Classical molecular dynamics simulations complemented with semiclassical perturbation theory have been applied to the study of the cyanide ion vibrational relaxation in liquid water. The model provides reasonable agreement with known experimental results as well as with ab initio calculations for small clusters. The role of Coulomb and non-Coulomb forces is studied in detail. A dominant role of the former in the vibrational energy (population) relaxation is found, while in contrast, the bandshape-and thus the dephasing-are determined by both forces. Further, and at variance with existing theories, the present model provides the first example in which nonlinear intermolecular terms in the vibration-solvent coupling art critical in the instantaneous frequency shift.