Anharmonic vibrational spectra of hydrogen bonded clusters: comparison between higher energy derivative and mean-field grid based methods

Abstract

We present a comparison between two different approaches for calculating anharmonic frequencies, namely mean-field grid-based methods (VSCF, CC-VSCF, VCI) and those obtained by second-order perturbation theory from the third and fourth energy derivatives. For a meaningful comparison between the two approaches we report the results for the anharmonic frequencies with the two approaches for the F−(H2O), Cl−(H2O), HNO3 and HNO3(H2O) molecular systems at the same level of electronic structure theory and basis set. The two methods were found to produce results of similar accuracy for the origin of the fundamental band and selected overtone and combination bands. The optimal equilibrium and vibrationally averaged geometries, spectroscopic constants and anharmonic frequencies of the (HF) n , n = 2–4 clusters are reported at the MP2/aug-cc-pVDZ level of theory based on calculations of higher energy derivatives and compared to previous results obtained from CC-VSCF calculations at the MP2/TZP level of theory. Contents PAGE 1. Introduction 720 2. Details of the electronic structure calculations 723 3. Results and discussion 724 3.1. Comparison between higher energy derivative and mean-field grid-based methods for model systems 724 3.2. Minimum energy and vibrationally averaged geometries of (HF) n , n = 2–𝜌4 clusters 726 3.3. Anharmonic frequencies 729 4. Conclusions 731 Acknowledgements 731 References 732