We compare different quantum-mechanical (QM) periodic models of ordinary ice with the objective to provide reference data for use in simulation studies. Four crystalline structures are considered derived from a cubic or hexagonal arrangement of the oxygen sublattice, both polar and apolar, and three Hamiltonians: HF, PW91 and B3LYP. Two periodic programs have been used, CRYSTAL and V-ASP. For each given QM method, the four structures are almost indistinguishable as far as energy and local bonding structure are concerned. Results are much more dependent on the QM technique concerning relative distances and activation barriers. (c) 2005 Elsevier B.V. All rights reserved.