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.

We investigate the temperature-pressure phase diagram of ${\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}$ using a first-principles effective-Hamiltonian approach. We find that the zero-point motion of the ions affects the form of the phase diagram dramatically. Specifically, when the zero-point fluctuations are included in the calculations, all the polar (tetragonal, orthorhombic, and rhombohedral) phases of ${\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}$ survive down to 0 K, while only the rhombohedral phase does otherwise. This behavior results from a practical equivalence between thermal and quantum fluctuations. Our work confirms the essential correctness of the phase diagram proposed by Ishidate et al. [Phys. Rev. Lett. 78, 2397 (1997)].

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First-principles study of the temperature-pressure phase diagram of BaTiO3.

We propose searching for Chern insulators by depositing atomic layers of elements with large spin-orbit coupling (e.g., Bi) on the surface of a magnetic insulator. We argue that such systems will typically have isolated surface bands with nonzero Chern numbers. If these overlap in energy, a metallic surface with large anomalous Hall conductivity will result; if not, a Chern-insulator state will typically occur. Thus, our search strategy reduces to looking for examples having the Fermi level in a global gap extending across the entire Brillouin zone. We verify this search strategy and identify several candidate systems by using first-principles calculations to compute the Chern number and anomalous Hall conductivity of a large number of such systems on MnTe, MnSe, and EuS surfaces. Our search reveals several promising Chern insulators with gaps of up to 140 meV.

http://mcc.uiuc.edu/workshops/electronicstructure/2013/abstracts/esw13_garrity.pdf

Chern Insulators from Heavy Atoms on Magnetic Substrates

We consider the flow of polarization current $\mathbf{J}=d\mathbf{P}/dt$ produced by a homogeneous electric field $\mathsc{E}(t)$ or by rapidly varying some other parameter in the Hamiltonian of a solid. For an initially insulating system and a collisionless time evolution, the dynamic polarization $\mathbf{P}(t)$ is given by a nonadiabatic version of the King-Smith--Vanderbilt geometric-phase formula. This leads to a computationally convenient form for the Schr\"odinger equation where the electric field is described by a linear scalar potential handled on a discrete mesh in reciprocal space. Stationary solutions in sufficiently weak static fields are local minima of the energy functional of Nunes and Gonze. Such solutions only exist below a critical field that depends inversely on the density of k points. For higher fields they become long-lived resonances, which can be accessed dynamically by gradually increasing $\mathsc{E}.$ As an illustration the dielectric function in the presence of a dc bias field is computed for a tight-binding model from the polarization response to a step-function discontinuity in $\mathsc{E}(t),$ displaying the Franz-Keldysh effect.

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Dynamics of Berry-phase polarization in time-dependent electric fields

The application of first-principles computational methods to the study of ferroelectric perovskites has greatly expanded our theoretical understanding of this important class of materials. These methods have most recently been applied to an increasingly wide range of ferroelectric materials, with special attention to their lattice dynamics, phase transitions, dielectric and piezoelectric properties, and to the study of defects and surfaces.

First-principles based modelling of ferroelectrics

We show that plane wave ultrasoft pseudopotential methods readily extend to the calculation of the structural properties of lanthanide and actinide containing compounds. This is demonstrated through a series of calculations performed on UO, UO2 ,U O 3 ,U 3O8 ,U C 2, a-CeC2, CeB6, CeSe, CeO2, NdB6, TmOI, LaBi, LaTiO3, YbO, and elemental Lu. PACS numbers: 61.50.Ah, 61.66.Fn The plane wave pseudopotential approach has developed steadily in applicability. Initially, empirical pseudopotentials were constructed for the so-called “easy” elements such as silicon and aluminum. These potentials were used mainly in the prediction and understanding of electronic band structures and were found to be particularly accurate for semiconductors. With the subsequent development of ab initio pseudopotentials and expressions for the accurate calculation of forces and stresses, the plane wave pseudopotential approach has come into its own as the method of choice for the first principles prediction of structural parameters. A combination of improved computing resources, algorithms, and pseudopotential formalisms has allowed the plane wave pseudopotential technique to be applied to ever “harder” elements, such as, initially, the first row elements (including carbon and oxygen), and later the transition metal elements. In this paper we will show

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David Vanderbilt

Papers

First-principles study of the temperature-pressure phase diagram of BaTiO3.

Chern Insulators from Heavy Atoms on Magnetic Substrates

Dynamics of Berry-phase polarization in time-dependent electric fields

First-principles based modelling of ferroelectrics

Structural Properties of Lanthanide and Actinide Compounds within the Plane Wave Pseudopotential Approach