We present a formalism for coupling a density-functional-theory-based quantum simulation to a classical simulation for the treatment of simple metallic systems. The formalism is applicable to multiscale simulations in which the part of the system requiring quantum-mechanical treatment is spatially confined to a small region. Such situations often arise in physical systems where chemical interactions in a small region can affect the macroscopic mechanical properties of a metal. We describe how this coupled treatment can be accomplished efficiently, and we present a coupled simulation for a bulk aluminum system.

Kinetic energy functionals of the electronic density are used to model large systems in the context of density functional theory, without the need to obtain electronic wavefunctions. We discuss the problems associated with the application of widely used kinetic energy functionals to non-periodic systems. We develop a method that circumvents this difficulty and allows the kinetic energy to be evaluated entirely in real space. We demonstrate that the method is efficient [O(N)] and accurate by comparing the results of our real-space formulation to calculations performed in reciprocal space, and to calculations using traditional approaches based on electronic states.Comment: 12 pages, 4 figure

We present the application of orbital-free density functional theory (OF-DFT) to NaAlH(4), a potential hydrogen storage material, and related systems. Although the simple Al and NaH structures are reproduced reasonably well by OF-DFT, the approach fails for the more complex NaAlH(4) structure. Calculations on AlH(3) show that the failure to describe the Al-H interaction is related to the kinetic energy functionals used rather than the local pseudopotentials which are required within the OF-DFT approach. Thus, systems such as NaAlH(4) present a challenge which awaits the development of more reliable orbital-free kinetic energy functionals.

We present new efficient (O(N log N )) methods for computing three quantities crucial to electronic structure calculations: the ionic potential, the electron-ion contribution to the Born-Oppenheimer forces, and the electron-ion contribution to the stress tensor. The present methods are applicable to calculations in which the electronic charge density is represented on a uniform grid in real space. They are particularly well-suited for metallic extended systems, where other O(N ) methodologies are not readily applicable. Based on a fast algorithm for determining the atomic structure factor, originally developed by Essmann et al. [1] for fast Ewald energy and force computation, the present methods involve approximations that can be systematically improved. The methods are tested on a representative metallic system (bulk Al), and their ability to simultaneously achieve high accuracy and efficiency is demonstrated. A wealth of efficient methods have recently been developed [2,3,4,5,6,7,8,9,10,11,12] for calculating electronic properties of an extended physical system that require an amount of computation that scales linearly with the size of the system N . This size can be defined to be the number of atoms or the number of valence electrons, or the volume of the system, all of which are linearly related for large condensed systems. In this article we present a quasi-linear-scaling (O(N log N )) method for computing the ionic potential, the ionic forces, and the stress tensor in electronic structure calculations. Atomic forces are necessary for the calculation of many physical properties of a system, including the determination of the optimal structure and simulation at a finite temperature. Some linear-scaling methods achieve linear computational scaling for the computation of the energy but not for the forces on all of the ions [8]. Other linear-scaling methods achieve efficient force calculations by working with a basis of localized functions [6,9,10], which are less efficient at representing delocalized electronic states found for example in metallic systems. The present method applies to calculations performed in a periodic parallelepiped supercell, not necessarily orthogonal, in which the electronic charge density ρ(r) is represented on a uniform grid.As a result of the Hellmann-Feynman [13,14] theorem, the force on the pth ion (within the Born-Oppenheimer approximation) is given by the sum of the partial derivatives of the ion-ion energy (the Ewald energy) and the electron-ion energy with respect to the atomic coordinates:where (2) and the ionic potential is defined as:where here the t p are the atomic positions within the unit cell; V psp (r) is the pseudopotential representing the ions; and the outer sum over R is over all lattice translation vectors R = n 1 a 1 + n 2 a 2 + n 3 a 3 for all integers n i , where the a i are the lattice vectors defining the unit cell. For simplicity of presentation, we will consider systems that involve only one type of pseudopotential, but the methods presented here generalize...