  A long-held belief is that shock energy induces initiation of an energetic
material through an indirect energy up-pumping mechanism involving phonon
scattering through doorway modes. In this paper, a 3-phonon theoretical
analysis of energy up-pumping in RDX is presented that involves both direct and
indirect pathways where the direct energy transfer dominates. The calculation
considers individual phonon modes which are then analyzed in bands. Scattering
is handled up to the third order term in the Hamiltonian based on Fermi's
Golden Rule. On average, modes with frequencies up to 90 cm-1 scatter quickly
and redistribute the energy to all the modes. This direct stimulation occurs
rapidly, within 0.16 ps, and involves distortions to NN bonds. Modes from 90 to
1839 cm-1 further up-pump the energy to NN bond distortion modes through an
indirect route within 5.6 ps. The highest frequency modes have the lowest
contribution to energy transfer due to their lower participation in
phonon-phonon scattering. The modes stimulated directly by the shock with
frequencies up to 90 cm-1 are estimated to account for 52 to 89\% of the total
energy transfer to various NN bond distorting modes.
