Calculations of vibrational energy levels by using a hybrid ab initio and DFT quartic force field: application to acetonitrile.

A hybrid quartic force field with quadratic force constants calculated at the CCSD(T)/cc-pVTZ level and cubic and quartic terms determined by a B3LYP/cc-pVTZ treatment is proposed to compute the vibrational energy levels of acetonitrile from a variational method. Fundamentals and overtones calculated in the range of 300-3200 cm(-1) are in fair agreement with the 31 observed data, with an absolute mean deviation of less than 0.8%. These results allow us to explain several Fermi resonances.