Crystal structure prediction and molecular dynamics methods were applied to the cyclotrimethylene trinitramine (RDX) crystal to explore the stability rankings of various polymorphs using a recently developed nonempirical potential energy function that describes the RDX dimer interactions. The energies of 500 high-density structures resulting from molecular packing were minimized and the 14 lowest-energy structures were subjected to isothermal–isostress molecular dynamics (NsT-MD) simulations. For both crystal structure prediction methods and molecular dynamics simulations, the lowest-energy polymorph corresponded to the experimental structure; furthermore, the lattice energy of this polymorph was lower than that of the other polymorphs by at least 1.1 kcal mol⁻¹. Crystal parameters and densities of the low-energy crystal produced by the NsT-MD simulations matched those of the experimental crystal to within 1% of density and cell edge lengths and 0.01° of the cell angle. The arrangement of the molecules within the time-averaged unit cell were in equally outstanding agreement with experiment, with the largest deviation of the location of the molecular mass centers being less than 0.07 Å and the largest deviation in molecular orientation being less than 2.8°. NsT-MD simulations were also used to calculate crystallographic parameters as functions of temperature and pressure and the results were in a reasonable agreement with experiment.