Calculation of spin densities in diatomic first-row hydrides

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Abstract
Fermi contact spin densities have been theoretically determined for the ground-state diatomic first-row hydrides CH, OH, and NH having open shell π electrons. Multiconfiguration self-consistent-field wave functions include the dominant configuration and single excitations from it describing the most important spin and orbital polarization effects. Optimization of the orbitals by precise numerical grid methods shows that this simple wave function model is capable of providing spin densities in satisfactory agreement with experiment. Gaussian basis sets suitable for use with this wave function model are determined by comparing to the precise numerical spin density results. Huzinaga’s popular (9s5p‖4s) primitive Gaussian basis provides a good starting point if augmented with diffuse and polarization functions and with a tight (high exponent) s function at hydrogen. Only the innermost few primitive functions may be contracted. Contraction coefficients may be determined on the basis of free atom Hartree–Fock calculations. These studies lead to economical contracted Gaussian basis sets that should be useful for spin density calculations on larger polyatomic radicals.