The details of a new all-atom force field designed to reproduce the phases of the native I-alpha and I-beta forms found in crystalline cellulose I are reported in this article. The energy differences, densities, unit cell parameters, and moduli are in close agreement with experimental evidence. Analyses of the modular dynamics simulations also included thermodynamic data and angle distributions as well as characterization of the intrachain, intrasheet, and intersheet hydrogen-bond networks for both phases under study.