Electric Dipole Moment of Diatomic Molecules by Configuration Interaction. II Fixed-Core Approximation

A configuration interaction function was calculated for the ground state of CS starting with the restricted Hartree–Fock function and including single and double excitations from the valence orbitals only. The dipole moment calculated from this function is compared to that from an open-core CI function, which includes single and double excitations from all the occupied Hartree–Fock orbitals, and to the experimental value; the fixed-core approximation is found to be unsatisfactory for calculating dipole moments. A perturbation theory approximation to the eigenvalue and eigenvector of the Hamiltonian matrix is developed for arbitrary order and is used to examine the way that single and double excitations couple to the Hartree–Fock function through second order in the perturbation series.