Spin–flip non-orthogonal configuration interaction with grouped baths: a method for charge transfer in transition metal complexes

We introduce a novel grouped-bath approach to the spin-flip non-orthogonal configuration interaction (SF-NOCI) method, named SF-GNOCI, which significantly reduces computational cost while preserving accuracy. SF-NOCI is a nearly ``black-box'' electronic structure theory developed by Mayhall et al., well-suited for studying charge transfer phenomena. This method allows for core-virtual orbital rotations for all configurations within the active space, efficiently capturing important orbital relaxation effect induced by electron transfer. However, the inclusion of this relaxation effect for all configurations results in a sharp increase in computational cost, especially for large active spaces commonly encountered in transition metal complexes. To address this challenge, we grouped configurations based on the number of electrons associated with each atom. Within each group, configurations share a common set of bath orbitals, significantly reducing the computational burden. We demonstrate the performance of SF-GNOCI through benchmark calculations on two systems: the avoided crossing of the lowest singlet states of LiF dissociation and the low-lying charge transfer states of [Fe(SCH_3)_4]^{2-/1-}. Our results show that SF-GNOCI maintains the accuracy comparable to the standard SF-NOCI while reducing a computational cost by a factor of about 1/10 for [Fe(SCH_3)_4]^{2-} and 1/14 for [Fe(SCH_3)_4]^{1-}. This SF-GNOCI method offers a promising reference wave-function ansatz for simulating charge transfer phenomena in transition metal complexes.