The POL term is the energy change associated with the relaxation of the MOs on each fragment in the presence of (the nuclei and occupied MOs of) the other fragment, while keeping the MOs of each fragment "absolutely localized" (i.e., only expanded by the atomic orbital basis functions belonging to that fragment) such that CT between fragments is forbidden. The CT term is then defined as the energy change from the polarized state to the fully relaxed complex, where the MOs are permitted to delocalize between fragments. Since it is possible for the spin state of the isolated adsorbate (i.e. in the FRZ stage) to differ from the spin state of the adsorbed species, an additional term corresponding to the spin flip (SF) energy will appear. It is defined as the difference between the fully relaxed DFT energies for the adsorbate-surface complex within two different spin states (e.g. triplet (T) and singlet (S)):