Fermi resonance in OH-stretch vibrational spectroscopy of liquid water and the water hexamer.

Vibrational spectroscopy of water contains a wealth of information about the structure and dynamics of this fascinating substance. Theoretical modeling of fundamental vibrational transitions in condensed water has proven difficult, and in many circumstances, one cannot reach even qualitative agreement with experiment. Due to the ability of water to form hydrogen bonds of various strengths, the OH stretching band spans several hundreds of wave numbers in the spectra, overlapping with the first overtone of the HOH bending band and triggering a resonance between these two vibrations. This effect, known as Fermi resonance, has been traditionally ignored in theoretical condensed-phase simulations due to the additional computational burden and its deemed low importance. Depending on a particular molecular environment, the Fermi resonance manifests itself from small spectral features in the spectra of liquid water to pronounced distinct peaks in the spectra of ice and water clusters. The goal of this work is to illustrate the effects of including the Fermi resonance coupling between the bending overtone and stretching fundamental vibrations in the mixed quantum-classical formalism developed by Skinner and co-workers on the IR and Raman spectra of liquid water and the water hexamer. We show that by adding the Fermi resonance coupling, we are able to reproduce the location of the peak and a shoulder on the red side of the IR spectrum as well as the bimodal structure of the polarized Raman spectrum of liquid water at 300 K. Very good agreement between theory and experiment is achieved for the IR spectra of the water hexamer as well. We suggest that the Fermi resonance should not be ignored if intricate features of spectra are of interest. In spite of these promising results obtained in the region of a spectrum where Fermi resonance is important, further development of spectroscopic maps is needed to improve agreement with the experiment outside of the frequency range affected by the Fermi resonance.