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

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.