Many-body contributions to the intermolecular potential in alkali halide crystals and clusters

Abstract

SCFMO calculations have been made on lithium fluoride clusters both within the crystal and as isolated species. Calculations have been made with different bases in order to separate exchange, charge-transfer and induction energies. Calculations confirm the conclusion from perturbation theory that charge-transfer is an important contribution to three-body energies and gives a small four-body energy. The three-body energies in the crystal environment are much smaller than for the isolated clusters. I. INTRODUCTIONThe considerable interest in the static and dynamic properties of ionic crystals, both perfect and defective lattices, and ionic melts, provides a continuing incentive to obtain accurate interionic potential functions. There has indeed been no lack of effort on this front; the first alkali halide potentials which gave a good fit to the lattice energy were obtained by Huggins and Mayer in 1933 [1].The total potential for a finite cluster of ions can be written as a many-body expansion. The first term will be the sum of pair potentials, the second we refer to as the three-body term, etc. The pair potentials contain the Coulomb energy, which is dominant at large distances and the exchange energy which provides the short range repulsion. There will be other smaller terms, some long range and some short, such as dispersion, induction and charge transfer, whose relative importance depends on the internuclear separation. There are however problems relating to whether these energies make separate and additive contributions to the two-body energy [2]. For polar molecules and particularly for ions it has generally been considered that the inductive energies are the most important contribution to the higher n-body terms. These energies are associated with the polarization of one atom or molecule in the total electric field arising from its neighbours. As the component fields must be combined vectorially the polarization depends on the relative positions of the neighbours.

Designing Ionic Materials Through Multiobjective Genetic Algorithms

“…Fracture toughness through molecular dynamics The potential energy for ionic materials is modeled using a combination of Born-Mayer and Coulomb potentials [1,12,13]. The former represents the repulsive force due to overlapping electron orbitals while the latter incorporates the attraction between unlike ions.…”

“…Design of ionic materials by interionic potential has been done extensively [1,2], while the potential use of Genetic Algorithms in materials design is well illustrated by recent work in this emerging area [3][4][5][6][7]. This study also involves atomic simulation of crack extension [8] and is a continuation of our previous work [9] which deals with the design of simple cubic materials using Lennard-Jones interatomic potentials and multiobjective Genetic Algorithms [10].…”

Anab initioapproach to many-body energies and Be2+V-dipoles in LiH crystal and clusters

“…To calculate the energy in the crystal environment, we follow the procedure reported by Murrell et al [12] and define a finite lattice whose Coulomb potential in the central region closely approximates the Coulomb potential in the unit cell of the infinite perfect crystal. Hartree-Fock calculations were carried out within an inner zone at the centre of the finite lattice.…”

Ground and excited state properties of many-body energies in Lin+nF-ions

“…Murrell, Tennyson and Kamel [1] performed SCFMO calculations on lithium fluoride both within the crystals and as isolated species at different bases, to separate exchange, charge transfer and induction energies. They concluded that charge transfer is an important contribution to 3-body energies and that 3-body energies in the crystal environment are much smaller than for the isolated clusters.…”