Model crystals containing as many as 37 cellooctaose molecules have been studied with both molecular dynamics and energy minimization. Both the AMBER/GLYCAM-04 and MM4 empirical force fields have been used, with information supplemented with B3LYP/6-31+G* quantum mechanics studies of cellobiose. The AMBER/GLYCAM-04 studies showed similar results with either molecular dynamics or energy minimization, allowing the detection of some of the problems with the force field with the much less expensive minimization method. Deficiencies with MM4 were not identified. Different hydrogen bonding strengths in AMBER/GLYCAM permitted improved agreement with the unit cell dimensions but problems deviations from two-fold screw axis symmetry remained until the strength was reduced so much that the unit cell grew too big. The quantum mechanics studies showed a number of stable conformations that correspond to a stable two-fold screw axis, and several crystal structures of cellulose analogs had pseudo two-fold screw axes.