PNNL

Abstract

We present a critical comparison between two different approaches for calculating anharmonic frequencies, namely grid-based fitting methods (VSCF, CC-VSCF, VCI) and the ones obtained by second-order perturbation theory from the third and fourth energy derivatives. 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 theory and basis set. The two methods were found to produce results of similar accuracy for the fundamental band origins 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. This work was performed under the auspices of the Division of Chemical Sciences, Geosciences and Biosciences, Office of Science, US Department of Energy with Battelle Memorial Institute, which operates the Pacific Northwest National Laboratory. Computer resources at the National Energy Research Scientific Computer Center (NERSC) were provided by the Division of Chemical Sciences Geosciences and Biosciences, US Department of Energy.