Using a fully flexible molecular potential in equilibrium molecular dynamics simulations, we study the alpha-and gamma-polymorphs of the energetic molecular crystal hexahydro-1,3,5-trinitro-s-triazine (RDX), their respective properties, and the conditions that contribute to the stress-induced gamma -> alpha solid-solid phase transition mechanisms. We find the pressure-dependent atomic structure, mechanical properties, and transition behavior to be described reasonably well. Uniaxial deformation of alpha-RDX along the crystal axes is shown to result in three different crystal responses where compression of the c-axis results in the alpha -> gamma transition, compression of the b-axis causes a transition with resulting structure similar to stacking faults observed by Cawkwell et al. [J. Appl. Phys. 2010, 107, 063512], and no transitions are observed for compression of the a-axis.