Short range interaction potentials between anions in crystals

Abstract

The ab initio computation of uncorrelated short range two-body anion–anion potentials Vs0(rAA) can yield two apparent anomalies. First, despite the common understanding that the repulsion between two closed shell species arises from the overlap of their wave functions, compression of the anion electron densities sometimes increases Vs0(rAA), even though the overlap is reduced. Second, attractive Vs0(rAA) are occasionally predicted at large ionic separations rAA. These apparent anomalies arise because Vs0(rAA) is the sum of a permutation term Vperm0(rAA) arising from interionic electron exchange plus a penetration term Vpen0(rAA), independent of such exchange, equal to the nonpoint Coulombic electrostatic interaction. This is attractive at realistic rAA and reduced in magnitude by ionic compression. Vperm0(rAA) is always repulsive and is decreased by ionic compression except occasionally at large rAA involving an attractive Vs0(rAA). The latter increases are explained by analyzing Vperm0(rAA) into two further terms: one involving Vpen0(rAA). Uniform electron gas density functional predictions of Vperm0(rAA) are oversensitive to the ion density, thereby missing compression-induced enhancements of Vs0(rAA). Ab initio predictions of Vpen0(rAA) and Vperm0(rAA) are presented both for “optimal” Vs0(rAA) computed using anion wave functions optimal for each crystal geometry and for “frozen” Vs0(rAA), where the entire potential is computed using the anion wave function optimal for a geometry very close to that of the crystal at equilibrium. This data plus the total “frozen” Vs(rAA) consisting of Vs0(rAA) plus an approximate electron correlation contribution were required to parametrize both previous compressible ion model studies and the refinements presented in the next paper.