In combination with the popular use of monomer basis sets and the initial guesses for FDE calculations, which usually are isolated-system calculations, this introduces implicit constraints and can be used to obtain quasi-diabatic states (and couplings between them) of the system. By solving the resulting small, effective eigenvalue problem, one gets access to solutions corresponding to linear combinations of the diabatic states. In the context of electron-transfer (ET) phenomena, this approach is known as FDE-ET. This strategy is conceptually related to methods like Multistate DFT and constrained DFT-CI (CDFT-CI) .