On the calculation of the vibrational frequencies of polycyclic aromatic hydrocarbons

The basis set dependence of the B3LYP approach for the calculation of the vibrational frequencies of naphthalene is shown. The dependence of the accuracy of the vibrational frequencies on the choice of functional is shown for naphthalene and naphthalene cation. Ten functionals are applied to large polycyclic aromatic hydrocarbons (PAHs); for the closed-shell neutral molecule, all of the methods perform well, while for two of the open-shell cations, all of the hybrid functionals tested fail, while all of the functionals using the generalized gradient approximation work. The implications on how best to study the vibrational frequencies of PAH molecules are discussed.