Technical Abstract: Powder and fiber diffraction patterns were calculated for model cellulose crystallites with chains 20 glucose units long. Model sizes ranged from four chains to 169 chains, based on cellulose I' coordinates, and were subjected to various combinations of energy minimization and molecular dynamics (MD) in water. Paracrystallinity induced by MD and solvation had small effects on the relative intensities, except that together they reduced the small angle scattering that was otherwise severe enough to shift the 1 -1 0 peak. Other shifts in the calculated peaks occurred because the empirical force field used for MD and minimization caused small discrepancies with the experimental intermolecular distances. Patterns were compared with experimental results. In particular, the calculated patterns revealed a potential for a larger number of experimental diffraction spots to be found for cellulose from higher plants when crystallites are well-oriented. Alternatively, further understanding of those structures is needed. One major use for patterns calculated from models is testing of various proposals for microfibril organization.