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A long-held belief is that shock energy induces initiation of an energeticmaterial through an indirect energy up-pumping mechanism involving phononscattering through doorway modes. In this paper, a 3-phonon theoreticalanalysis of energy up-pumping in RDX is presented that involves both direct andindirect pathways where the direct energy transfer dominates. The calculationconsiders individual phonon modes which are then analyzed in bands. Scatteringis handled up to the third order term in the Hamiltonian based on Fermi'sGolden Rule. On average, modes with frequencies up to 90 cm-1 scatter quicklyand redistribute the energy to all the modes. This direct stimulation occursrapidly, within 0.16 ps, and involves distortions to NN bonds. Modes from 90 to1839 cm-1 further up-pump the energy to NN bond distortion modes through anindirect route within 5.6 ps. The highest frequency modes have the lowestcontribution to energy transfer due to their lower participation inphonon-phonon scattering. The modes stimulated directly by the shock withfrequencies up to 90 cm-1 are estimated to account for 52 to 89\% of the totalenergy transfer to various NN bond distorting modes.

3-phonon scattering pathways for vibrational energy transfer in crystalline RDX