First-principles calculations of the electronic, vibrational, and thermodynamic properties of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

LLM-105 is one of the insensitive high explosives (IHE), and has extensive prospective applications in the military and civilian fields. It is well-known that the theoretical method based on first-principles calculations in energetic compounds properties (sensitivity, detonation) analysis is important for us to comprehensively understand energetic materials. Thus, density functional theory (DFT) is employed to calculate the electronic, vibrational and thermodynamic properties of LLM-105 crystal. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) and the Grimme method for DFT-D correction were adopted to describe the exchange–correlation potential and van der Waals interactions. The lattice parameters are in excellent agreement with reported experimental and calculated values. Moreover, the theoretical level (GGA-PBE + G) matches the experimental data batter. Band gap related with sensitivity is 1.301 eV. The electronic DOS and phonon dispersion show that LLM-105 crystal with covalent bonds is dynamically stable. The vibrational modes of bonds and chemical groups are described through symmetry system based on point group C_2h of LLM-105 crystal, which is used to discuss the Infrared and Raman active. The vibrational modes analyses in high frequency region show that two amino groups (NH₂ (1), NH₂ (2)) of molecule are different from each other by comparing the bond length, angle and intramolecular hydrogen-bonds length. The thermodynamic properties such as Helmholtz free energy (F), entropy (S), enthalpy (H), Gibbs free energy (G) and heat capacity (C_V) are obtained. The calculated results of several kinds of properties are in agreement with experimental values. We hope providing some reference and guidance for deeper future research.