EXPLICITLY CORRELATED COUPLED-CLUSTER CALCULATIONS OF THE DISSOCIATION-ENERGIES AND BARRIERS TO CONCERTED HYDROGEN-EXCHANGE OF (HF)(N) OLIGOMERS (N = 2, 3, 4, 5)

The electronic dissociation energies and barriers to concerted hydroge n exchange of (HF), oligomers with n = 2,...,5 are computed by means o f a many-body decomposition of the total electronic energy. The one-an d two-body terms are obtained from explicitly correlated coupled clust er 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 t he 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 s ets with 305 (tetramer) and 380 (pentamer) functions. Since the many-b ody terms are computed using the same basis set (i.e. the basis of the largest fragment) for all fragments and subfragments, our calculation s implicitly include a counterpoise correction. The results of the cal culations are compared with semi-empirical one-, two-, and three-body potentials, and new best estimates of the electronic dissociation ener gies 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)kJ mol(-1), and for th e electronic barriers to concerted hydrogen exchange 175(10), 85(10), 60(10) and 65(10) kJ mol(-1). The results are shown to be consistent w ith NMR line broadening data within the framework of transition state theory.