Charge Transfer from Regularized Symmetry-Adapted Perturbation Theory

The charge-transfer (CT) together with the polarization energy appears at second and higher orders in symmetry-adapted perturbation theory (SAPT). At present there is no theoretically compelling way of isolating the charge-transfer energy that is simultaneously basis-set independent and applicable for arbitrary intermolecular separation. We argue that the charge-transfer can be interpreted as a tunneling phenomenon and can therefore be defined via regularized SAPT. This leads to a physically convincing, basis-independent definition of the charge-transfer energy that captures subtleties of the process, such as the asymmetry in the forward and backward charge transfer, as well as secondary transfer effects. With this definition of the charge-transfer the damping needed for polarization models can be determined with a level of confidence hitherto not possible.