Accurate vibrational spectra of large molecules by density functional computations beyond the harmonic approximation: the case of uracil and 2-thiouracil

We present harmonic and anharmonic frequencies of uracil and 2-thiouracil computed using the BLYP and B3LYP density functionals and medium size basis sets. Anharmonic corrections have been obtained by a second order perturbative treatment (PT2) recently introduced by one of the authors in the Gaussian program. At the B3LYP level inclusion of anharmonic contributions reduces the absolute average error by nearly an order of magnitude, whereas BLYP results are not equally satisfactory due to an overall underestimation of harmonic frequencies. The agreement of B3LYP/PT2 results with experiments paves the route for reliable yet affordable studies of molecules of biological interest by a completely automatic procedure without any scaling factor.