Overcoming Artificial Multipoles in Intramolecular Symmetry-Adapted Perturbation Theory

TL;DR

This paper introduces new partitioning algorithms for intramolecular symmetry-adapted perturbation theory (ISAPT) to correct artificial dipole moments at interfragment boundaries, improving accuracy and convergence in energy decomposition.

Contribution

The authors propose several novel link partitioning algorithms, especially ISAPT(SIAO1), that address issues of unphysical dipole moments and improve ISAPT's reliability across various molecular fragmentation patterns.

Findings

ISAPT(SIAO1) yields reasonable energy corrections for all tested patterns.

The new algorithms reduce unphysical dipole moments at boundaries.

Improved ISAPT shows better basis set convergence and accuracy.

Abstract

Intramolecular symmetry-adapted perturbation theory (ISAPT) is a method to compute and decompose the noncovalent interaction energy between two molecular fragments A and B connected via a linker C. The existing ISAPT algorithm displays several issues for many fragmentation patterns, including an artificially repulsive electrostatic energy (even when the fragments are hydrogen-bonded) and very large and mutually cancelling induction and exchange-induction terms. We attribute those issues to the 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. Therefore, we propose several new partitioning algorithms which reassign one electron, on a singly occupied link hybrid orbital, from C to each of A/B. Once the contributions from these link orbitals are added to fragment density matrices, the computation of ISAPT electrostatic, induction, and dispersion energies proceeds exactly as normal, and the exchange energy expressions need only minor modifications. Among the link partitioning algorithms introduced, the ISAPT(SIAO1) approach (in which the link orbital is obtained by a projection onto the intrinsic atomic orbitals of a given fragment followed by orthogonalization to this fragment's occupied space) leads to reasonable values of all ISAPT corrections for all fragmentation patterns and exhibits fast and systematic basis set convergence. This improvement is made possible by a significant reduction in magnitude (even though not a complete elimination) of the unphysical dipole moments at the interfragment boundaries. We demonstrate the utility of the improved ISAPT partitioning by examining intramolecular interactions in several pentanediol isomers, examples of linear and branched alkanes, and the open and closed conformations of a family of N-arylimide molecular torsion balances.