SCC-TB, DFT/B3LYP, MP2, AM1, PM3 and RHF study of ethylene oxide and propylene oxide structures, VA and VCD spectra

Abstract

Recent self-consistent-charge tight-binding studies of equilibrium structures, vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of N-acetyl- l-alanine- N ′-methyl-amide have shown promising results. Here we present further testing of the self-consistent-charge tight-binding method for equilibrium structures, VA and VCD spectra of ethylene oxide and propylene oxide in the gas-phase. Comparison to conventional methods AM1, PM3, MP2, RHF and DFT/B3LYP is carried out. We report results over a wider range of frequencies than previous work. In particular, we find indications that the self-consistent-charge tight-binding method, combined with DFT/B3LYP atomic polar tensors and atomic axial tensors, compares favourably with competing methods tendency to overestimate the location of spectral peaks with respect to frequencies, the latter observation being most pronounced in the higher frequency regions. Our findings produce additional support for the self-consistent-charge tight-binding method as a fast computational method for small and larger molecules, however, also that improved parameterisations are needed to reach accuracies of MP2 and DFT/B3LYP.