Density Functional Expression Research Articles

Positron-electron correlation-polarization potential model for positron binding in polyatomic molecules.

Positron binding energies (PBEs) of 41 polyatomic molecules were calculated using the positron-electron correlation-polarization potential (CPP) approach and compared with experimentally measured values. In this approach, the short-range positron-electron potential is modeled using the density-functional expression, whereas the long-range potential is approximated by the attractive polarization potential. The positron-electron CPP model based on local-density approximation yields larger PBEs than experimental values; however, the calculated values can be substantially improved by introducing generalized gradient approximation. We also investigated the conformational dependence of PBEs for representative molecules.

Positron binding to hydrocarbon molecules: calculation using the positron–electron correlation polarization potential

Positron binding energies of naphthalene and alkanes (CnH2n+2 with n = 3 − 16) are calculated using the correlation polarization potential approach, where the short-range positron–electron correlation potential is modeled by the density-functional expression taken from a homogenous electron gas model. In the case of naphthalene, the calculated positron binding energy is found to reasonably agree with the experimental measurement. In the case of CnH2n+2 with the linear all-trans conformation we found positive positron binding energies for n ≥ 8 while the positron is not bound for n ≤ 7. This result cannot reproduce the previous experimental study, where the positron was bound for all alkanes with 3 ≤n ≤ 16. In addition, our calculated positron binding energies for n ≥ 9 are much larger than the experimental values although the generalized gradient approximation could improve the calculated values. We also investigated the conformer dependence of the positron binding energy for C16H34 and found that the positron binding energies significantly depend on the conformational structure; hairpin-like and crown-like structures generally have large positron binding energies, while single and multiple gauche structures have smaller binding energies.

Ultra-nonlocality in density functional theory for photo-emission spectroscopy.

We derive an exact expression for the photocurrent of photo-emission spectroscopy using time-dependent current density functional theory (TDCDFT). This expression is given as an integral over the Kohn-Sham spectral function renormalized by effective potentials that depend on the exchange-correlation kernel of current density functional theory. We analyze in detail the physical content of this expression by making a connection between the density-functional expression and the diagrammatic expansion of the photocurrent within many-body perturbation theory. We further demonstrate that the density functional expression does not provide us with information on the kinetic energy distribution of the photo-electrons. Such information can, in principle, be obtained from TDCDFT by exactly modeling the experiment in which the photocurrent is split into energy contributions by means of an external electromagnetic field outside the sample, as is done in standard detectors. We find, however, that this procedure produces very nonlocal correlations between the exchange-correlation fields in the sample and the detector.

Alternative equations of motion for dynamical simulated annealing of the density functional.

An alternative scheme for ab initio molecular-dynamics simulations using the density-functional expression for the total electronic energy is proposed. A constrained procedure emerges where only the total electronic density need be dynamically propagated. When energy dissipation is removed from the minimization dynamics, the method becomes equivalent to the microcanonical sampling of the functional integral representation of interacting fermions and bosons. \textcopyright{} 1996 The American Physical Society.

Electronic structure and Schottky-barrier heights of (111) NiSi2/Si A- and B-type interfaces.

The electronic structures of ${\mathrm{NiSi}}_{2}$/Si(111) A- and B-type interfaces are calculated within the local density approximation (LDA) using large supercells and the LMTO full-potential and ASA methods. The Schottky-barrier heights for the two interfaces differ by 0.14 eV, in agreement with experiment. The difference is caused by a partly filled interface band present in both structures. The LDA barrier heights ${E}_{F}$-${E}_{v}$ are both 0.4 eV too low, and insensitive to interface relaxations and to external potentials. The correct density-functional expression for the barrier height is ${E}_{F}$-${E}_{v}$+\ensuremath{\Delta}${v}_{\mathrm{xc}}$, where \ensuremath{\Delta}${v}_{\mathrm{xc}}$ is a nonlocal correction.

Density-functional analysis of Wigner crystallization.

When the density of the electrons in jellium is modulated sinusoidally, the density-functional expression for the energy depends on the wave number of the modulation. It is shown that this dependence on the wave number can lead to the modulated system having lesser energy than the case of uniform density at high values of ${r}_{s}$, thereby suggesting the possibility of Wigner crystallization.