Journal
MOLECULAR PHYSICS
Volume 111, Issue 16-17, Pages 2570-2584Publisher
TAYLOR & FRANCIS LTD
DOI: 10.1080/00268976.2013.827253
Keywords
intermolecular interactions; symmetry-adapted perturbation theory; single-exchange approximation; overlap expansion; exchange-induction energy; exchange-dispersion energy
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It is shown that the exchange-dispersion energy contribution of symmetry-adapted intermolecular perturbation theory (SAPT) can be evaluated without invoking the single-exchange approximation in the case of single-determinant ground-state references. A succinct expression is derived which allows for efficient implementation. Making use of the corresponding exchange-induction contribution, obtained recently, this enables DFT-SAPT calculations, i.e., SAPT based on a density-functional theory description of the monomers, without an expansion in orders of the intermonomer overlap S. The approach is used to test the quality of the S-2 or single-exchange approximation for the systems He-2, Ar-2, N-2-Ne, H2O-Ar, (H2O)(2) and LiF, i.e., van der Waals and hydrogen-bridged complexes as well as an ionic molecule. While employing the S-2 approximation for the exchange-dispersion contribution is found to have minor impact on total SAPT interaction energies, its use to evaluate the exchange-induction contribution significantly affects the quality of the repulsive branches of the potential energy curves for the interacting partners. Adding appropriate Hartree-Fock level estimates of higher-order induction plus exchange-induction energies, however, restores good agreement with complete basis set limit coupled-cluster results.
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