaTiO3, and NaNbO3, that are known to undergo antiferrodistortive phase transitions. We calculate their transition sequences and transition temperatures at the experimental lattice constants. For SrTiO3, we find our results agree well with experiment. For more complicated compounds like CaTiO3 and NaNbO3, which can have many different structures with very similar energy, the agreement is somewhat less.

First-principles theory of structural phase transitions for perovskites: competing instabilities

A theoretical investigation of disorder in undoped polyacetylene indicates that a substantial amount of topological and structural disorder is consistent with the experimental work done to date. Certain topological defects (chain ends and cross links) are found to be unstable to soliton emission, with the remarkable consequence that odd-membered finite chains always contain a soliton. Relaxations of the stable defects are determined. Various kinds of structural disorder are studied; these include admixtures of cis-isomerization, bending and twisting of chains, local interchain interactions, and stochastic bond-length fluctuations. The effect upon the electronic density of states is calculated in each case. When some chain bending is included, that a distribution of interchain interaction and a plausible amount of bond-length disorder may explain the observed broadening of the Peierls edges.

David Vanderbilt

Papers

Theory of hypothetical ferroelectric superlattices incorporating head-to-head and tail-to-tail 180° domain walls

Atomic scale calculations in materials science

Core reconstruction of the 90-degree partial dislocation in non-polar semiconductors

First-principles theory of structural phase transitions for perovskites: competing instabilities

Effects of disorder on the electronic structure of undoped polyacetylene