A flexible method for calculating vibrational spectra of clusters applied to methanol

A method is presented for calculating vibrational spectra of clusters using intra- and intermolecular potential models of different origin. The two types of potentials are tuned and coupled to construct a total cluster potential. The determination of minima on this cluster potential energy surface, and the expansion of the potential about the minimum configurations makes it possible to apply standard spectroscopic methods. Starting from a normal mode analysis, harmonic and anharmonic frequency shifts are calculated using perturbational and variational methods for methanol clusters from the dimer to the hexamer. The results using the empirical OPLS potential model are compared with each other, with the harmonic line shift calculations for a calculated potential in the SCF approximation, and with the experimental data. There is an improvement with respect to the results obtained so far, however, the need for a better description of the intermolecular potential is pointed out