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.