Understanding nonbonded interactions between molecular fragments

The insights from computational chemistry into noncovalently interacting systems go far beyond a single number that is the interaction energy. Quantum chemistry methods that provide a decomposition of this number in terms of both physical modes of interaction (such as electrostatics and dispersion) and specific molecular fragments provide a wealth of useful data for understanding and rationally designing weakly bound complexes. Symmetry-adapted perturbation theory (SAPT) is a well-grounded and robust approach that can provide such a dual interaction energy decomposition. This review describes novel variants of SAPT that can elucidate noncovalent interactions involving fragments on different molecules, fragments on the same molecule, complexes embedded in an environment, and complexes of three or more interacting subsystems.