Hydrogen Bonds and van der Waals Forces in Ice at Ambient and High Pressures

The balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high pressure phases of ice has been examined with the first principles approaches, density-functional theory (DFT) and quantum Monte Carlo. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that the transition pressures obtained from DFT functionals which neglect vdW forces are greatly overestimated [Phys. Rev. Lett. \textbf{107}, 185701 (2011)].