All experimental and theoretical studies76–79 on the geometry of nitromethane agree that the barrier (ΔE = 9.6μE h 76) for the rotation of the methyl group around the CN axis is very small, with the staggered conformation being slightly more stable. Our results reproduce quantitatively the barrier height (ΔE B3LYP = 10μE h). Such a low barrier makes it necessary to consider several rotamers when modeling the theoretical spectra, and for this reason all the geometries corresponding to the extrema of the energy along the rotation of the methyl group are considered in this study (cf. Fig. 1). A Boltzmann averaging at room temperature of these rotamers would give a quasi-equal weight for each of the conformers, and for this reason all rotamers will thus be considered of equal weight in the averaging of the spectra from the different rotamers. We note that such a treatment for the low-frequency internal rotation of the methyl group has to be considered approximate, and that this vibration mode probably should be treated by a non-local representation going beyond the normal-mode approximation. For this reason, we will in the following not include this mode in the anharmonic treatment.