Accurate vibrational spectra of large molecules by density functional computations beyond the harmonic approximation: The case of azabenzenes

This paper compares harmonic and anharmonic frequencies of pyridine, pyridazine, pyrimidine, pyrazine, 1,2,3-triazine, 1,2,4-triazine, and s-tetrazine computed using the B3LYP density functional method and medium size basis sets. Anharmonic corrections cannot be neglected for quantitative studies but can be obtained quite effectively by a perturbative treatment including cubic force constants to the second order and semidiagonal quartic constants to the first order. Simple finite difference equations provide all the necessary terms by at most 6N-11 Hessian evaluations, where N is the number of atoms in the system. This approach has been recently coded in the Gaussian package and provides for all the azabenzenes mentioned above an absolute average error of fundamental bands below 10 cm(-1) and a maximum error never exceeding 40 cm(-1). This level of agreement, obtained by a black box procedure without any scaling factor, allows to propose some revision of current spectral assignments based on less reliable harmonic computations.