The Improvement of Intramolecular Symmetry-Adapted Perturbation Theory

Abstract

Symmetry-adapted perturbation theory (SAPT) is a popular and versatile tool to compute and decompose noncovalent interaction energies between molecules. The intramolecular SAPT (ISAPT) variant provides a similar energy decomposition between two nonbonded fragments of the same molecule, covalently connected by a third fragment. The presence of artificial dipole moments at the interfragment boundary, as the atoms of A and B directly connected to C are missing electrons on one of their hybrid orbitals, displays several issues for many fragmentation patterns (that is, specific assignments of atoms to the A/B/C subsystems), including an artificially repulsive electrostatic energy (even when the fragments are hydrogen-bonded) and very large and mutually cancelling induction and exchange-induction terms. The improved ISAPT(SIAO1) approach reassigns one electron on a singly occupied link hybrid orbital from C to each of A/B, providing reasonable values of all ISAPT corrections for all fragmentation patterns, and a fast and systematic basis set convergence. An alternative approach to study the SAPT/ISAPT interaction energy is established by singling out the noncovalent interaction using the long-ranged part of the Coulomb potential based on the Gaussian or error-function range separation. The long-ranged Coulomb potential is evaluated with either the entire intermolecular/interfragment interaction or only its attractive terms. The energy corrections from range-separated SAPT/ISAPT are in reasonable agreement with complete SAPT/ISAPT data. The best consistency is attained for the error-function separation applied to all interaction terms, both attractive and repulsive. The range separation appears to be a potentially promising technique towards a fragmentation-free decomposition of intramolecular nonbonded energy.