Vibrational spectra of large molecules by density functional computations beyond the harmonic approximation: the case of pyrrole and furan

This paper compares harmonic and anharmonic frequencies of pyrrole and furan computed using the B3LYP density functional and medium size basis sets. Anharmonic corrections have been obtained by a perturbative treatment recently introduced by the author in the Gaussian program. Inclusion of anharmonic contributions reduces the absolute average error by nearly an order of magnitude down to about 5 cm−1. This level of agreement is at least comparable with that of the best scaled quantum mechanical force fields, but is obtained by a completely automatic procedure without any scaling factor.