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.

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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.

A realistic approach to the electronic theory of bond-coordination defects in chalcogenides, based on self-consistent pseudopotential calculations, is used to study glassy Se. The results of the pseudopotential calculations are interpreted in terms of simpler tight-binding models. The onefold and threefold coordination defects are both found to give rise to nondegenerate, nonhydrogenic gap states, whose properties are unique to the chalcogenides in several respects. The existence of $\ensuremath{\pi}$ interactions between nonbonding orbitals at defect sites is found to be crucial to an understanding of the electronic structure. These interactions are responsible for large charge transfers between atoms and consequently large energy shifts of atomic valence orbitals, which make these defects quite unlike those in other semiconductors. The origin, character, energy location, and localization of the defect states associated with bond-coordination defects, and with defect pairs and certain relaxed defects, are discussed.

Theory of defect states in glassy selenium

We investigate the energetics of the single-period and double-period core reconstructions of the $90\ifmmode^\circ\else\textdegree\fi{}$ 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 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.

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Core reconstruction of the 90 ° partial dislocation in nonpolar semiconductors

Perovskite and double perovskite oxides are perhaps the most studied ferroelectric and multiferroic materials. The authors present in this manuscript an extensive analysis of another promising family of ferroelectrics, the corundum derivatives $A\phantom{\rule{0}{0ex}}B\phantom{\rule{0}{0ex}}{O}_{3}$ and ${A}_{2}\phantom{\rule{0}{0ex}}B\phantom{\rule{0}{0ex}}{B}^{\ensuremath{'}}\phantom{\rule{0}{0ex}}{O}_{6}$. Despite a superficial similarity of the chemical formulas to single and double perovskites, the corundum derivatives have an entirely different structure and the mechanism for the polarization reversal. The authors perform a detailed first-principles study and arrive at empirical rules for evaluating the ferroelectric properties of materials in this class.

Ferroelectricity in corundum derivatives

Ferroelectric domain walls (FDWs) are usually considered to be of Ising type, but there have been suggestions in recent years that Bloch-type FDWs are also possible in some cases, e.g., in the rhombohedral phase of BaTiO${}_{3}$. The mechanically compatible and electrically neutral FDWs in rhombohedral BaTiO${}_{3}$ are of 71${}^{\ensuremath{\circ}}$, 109${}^{\ensuremath{\circ}}$, and 180${}^{\ensuremath{\circ}}$ type. We have investigated these FDWs based both on first-principles calculations and on a Ginzburg-Landau-Devonshire (GLD) model [P. Marton, I. Rychetsky, and J. Hlinka, Phys. Rev. B 81, 144125 (2010)]. The results from both approaches confirm the Ising nature of the 71${}^{\ensuremath{\circ}}$ FDW and the Bloch nature of the 180${}^{\ensuremath{\circ}}$ FDW, and predict both Ising-type and Bloch-type FDWs are possible for the 109${}^{\ensuremath{\circ}}$ case. Considering the relatively small rhombohedral strain in BaTiO${}_{3}$, the competition between the energies of Bloch and Ising FDWs can be discussed in terms of a picture in which a Bloch wall is regarded as being composed of a pair of smaller-angle Ising ones. A reduction by 40$%$ in the parameters describing the gradient term in the GLD model brings it into better agreement with the first-principles results for detailed properties such as the energies and widths of the FDWs.

Bloch-type domain walls in rhombohedral BaTiO 3

Although ferroelectric compounds containing hydrogen bonds were among the first to be discovered, organic ferroelectrics are relatively rare. The discovery of high polarization at room temperature in croconic acid [Horiuchi et al., Nature (London) 463, 789 (2010)] has led to a renewed interest in organic ferroelectrics. We present an ab initio study of two ferroelectric organic molecular crystals, 1-cyclobutene-1,2-dicarboxylic acid (CBDC) and 2-phenylmalondialdehyde (PhMDA). By using a distortion-mode analysis we shed light on the microscopic mechanisms contributing to the polarization, which we find to be as large as 14.3 and 7.0 $\ensuremath{\mu}$C/cm${}^{2}$ for CBDC and PhMDA, respectively. These results suggest that it may be fruitful to search among known but poorly characterized organic compounds for organic ferroelectrics with enhanced polar properties suitable for device applications.

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David Vanderbilt

Papers

Theory of defect states in glassy selenium

Core reconstruction of the 90 ° partial dislocation in nonpolar semiconductors

Ferroelectricity in corundum derivatives

Bloch-type domain walls in rhombohedral BaTiO 3

Polar distortions in hydrogen bonded organic ferroelectrics