State-Specific Configuration Interaction for Excited States

We introduce and benchmark a systematically improvable route for excited-state calculations, labeled state-specific configuration interaction (Delta CI), which is a particular realization of multiconfigurational self-consistent field and multireference configuration interaction. Starting with a reference built from optimized configuration state functions, separate CI calculations are performed for each targeted state (hence, state-specific orbitals and determinants). Accounting for single and double excitations produces the Delta CISD model, which can be improved with second-order Epstein-Nesbet perturbation theory (Delta CISD+EN2) or a posteriori Davidson corrections (Delta CISD+Q). These models were gauged against a vast and diverse set of 294 reference excitation energies. We have found that Delta CI is significantly more accurate than standard ground-state-based CI, whereas close performances were found between Delta CISD and EOM-CC2 and between Delta CISD+EN2 and EOM-CCSD. For larger systems, Delta CISD+Q delivers more accurate results than EOM-CC2 and EOM-CCSD. The Delta CI route can handle challenging multireference problems, singly and doubly excited states, from closed-and open-shell species, with overall comparable accuracy and thus represents a promising alternative to more established methodologies. In its current form, however, it is reliable only for relatively low-lying excited states.