A recent paper (Phys. Rev. Lett. 109, 173201, 2012) has introduced the concept of vibrational conical intersections as a potential source of ultrafast vibrational relaxation, using the coupling between high-frequency OH modes and low-frequency intramolecular hydrogen bonding modes of
malonaldehyde as an example. Here, the question is addressed whether such conical intersections may also appear for intermolecular hydrogen bonds. To that end, the water dimer is studied as a minimal model for the hydrogen bonding in liquid water. Although a significant separation of time scales between intra- and intermolecular degrees of freedom exists in (H2O)2, a standard normal mode description is found to lead to a complete break-down of the adiabatic ansatz. This is due to strong nonlinear couplings between high- and low-frequency normal modes, which in turn give rise to large overall nonadiabatic couplings. A valid adiabatic picture is obtained, on the other hand, when internal coordinates are employed. The resulting adiabatic potential energy surfaces indeed
exhibit low-lying conical intersections, whose possible relevance for ultrafast relaxation and energy transfer in water is discussed.