Calculating electron transfer couplings by the Spin-Flip approach: energy splitting and dynamical correlation effects

We propose to use Spin-Flip (SF) methods for calculating electron transfer coupling strengths in terms of energy gaps between adiabatic states. The SF variant of Configuration-Interaction-Singles (SF-CIS) significantly improves descriptions of adiabatic states in charge-transfer systems and yields reliable energy splittings at the electron-transfer (ET) transition state. The SF-CIS scheme is more robust than the Hartree–Fock Koopmans Theorem, and its coupling dependence on intermolecular separation is correct even in diffuse basis sets. The SF approach allows for the systematic inclusion of dynamical correlation effects in electron transfer couplings. Our results indicate a small dynamical correlation effect for intermolecular electron transfer couplings.