Comparison of full-configuration interaction and coupled-cluster harmonic and fundamental frequencies for BH and HF

The harmonic and fundamental frequencies are calculated for the potential-energy curves of BH and HF using the full-configuration interaction model and two hierarchies of coupled-cluster wavefunction models. The anharmonic contributions are also obtained using second-order vibrational perturbation theory. A consistent and systematic improvement is seen for both the harmonic and anharmonic contributions when increasing the level of the correlation treatment. The changes are largest for the harmonic contributions. This is also the case when including valence or diffuse functions in the basis set. Second-order perturbation theory gives a good approximation to the anharmonic contribution and introduces errors that are small compared to the ones that occur in the harmonic Frequencies. Without significant loss of accuracy the anharmonic contribution can be computed at a lower level of correlation and using smaller basis sets than for the harmonic contribution. (C) 2001 Elsevier Science B.V. All rights reserved.