All-atom simulations are used to derive effective parameters for a coarse-grained description of the crystalline cellulose I α. In this description, glucose monomers are represented by the C4 atoms and non-bonded interactions within the cellulose sheets and between the sheets by effective Lennard-Jones interactions. The parameters are determined by two methods: the Boltzmann inversion and through monitoring of the energies associated with changes of the coarse-grained degrees of freedom. We find that the stiffness-related parameters for cellulose I α are nearly the same as for I β allomorph. However, the non-bonded terms are placed differently and are weaker leading to an overall lower energy, and free energy, of I β compared to I α. We apply the coarse-grained description to determine amorphous transition states for the room-temperature conversion process between the I α and I β allomorphs and to characterize the interface between the crystalline forms of the allomorphs.