A recent structure determination of native cellulose has shown that it is composed of two different crystal structures, a two-chain monoclinic phase and a single-chain triclinic phase. In this article a molecular dynamics study of the two allomorphs is presented, and a general picture of structure and energetics is provided. Consistent with experimental data, the monoclinic phase is more stable than the triclinic phase by −8.7 kJ mol−1 cellobiose−1. In the monoclinic phase a small angle is observed between glucose residues that belong to alternate (200) planes. The glucose residues in every second plane are parallel to the (200) plane, and form more favorable intermolecular hydrogen bonds. In the triclinic phase the glucose residues are not parallel to the (200) plane. The 13C NMR shifts for C-6 are fully accounted for by the distribution of the C-6 dihedral angle. The nonbonded environment is important to the splitting for C-1. The fine structure of IR spectra in the OH-stretching region can be qualitatively correlated with the number of different hydrogen bonds observed. Results indicate that chains in one set of alternating (200)-planes in the monoclinic phase resemble the triclinic phase.