Basis sets and restricted Hartree-Fock instabilities in thiocarbonyls

Abstract

While calculating zero-field splitting terms for the lowest triplet state of 4H-pyran-4-thione, at the experimental geometry, it was found that the ground state was unstable with respect to an unrestricted Hartree-Fock perturbation. The smallest fragment of the molecule retaining this instability was found to be diatomic CS. The instability is dependent on the basis set used for the calculation. The instability and its relationship to basis set, equilibrium geometry, and the importance of polarization functions for second row elements has been explored for CS and H,CS as a function of internuclear distance. Minimal basis set calculations involving second row elements may be particularly prone to such instabilities.