Assessing the accuracy of first-principles DFT in describing van-der-Waals interactions in energetic molecular crystals at ambient and high pressure conditions

Various density functional theory (DFT) functionals are evaluated in their ability to accurately describe vdW interactions in energetic molecular crystals, which are of critical importance to obtain an accurate isothermal equation of state as well as other basic thermodynamic properties of energetic materials. Equilibrium volumes of seven EM crystals are evaluated using several DFT functionals with or with- out semi-empirical Van der Waals corrections, as well as non-local van der Waals density functionals, and compared with available experimental data. By calculating vibrational spectra of energetic molecular crystals, zero point energy (ZPE) and thermal contributions at finite temperature to cold DFT data, pressure-dependent lattice parameters and equation of state are obtained at experimental temperatures, thus allowing direct quantitative comparison with experiment. Newly developed meta-GGA SCAN functional provides the best description of vdW interactions, and delivers an accurate EOS of EMs in a wide range of pressures and temperatures.