We have carried out ab initio simulations on the high-pressure polymorph of solid water, ice II, a phase for which there is a surprising lack of experimental data. We report our calculated third-order Birch-Murnaghan equation of state for ice II: the zero pressure and temperature density, rho(0)=1240.27+/-0.62 kg m(-3), bulk modulus, K-0=16.18+/-0.12 GPa, with the first pressure derivative of the bulk modulus, K-0('), fixed equal to 6.0. These parameters, the unit cell dimensions, and the atomic positions are in good agreement with experimental values. We also describe the way in which the change in unit cell volume is accommodated within the structure, primarily by contraction of the distance between neighboring hexagonal tubes-the principal structural element of ice II. This is in agreement with existing experimental data. (C) 2003 American Institute of Physics.