Competing factors on the frequency separation between the OH stretching modes in water

•We simulated liquid water with ab initio molecular dynamics simulations.•We examined its vibrational spectra with effective normal mode analysis.•Increasing of the local asymmetry leads to greater OH frequency splitting.•Decreasing of the intramolecular coupling leads to lesser OH frequency splitting.•In liquid water, these two competing factors are found.

Recent simulations demonstrated that the inhomogeneous broadening as observed in the vibrational spectra of liquid water at ambient conditions can be viewed as a large vibrational splitting of symmetric and asymmetric OH stretching modes, due to the asymmetry of the local hydrogen-bonding network [J. Phys. Chem. Lett., 2013, 4(19), pp 3245-3250]. In this work, we show that the finite temperature and the liquid phase do not only modulate the local hydrogen-bonding asymmetry of water molecules, but also the intramolecular coupling strength. These two factors compete together in the determination of the overall magnitude of the frequency separation between the two OH stretching modes in water at ambient conditions.