The spin optimized separated pair method

Abstract

Spin optimized pair wavefunctions, and their energies, are determined for the ¹ A ₁ ground state and the lowest energy ³ A ₁ and ³ T ₂ excited states of methane, and for the lowest energy ³ A ₁ and ² T ₂ states of the methane positive ion. The spatial pair functions, in both bond orbital and separated pair forms, are expanded in terms of a limited basis set of orthogonalized gaussian fitted hydrogen and carbon hybrid atomic orbitals. The pair function coefficients for methane are not very sensitive to the symmetry and multiplicity of the electronic state; but significant deviations from the methane values, and a dependence on the nature of the electronic state, are observed for the methane positive ion. In all cases, the structure with the greatest number of singlet coupled pairs dominates the expansion of the total wave-function. The calculated ionization energies are in reasonable agreement with experiment, but the triplet excitation energies are poor.