Ab Initio Calculation of Harmonic Force Constants

A method is presented which allows one to obtain analytically the harmonic force constants of a molecule solely from the single‐center wavefunctions for the molecule in its equilibrium nuclear configuration. The dependence of the nonlinear parameters of the wavefunction upon the nuclear coordinates is explicitly included. The method is applied to a simple single‐determinantal wavefunction for water and an analysis of the terms contributing to the force constants is made. The extension of the method to more complex single‐center wavefunctions is also discussed. Methods for obtaining the derivatives of the various one‐ and two‐electron integrals which are required in this treatment are given in the Appendix.