Fundamentally, polarization is orbital relaxation in response to a perturbing influence, in this case, from a nearby nonbonded partner. Mathematically, orbital relaxation can be cast as a unitary transformation from the isolated-monomer MOs to the relaxed orbitals obtained in the presence of the partner. Such a transformation can always be written in terms of singly-substituted Slater determinants expressed in the original, unrelaxed basis. In a compact basis set, where the atomic orbital basis functions centered on monomer A have little overlap with those centered on B, one can readily identify the A-centered excitations that relax A's MOs in the presence of B, and distinguish them from A → B excitations. That dichotomy could provide a basis for separating E pol from E CT . In larger basis sets, however, functions centered on one monomer extend significantly over the other monomer and the distinction between orbital relaxation and CT is obscured. As a result, orbital-based definitions of CT are notoriously unstable with respect to expansion of the atom-centered Gaussian basis set. Within SAPT, it has been suggested to define E CT as the difference between E