Ab Initio Calculations of Harmonic Force Constants. V. Application to Natural Expansions

Abstract

The analytic method of calculating force constants is extended to handle wavefunctions built from the natural orbitals obtained from a configuration interaction wavefunction. The method is applied to a series of wavefunctions for H₂. No correlation between the force constant and the degree of truncation of the natural expansion is detected. In a study of some rank 1 functions for H₂ (considered to be close to the SCF wavefunctions) it is found that the contribution to the force constant $\mathrm{(k)}$ from allowance for flexibility in the nonlinear parameters is important and that the $k$ 's for the rank 1 functions and CI wavefunctions (from which they are derived) are very similar.