On the interplay of the potential energy and dipole moment surfaces in controlling the infrared activity of liquid water.

Infrared vibrational spectroscopy is a valuable tool for probing molecular structure and dynamics. However, obtaining an unambiguous molecular-level interpretation of the spectral features is made difficult, in part, due to the complex interplay of the dipole moment with the underlying vibrational structure. Here, we disentangle the contributions of the potential energy surface (PES) and dipole moment surface (DMS) to the infrared spectrum of liquid water by examining three classes of models, ranging in complexity from simple point charge models to accurate representations of the many-body interactions. By decoupling the PES from the DMS in the calculation of the infrared spectra, we demonstrate that the PES, by directly modulating the vibrational structure, primarily controls the width and position of the spectroscopic features. Due to the dependence of the molecular dipole moment on the hydration environment, many-body electrostatic effects result in a ∼100 cm(-1) redshift in the peak of the OH stretch band. Interestingly, while an accurate description of many-body collective motion is required to generate the correct (vibrational) structure of the liquid, the infrared intensity in the OH stretching region appears to be a measure of the local structure due to the dominance of the one-body and short-ranged two-body contributions to the total dipole moment.