The influence of improved dipole-quadrupole dispersion coefficients on the predicted cohesion of ionic oxides

The coefficients governing the dipole-quadrupole dispersive attractions between all the ions in solid magnesium oxide are computed ab initio using the coupled Hartree-Fock method. These confirm for oxides the previous conclusion that the Starkschall-Gordon formula provides the most reliable values for systems not amendable to ab initio computation. Combination of the ab initio results with semiempirical estimates of the much smaller contributions from electron correlation produces improved coefficients. The cohesion predicted for MgO with the fully ionic model remains essentially unchanged on replacing the Starkschall-Gordon dipole-quadrupole dispersion coefficients with the improved values. The use of improved predictions for C8(O2-... O2-) in solid CaO, ThO2 and UO2 in place of Starkschall-Gordon values leaves essentially unchanged the cohesion predicted for CaO whilst very slightly but significantly increasing that predicted for ThO2 and UO2. Theory and experiment for ThO2 then agree as excellently as for MgO and CaO, thus showing no evidence for any covalency.