The theoretical decomposition of intermolecular interaction energies in various physical components is an indispensable tool to the understanding of various chemical and biochemical processes. Although all decompositions into non-observable quantities are in essence arbitrary, Symmetry-Adapted Perturbation Theory (SAPT) is one of the most successful as it naturally yields electrostatics, exchange, induction and dispersion from the interaction energy. Until recently, most SAPT implementations were limited to closed-shell systems, precluding the analysis of interactions between high-spin radical systems or between radicals and closed-shell molecules. In the present work, we implement in the software Psi4 [psicode.org] the open-shell version of low-order SAPT energy terms: Eelst(10), Eexch(10), Eind,r(20) and Eexch-ind,r(20) with orbital relaxation, Edisp(20) and Eexch-disp(20). All terms make efficient use of the density-fitting approximation, thereby allowing application to system sizes beyond the reach of existing implementations. We then evaluate the accuracy of open-shell SAPT on a test set of small and medium-sized radical dimers. We examine in particular the benzene dimer radical cation at different configurations and intermonomer distances. Our results indicate that open-shell SAPT yields quantitative results, except when extensive electronic reorganization of the monomers occur. Finally, we demonstrate application of our code to biologically relevant systems by examining how ionization affects π-π interactions in DNA base pair steps.

Efficient implementation and assessment of open-shell symmetry-adapted perturbation theory

253rd National Meeting (2017)

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