Short range interaction potentials between anions in crystals

The ab initio computation of uncorrelated short range two-body anion-anion potentials V-s(0)(r(AA)) 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 V-s(0)(r(AA)), even though the overlap is reduced. Second, attractive V-s(0)(r(AA)) are occasionally predicted at large ionic separations r(AA). These apparent anomalies arise because V-s(0)(r(AA)) is the sum of a permutation term V-perm(0)(r(AA)) arising from interionic electron exchange plus a penetration term V-pen(0)(r(AA)), independent of such exchange, equal to the nonpoint Coulombic electrostatic interaction. This is attractive at realistic r(AA) and reduced in magnitude by ionic compression. V-perm(0)(r(AA)) is always repulsive and is decreased by ionic compression except occasionally at large r(AA) involving an attractive V-s(0)(r(AA)). The latter increases are explained by analyzing V-perm(0)(r(AA)) into two further terms: one involving V-pen(0)(r(AA)). Uniform electron gas density functional predictions of V-perm(0)(r(AA)) are oversensitive to the ion density, thereby missing compression-induced enhancements of V-s(0)(r(AA)). Ab initio predictions of V-pen(0)(r(AA)) and V-perm(0)(r(AA)) are presented both for "optimal" V-s(0)(r(AA)) computed using anion wave functions optimal for each crystal geometry and for "frozen" V-s(0)(r(AA)), 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" V-s(r(AA)) consisting of V-s(0)(r(AA)) plus an approximate electron correlation contribution were required to parametrize both previous compressible ion model studies and the refinements presented in the next paper. (C) 2001 American Institute of Physics.