**4. Conclusions**

We calculated second-order elastic coefficients of *β*-HMX (P21/n space group setting) as derivatives of the Cauchy stress with respect to the linear strain using MD with a well-validated fully flexible force field. A set of pressures between 1 atm (10−<sup>4</sup> GPa) and 30 GPa and temperatures between 300 and 1100 K was considered. The resulting elastic coefficients and bulk and shear elastic moduli are reported in tabular form. The following dominant trends are observed. For a given temperature, magnitudes of the elastic coefficients increase substantially with increasing pressure. For a given pressure, magnitudes of the elastic coefficients decrease with increasing temperature. The strong dependence of the elastic coefficients on pressure should be taken into account in mesoscale continuum modeling and simulations.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/10/12/1123/ s1, Table S1: Equilibrium unit-cell volumes and lattice parameters for *β*-HMX as functions of temperature and pressure. Table S2: Full 6 × 6 matrix of elastic coefficients *Cij* of *β*-HMX in units of GPa at 300 K and 1 atm (10−<sup>4</sup> GPa). The first column on the left and the top row are indices *i* and *j*. The matrix is very close to being symmetric. Sixteen coefficients are close to zero as expected for the monoclinic crystal. Table S3: Full 6 × 6 matrix of elastic coefficients *Cij* of *β*-HMX in units of GPa at 700 K and 1 atm (10−<sup>4</sup> GPa). The first column on the left and the top row are indices *i* and *j*. The matrix is very close to being symmetric. Sixteen coefficients are close to zero as expected for the monoclinic crystal. Table S4: Full 6 × 6 matrix of elastic coefficients *Cij* of *β*-HMX in units of GPa at 1100 K and 30 GPa. The first column on the left and the top row are indices *i* and *j*. The matrix is very close to being symmetric. Sixteen coefficients are close to zero as expected for the monoclinic crystal.

**Author Contributions:** Conceptualization, T.S. and A.P.; methodology, A.P. and T.S.; software, A.P.; validation, A.P. and T.S.; formal analysis, A.P.; investigation, A.P.; resources, T.S.; writing—original draft preparation, A.P.; writing—review and editing, A.P. and T.S.; visualization, A.P.; project administration, T.S.; and funding acquisition, T.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Air Force Office of Scientific Research grant number FA9550-19-1-0318.

**Conflicts of Interest:** The authors declare no conflict of interest.
