Standard vibrational frequency analysis employs the double harmonic approximation, the simplest description of the vibrations and the molecule’s interaction with the electromagnetic field of the incident photons. It treats nuclear vibrations as simple harmonic motion about the minimum energy structure and uses a correspondingly simplified model for the electromagnetic field interaction. For many systems, this is sufficient, but sometimes the approximation reduces the accuracy of the predicted spectra.

Anharmonic frequency analysis relaxes both parts of the double harmonic approximation by introducing additional mathematical terms: higher derivatives of the energy, dipole moment, polarizability (as appropriate to the type of spectroscopy being modeled). Doing so results in changes in the locations of the fundamental frequencies and enables the prediction of overtone and combination bands. Later revisions of Gaussian 09 included anharmonic IR and Raman spectra, and Gaussian 16 adds anharmonic VCD and ROA spectra.

### Predicted ROA and VCD Spectra for R-methyloxirane

The experimental spectrum is in green [C. Merton et al., * J. Phys. Chem. Lett. *** 4** (2013) 3424]. The spectra predicted with harmonic and anharmonic analyses are in pink and blue (respectively). In both cases, the anharmonic results are in better agreement with the observed spectrum.