Getting down to the Fundamentals of Hydrogen Bonding: Anharmonic Vibrational Frequencies of (HF)2 and (H2O)2 from Ab Initio Electronic Structure Computations

This work presents a systematic investigation into the basis set convergence of harmonic vibrational frequencies of (H2O)(2) and (HF)(2) computed with second-order Moller-Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles method with perturbative connected triples, CCSD(T), while employing correlation-consistent basis sets as large as aug-cc-pV6Z. The harmonic vibrational frequencies presented here are expected to lie within a few cm-1 of the complete basis set (CBS) limit. For these important hydrogen-bonding prototype systems, a basis set of at least quadruple-zeta quality augmented with diffuse functions is required to obtain harmonic vibrational frequencies within 10 cm(-1) of the CBS limit. In addition, second-order vibrational perturbation theory (VPT2) anharmonic corrections yield CCSD(T) vibrational frequencies in excellent agreement with experimental spectra, differing by no more than a few cm-1 for the intramonomer fundamental vibrations. D0 values predicted by CCSD(T) VPT2 computations with a quadruple-zeta basis set reproduce the experimental values of (HF)(2) and (H2O)(2) to within 2 and 21 cm(-1), respectively.