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 t he 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 overl ap 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)), indep endent of such exchange, equal to the nonpoint Coulombic electrostatic inte raction. 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 attracti ve 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 elec tron gas density functional predictions of V-perm(0)(r(AA)) are oversensiti ve 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 optim al for a geometry very close to that of the crystal at equilibrium. This da ta plus the total "frozen" V-s(r(AA)) consisting of V-s(0)(r(AA)) plus an a pproximate electron correlation contribution were required to parametrize b oth previous compressible ion model studies and the refinements presented i n the next paper. (C) 2001 American Institute of Physics.