ABSTRACT
A generalized least-squares fitting procedure which jointly uses the energy and gradient data obtained at B3LYP/cc-pVTZ level is applied to construct the anharmonic force field of acetonitrile, methyllithium and its dimer. From a pure variational method we obtain all vibrational energy transitions expected in the medium IR region. For acetonitrile our results are in perfect agreement with the observed data. For methyllithium the disagreements observed suggest a new interpretation of the experimental spectra.
