Molecular dynamics study of the pressure-dependent terahertz infrared absorption spectrum of α- and γ-RDX

Terahertz infrared absorption spectra of the alpha and gamma polymorphs of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) were predicted using two different theoretical approaches based on molecular dynamics simulations. The thermodynamic conditions studied were T = 298 K and hydrostatic pressures P = 0.0, 1.0, and 2.0 GPa for alpha-RDX and P = 3.0, 5.2, and 7.0 GPa for gamma-RDX. The spectra obtained using the two methods are similar but not identical. In the case of alpha-RDX for pressure P = 0.0 GPa both spectra agree reasonably well with experimental data. The predicted spectra for alpha-RDX exhibit red-shifting (mode softening) of the main absorption peak with increasing pressure while for gamma-RDX the spectra exhibit overall blue-shifting with increasing pressure. (C) 2013 AIP Publishing LLC.