Explicitly correlated coupled cluster calculations of the dissociation energies and barriers to concerted hydrogen exchange of (HF)n oligomers (n=2,3,4,5)

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Abstract
The electronic dissociation energies and barriers to concerted hydrogen exchange of (HF) n oligomers with n = 2,...,5 are computed by means of a many-body decomposition of the total electronic energy. The one- and two-body terms are obtained from explicitly correlated coupled cluster calculations including singles, doubles, and a perturbative triples correction (CCSD(T)-R12), in a large Gaussian basis set consisting of 276 contracted atomic functions. The three-body term is computed at the conventional CCSD(T) level in a basis set containing 228 functions. The four- and five-body terms are obtained from explicitly correlated second-order perturbation theory calculations (MP2-R12), using basis sets with 305 (tetramer) and 380 (pentamer) functions. Since the many-body terms are computed using the same basis set (i.e. the basis of the largest fragment) for all fragments and subfragments, our calculations implicitly include a counterpoise correction. The results of the calculations are compared with semi-empirical one-, two-, and three-body potentials, and new best estimates of the electronic dissociation energies and barriers are inferred. For (HF)2, (HF)3, (HF)4, and (HF)5, respectively, we obtain for the electronic dissociation energies into monomers 19.1(2), 64(2), 116(3) and 158(4)kJmol-1, and for the electronic barriers to concerted hydrogen exchange 175(10), 85(10), 60(10) and 65(10)kJmol-1. The results are shown to be consistent with NMR line broadening data within the framework of transition state theory.

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