Localized Active Space State Interaction Singles.

We introduce localized active space state interaction singles (LASSIS), a multireference electronic structure method that uses two-step diagonalization to model low-energy electronic states of systems characterized by multiple distinct localized centers of strong electron correlation, with weaker but not negligible electron correlation between the centers. LASSIS is a specific variant of localized active space state interaction (LASSI), which restores interfragment interactions omitted by a LASSCF reference wave function by expanding the interacting wave function in a basis of model states characterized by various charge and spin distributions. These distributions, and the number of states of each type, are determined automatically, without any user input, in contrast to previous work with the LASSI formalism. LASSIS combined with multiconfiguration pair-density functional theory (MC-PDFT) energy calculation is shown in test calculations to qualitatively reproduce the results of converged DMRG-PDFT calculations on multimetallic transition metal compounds.