A perturbative correction to restricted open shell configuration interaction with single substitutions for excited states of radicals

A perturbative correction, termed RCIS(D), to restricted open shell single excitation configuration interaction (ROCIS) is formulated and implemented for excited states of radicals. RCIS(D) gives spin-pure energies (but not wavefunctions), is size-consistent, and has computational complexity scaling with the fifth power of molecular size. All these properties are analogous to second order restricted Mّller-Plesset theory for the ground state. Comparison of calculated vertical excitation energies against experiment for a diverse series of small radicals, BeH, BeF, CH, CH3, CN, CO+, NO and OH, suggests that RCIS(D) constitutes a useful improvement over ROCIS.