On the nature of electronic transitions in radicals: An extended single excitation configuration interaction method

We propose that the generalization of pure spin single reference excited state theories from molecules with closed shell ground states to radicals be based on a redefinition of the meaning of excitation levels. Open shell single substitutions are expanded to include spin-adapted double excitations which traverse the singly occupied orbitals. These excitations are deleted from the expanded open shell double substitutions, which include the corresponding spin-adapted triple substitutions (e.g., a pair of the excitations traverses the singly occupied orbitals), etc. Applied to single excitation configuration interaction (CIS), this approach defines a spin-adapted extended CIS (XCIS) theory for the excited states of radicals. XCIS is size consistent and variational and has computational complexity that scales in the same way as CIS with molecular size. More low-lying excited states of radicals are correctly described by XCIS than by the restricted open shell CIS method. This is demonstrated by calculations on excited states of seven diatomic radicals and the methyl, nitromethyl, benzyl, phenoxyl, and anilino radicals.