Extended basis set calculations of nonlinear susceptibilities of conjugated hydrocarbons

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Abstract
We reconsidered perturbation treatments of molecular eigenstates by expanding in terms of other molecular eigenstates. This constitutes an improvement over a previously reported treatment where we considered molecular orbitals which were expanded in terms of other molecular orbitals. In the present paper, we take the ground state molecular wavefunction as a single determinant SCF function. Fourth order energy perturbations are then obtained as a sum of contributions from singly and doubly excited molecular configurations. We evaluated the various fourth‐order energy perburbations and corresponding third‐order electric susceptibilities for a group of about 30 aromatic hydrocarbons and for a group of linear conjugated hydrocarbons. Our results are roughly consistent with the limited experimental information that is available for these molecules.