Dispersion-interaction corrected DFT simulations are performed to study the structure of two allomorphs of native cellulose I. Good agreement between theoretical and experimental data is achieved. Two H-bond patterns, previously identified experimentally, are examined for both allomorphs. The transition mechanism for the conversion between the phase I-alpha and I-beta is studied by means of constrained relaxations. New metastable intermediate phase occurring on the I-alpha -> I-beta route is identified, and the corresponding structural data are reported.