IR spectroscopy of condensed phase systems: Can the environment induce vibrational mode coupling?

The Perturbed Matrix Method is one of the methods to carry out theoretical studies on infrared spectroscopy at a low computational cost. One of the main assumptions adopted up to now is the invariant mode approximation: the actual mass-weighted Hessian eigenvectors of a vibrational center embedded in a chemical environment can be well approximated by the vacuum Hessian eigenvectors with the environment perturbation affecting only the mode frequencies. Herein, we investigate the accuracy of this approximation considering different scenarios. The results demonstrate the accuracy of this approximation providing non-negligible mode coupling only when dealing with extremely intense perturbation fields.