Curvilinear coordinates for full-core atoms

Curvilinear coordinates, first introduced by F. Gygi for valence-only electronic systems within the local-density functional theory, can be used to describe both core and valence electrons in electronic-structure calculations. A simple and quite general coordinate transformation results in a large, yet affordable plane-wave energy cutoff for full-core systems (e.g., about 120 Ryd for carbon or silicon) within the local-density functional theory, and in a reduced correlation time for full-core variational Monte Carlo calculations. Numerical tests for isolated Li, C, and Si atoms are presented.Comment: 14 pages, 8 Postscript figures; acknowledgements and two refs. adde

Chemical Hardness, Linear Response, and Pseudopotential Transferability

We propose a systematic method of analyzing pseudopotential transferability based on linear-response properties of the free atom, including self-consistent chemical hardness and polarizability. Our calculation of hardness extends the approach of Teter\cite{teter} not only by including self-consistency, but also by generalizing to non-diagonal hardness matrices, thereby allowing us to test for transferability to non-spherically symmetric environments. We apply the method to study the transferability of norm-conserving pseudopotentials for a variety of elements in the Periodic Table. We find that the self-consistent corrections are frequently significant, and should not be neglected. We prove that the partial-core correction improves the pseudopotential hardness of alkali metals considerably. We propose a quantity to represent the average hardness error and calculate this quantity for many representative elements as a function of pseudopotential cutoff radii. We find that the atomic polarizabilities are usually well reproduced by the norm-conserving pseudopotentials. Our results provide useful guidelines for making optimal choices in the pseudopotential generation procedure.Comment: Revtex (preprint style, 33 pages) + 9 postscript figures A version in two-column article style with embedded figures is available at http://electron.rutgers.edu/~dhv/preprints/index.html#l

Pair-distribution functions of the two-dimensional electron gas

Based on its known exact properties and a new set of extensive fixed-node reptation quantum Monte Carlo simulations (both with and without backflow correlations, which in this case turn out to yield negligible improvements), we propose a new analytical representation of (i) the spin-summed pair-distribution function and (ii) the spin-resolved potential energy of the ideal two-dimensional interacting electron gas for a wide range of electron densities and spin polarization, plus (iii) the spin-resolved pair-distribution function of the unpolarized gas. These formulae provide an accurate reference for quantities previously not available in analytic form, and may be relevant to semiconductor heterostructures, metal-insulator transitions and quantum dots both directly, in terms of phase diagram and spin susceptibility, and indirectly, as key ingredients for the construction of new two-dimensional spin density functionals, beyond the local approximation.Comment: 12 pages, 10 figures; misprints correcte

Ab initio Molecular Dynamics in Adaptive Coordinates

We present a new formulation of ab initio molecular dynamics which exploits the efficiency of plane waves in adaptive curvilinear coordinates, and thus provides an accurate treatment of first-row elements. The method is used to perform a molecular dynamics simulation of the CO_2 molecule, and allows to reproduce detailed features of its vibrational spectrum such as the splitting of the Raman sigma+_g mode caused by Fermi resonance. This new approach opens the way to highly accurate ab initio simulations of organic compounds.Comment: 11 pages, 3 PostScript figure

Tailoring strain in SrTiO3 compound by low energy He+ irradiation

The ability to generate a change of the lattice parameter in a near-surface layer of a controllable thickness by ion implantation of strontium titanate is reported here using low energy He+ ions. The induced strain follows a distribution within a typical near-surface layer of 200 nm as obtained from structural analysis. Due to clamping effect from the underlying layer, only perpendicular expansion is observed. Maximum distortions up to 5-7% are obtained with no evidence of amorphisation at fluences of 1E16 He+ ions/cm2 and ion energies in the range 10-30 keV.Comment: 11 pages, 4 figures, Accepted for publication in Europhysics Letter (http://iopscience.iop.org/0295-5075

Ab initio simulations of liquid systems: Concentration dependence of the electric conductivity of NaSn alloys

Liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80% sodium) are studied using density functional calculations combined with molecular dynamics(Car-Parrinello method). The frequency-dependent electric conductivities for the systems are calculated by means of the Kubo-Greenwood formula. The extrapolated DC conductivities are in good agreement with the experimental data and reproduce the strong variation with the concentration. The maximum of conductivity is obtained, in agreement with experiment, near the equimolar composition. The strong variation of conductivity, ranging from almost semiconducting up to metallic behaviour, can be understood by an analysis of the densities-of-states.Comment: LaTex 6 pages and 2 figures, to appear in J.Phys. Cond. Ma

Local-spin-density functional for multideterminant density functional theory

Based on exact limits and quantum Monte Carlo simulations, we obtain, at any density and spin polarization, an accurate estimate for the energy of a modified homogeneous electron gas where electrons repel each other only with a long-range coulombic tail. This allows us to construct an analytic local-spin-density exchange-correlation functional appropriate to new, multideterminantal versions of the density functional theory, where quantum chemistry and approximate exchange-correlation functionals are combined to optimally describe both long- and short-range electron correlations.Comment: revised version, ti appear in PR

A mixed ultrasoft/normconserved pseudopotential scheme

A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the normconservation is released for only one or a few angular channels, is presented. Within this scheme some difficulties of the truly ultrasoft pseudopotentials are overcome without sacrificing the pseudopotential softness. i) Ghost states are easily avoided without including semicore shells. ii) The ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The number of nonlocal operators is reduced. The scheme will be most useful for transition metals, and the feasibility and accuracy of the scheme is demonstrated for the 4d transition metal rhodium.Comment: 4 pages, 2 figure

Ab initio simulations of liquid NaSn alloys: Zintl anions and network formation

Using the Car-Parrinello technique, ab initio molecular dynamics simulations are performed for liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80 % sodium). The obtained structure factors agree well with the data from neutron scattering experiments. The measured prepeak in the structure factor is reproduced qualitatively for most compositions. The calculated and measured positions of all peaks show the same trend as function of the composition.\\ The dynamic simulations also yield information about the formation and stability of Sn$_4$ clusters (Zintl anions) in the liquid. In our simulations of compositions with 50 and 57 % sodium we observe the formation of networks of tin atoms. Thus, isolated tin clusters are not stable in such liquids. For the composition with 20 % tin only isolated atoms or dimers of tin appear, ``octet compounds'' of one Sn atom surrounded by 4 Na atoms are not observed.Comment: 12 pages, Latex, 3 Figures on reques

Variational finite-difference representation of the kinetic energy operator

A potential disadvantage of real-space-grid electronic structure methods is the lack of a variational principle and the concomitant increase of total energy with grid refinement. We show that the origin of this feature is the systematic underestimation of the kinetic energy by the finite difference representation of the Laplacian operator. We present an alternative representation that provides a rigorous upper bound estimate of the true kinetic energy and we illustrate its properties with a harmonic oscillator potential. For a more realistic application, we study the convergence of the total energy of bulk silicon using a real-space-grid density-functional code and employing both the conventional and the alternative representations of the kinetic energy operator.Comment: 3 pages, 3 figures, 1 table. To appear in Phys. Rev. B. Contribution for the 10th anniversary of the eprint serve