  The first principles approaches, density functional theory (DFT) and quantum
Monte Carlo, have been used to examine the balance between van der Waals (vdW)
forces and hydrogen (H) bonding in ambient and high pressure phases of ice. At
higher pressure, the contribution to the lattice energy from vdW increases and
that from H 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 DFT functionals which neglect vdW
forces are greatly overestimated.
