1,721,062 research outputs found
First-principle molecular dynamics with ultrasoft pseudopotentials: Parallel implementation and application to extended bioinorganic systems
No full textA plane-wave ultrasoft pseudopotential implementation of first-principle molecular dynamics was described. An approach to model molecular systems with a net charge or large dipole moments were also described. It was observed that the method is well suited to model large molecular systems containing transition metal centers. The results show that the accurate density-functional theory calculations on systems with several hundred atoms are feasible with access to moderate computational resources
Hartree-Fock energy bands in molecular crystals : Solid hydrogen in the cubic phase
No full textWe calculate the Hartree-Fock energy bands of solid hydrogen in the cubic phase, using a plane-wave-basis set. The novelty of our approach is that, by the expression of the crystal density matrix in terms of localized Gaussian functions, all the matrix elements necessary to calculate the energy bands (including those of the nonlocal exchange operator) can be computed in closed analytical form. This is achieved by the extension to molecular crystals of a technique previously applied to solid rare gases. We compare our results with previous theoretical work, and with available experimental data. In particular, we provide a new interpretation of recent photoemission measurements on thick H2 films deposited on noble metals. The effects of the molecular steric order on the electron states are also briefly discussed
Relationship between electronic conduction states and indirect magnetic exchange
No full textn attempt to find a quantitative relationship between magnetic transition temperature and conductionstates properties in metallic materials is presented. The exchange interaction among magnetic moments is assumed to be of the Anderson's s-f type and the conductionstates have s-d orbital character. The results are analyzed vs. different amount of s-d hybridization within LCAO schem
Measuring shared electrons in extended molecular systems: Covalent bonds from plane-wave representation of wave function
Full text linkIn the study of materials and macromolecules by first-principle methods, the bond order is a useful tool to represent molecules, bulk materials and interfaces in terms of simple chemical concepts. Despite the availability of several methods to compute the bond order, most applications have been limited to small systems because a high spatial resolution of the wave function and an all-electron representation of the electron density are typically required. Both limitations are critical for large-scale atomistic calculations, even within approximate density-functional theory (DFT) approaches. In this work, we describe our methodology to quickly compute delocalization indices for all atomic pairs, while keeping the same representation of the wave function used in most compute-intensive DFT calculations on high-performance computing equipment. We describe our implementation into a post-processing tool, designed to work with Quantum ESPRESSO, a popular open-source DFT package. In this way, we recover a description in terms of covalent bonds from a representation of wave function containing no explicit information about atomic types and positions
High pressure lattice instabilities and structural phase transformations in solids from ab-initio lattice dynamics
No full textThis paper is devoted to some recent applications of density-functional perturbation theory to the prediction of lattice instabilities occurring in ionic solids at high pressure. We first briefly review some work aimed at understanding the interplay between shear instabilities and phonon softening in the pressure-induced phase transformations of Cesium halides and hydride. We then report on preliminary results of an attempt of ours to apply similar techniques to the long-standing problem of the pressure-induced amorphization of quartz
TOWARDS VERY LARGE-SCALE ELECTRONIC-STRUCTURE CALCULATIONS
No full textWe present a new approach to density functional theory, which does not require the calculation of Kohn-Sham orbitals. The computational workload required by our method-which is based on the calculation of selected elements of the Green's function-scales linearly with the volume of the system, thus opening the way to first-principles calculations for very large systems. Some of the problems which still hinder the achievement of this goal are discussed, and possible solutions are outlined. As an application, we calculate the charge density of a model silicon supercell containing 64 atoms slightly displaced at random from equilibrium
Simulation study of Fermi level depinning in metal-MoS2 contacts
Full text linkWe used Density Functional Theory (DFT) to study the Fermi level pinning and Schottky barrier height in metal-MoS2 contacts. We showed that the Fermi level de-pinning could be attained by controlling the distance between the metal and MoS2. In particular, with proper buffer layers and the use of back-gated structures, the Schottky barrier height can be practically zeroed in some metal-MoS2 stacks, which is important to attain Ohmic contacts
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