The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces [电子孤对] and hydrogen bonding [实键] in ambient and high-pressure phases of ice. 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 transition pressures obtained from density-functional theory exchange-correlation functionals which neglect vdW forces are greatly overestimated.
Biswajit Santra et al., Hydrogen Bonds and van der Waals Forces in Ice at Ambient and High Pressures, Phys. Rev. Lett. 107, 185701 (2011)