*2.2. Force Field and MD Simulation Details*

All MD simulations were performed using the LAMMPS package [19]. The nonreactive, fully flexible molecular potential for nitramines proposed by Smith and Bharadwaj [21] and further developed by Bedrov et al. [22] was employed in all of the MD simulations. This force field is well-validated and has been used in numerous previous studies of HMX [14,15,22–26]. In the current study, we modified the original nitramine force field used in [22] by adjusting the N-O and C-H harmonic bond stretching force constants to better reproduce the corresponding experimental vibrational mode frequencies (see [25] for more details). We further modified the original force field by adding a 1/*r*<sup>12</sup> repulsive-core term to the non-bonded pair interaction potential. The reason for this latter modification is to eliminate the unphysical very-short-range attractive well in the Buckingham pair potential used in the original force field. The parameters of the 1/*r*<sup>12</sup> repulsive core are chosen

in such a way that the system dynamics is practically unaffected under the conditions we study in this paper. More details can be found in the supporting information for the work by Zhao et al. [27]. A cut-off distance of 11 Å was used for repulsion, dispersion, and short-range Coulomb interactions. Long-range electrostatic interactions were calculated using the PPPM method with the relative error in the forces set to 1 × <sup>10</sup><sup>−</sup>6. A time step of 0.2 fs was used. Sample input decks including all force-field parameters and the crystal cell description are included in the Supplementary Materials.
