ABINITIO STUDIES OF THE GEOMETRIES AND ELECTRONIC-STRUCTURES OF CAF2 AND CACL2

Ab initio calculations with basis sets approaching the HF limit do not converge monotonically towards a certain bond angle. The energy gap between the linear and the bent geometry for CaF2 at both the SCF and the MP2 level is extremely small. Small basis sets lead to bent structures and bond lengths that are much too short, indicating a large BSSE. The inclusion of d orbitals is a precondition to obtain bent geometries, but a large set of 4p orbitals is crucial for the geometry determination as well, since p orbitals favour linear structures. Thus, near the HF limit, basis sets do not indicate significant bending if the 3d and the 4p orbitals are equally well represented. Correlation effects are of minor importance. No significant difference from CaCl2 is detected which might explain the claimed experimental evidence that CaF2 is bent and CaCl2 linear. © 1990.