Symmetry-adapted perturbation theory

Symmetry-Adapted Perturbation Theory (SAPT) is a theory developed to describe non-covalent interactions between molecular fragments. SAPT provides a perturbative expression for the interaction energy. As this perturbative expression can be truncated at any level, the various terms arising from this truncated expansion provides insight into the different components of interaction energy. It is to be noted that SAPT makes use of special type of basis functions named dimer-centered basis set (DCBS) so as to avoid basis set superposition error (BSSE).