Nonequilibrium formulation of infrared frequencies and intensities in solution: Analytical evaluation within the polarizable continuum model

We present a nonequilibrium approach to the analytical evaluation of infrared frequencies and intensities for molecules in solution within the polarizable continuum model framework. Vibrational frequencies and intensities are obtained in the harmonic approximation as the second derivatives of the suitable nonequilibrium free energy. A numerical application to the calculation of carbonyl stretching frequencies and intensities of a series of ketones at the density functional level is shown. In order to effectively compare theoretical and experimental data the coupling between the solvent and the probing field is also taken into account. The dependence of the results on the choice of the functional and of the basis set, as well as on the cavity geometry, is considered. (C) 2000 American Institute of Physics. [S0021-9606(00)51948-5].