Performances of different density functionals in the computation of vibrational spectra beyond the harmonic approximation

The performances of different density functionals and basis sets in the computation of anharmonic frequencies of semirigid molecules by a second-order perturbative treatment have been evaluated with reference to coupled cluster computations. Our results show that most of the errors are related to the harmonic part of the force field, so that hybrid models in which harmonic frequencies computed by coupled cluster methods are coupled to anharmonic contributions obtained by proper density functionals perform a very good job. In this connection the B3LYP/6-31+G(d,p) model appears particularly effective, whereas conventional functionals are not sufficiently reliable.