Molecular properties by a b i n i t i o quasidegenerate many-body perturbation theory effective Hamiltonian method: Dipole and transition moments of CH and CH+

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Abstract
The ab initio effective valence shell Hamiltonian method, based on quasidegenerate many‐body perturbation theory, is generalized to calculate molecular properties as well as the valence state energies which have previously been determined for atoms and small molecules. Our approach is applicable to both expectation values and transition moments of any molecular property within and between the valence states, respectively. The procedure requires the evaluation of effective operators for each molecular property. Effective operators are perturbatively expanded in powers of correlation and contain contributions from excitations outside of the large multireference valence space. Expectation values and transition moments are the diagonal and off‐diagonal matrix elements, respectively, of the effective property operators between the eigenfunctions of the correlated effective Hamiltonian. Calculations for dipole moments of and transition moments between several low lying states of CH and CH+ to first order in the correlation corrections are compared with large configuration interaction calculations to show that our methods provide a useful ab initio formalism for dipole moments.