The vibrational spectrum of a computer model of crystalline RDX was studied using a 216-molecule periodic supercell, allowing for intra- and intermolecular degrees of freedom using the force field by Boyd [J. Chem. Phys. 124, 104508 (2006)]. The normal modes were analyzed with regard to their activity involving molecule center-of-mass translations and rotations, as well as 15 intramolecular degrees of freedom, including bond stretches, bend and dihedral angle variations, and out-of-plane motions of the nitro groups. We correlate center-of-mass motions with the occupation of internal degrees of freedom for all of the normal modes in the model with particular attention to correlations between nitro rotations and lattice modes. Transfer of lattice energy to internal degrees of freedom can occur through doorway modes and is significant for the initiation of detonation. Several clusters of potential doorway modes are found which involve significant lattice motion as well as nitro rotations. Such groups of modes have been found in the ranges between 186 and 220 and between 420 and 434 cm-1. Symmetry properties and details of the involved molecular motions are described.