Cubic crystal cohesion and geometry-dependent spherical deformations ofions

Abstract

The cohesive properties of cubic ionic crystals are, in principle, affected by theresponse of anions to symmetry-preserving changes of their environment awayfrom the equilibrium nuclear geometry. The importance of such a response isinvestigated by performing non-empirical computations with the RelativisticIntegrals program. For each crystal, the properties computed using anionwavefunctions optimal for each nuclear geometry are compared with thosepredicted using ‘frozen’ potentials in which the same anion wavefunction, oneoptimal for a near-equilibrium nuclear geometry, is used to compute all theinter-ionic interactions. Use of such ‘frozen’ potentials leaves essentiallyunchanged the excellent predictions of the fully optimal computations for boththe lattice energy and the closest equilibrium cation–anion separation.The bulk compressibility is significantly overestimated by using ‘frozen’potentials, often by 50% or even 80%, thus destroying the agreementbetween experiment and the predictions of the fully optimal computations.