First-principles investigation of anisotropic constitutive relationships in pentaerythritol tetranitrate

First- principles density functional theory ( DFT ) calculations have been used to obtain the constitutive relationships of pentaerythritol tetranitrate ( PETN- I ), a crystalline energetic material. The isotropic equation of state ( EOS ) for hydrostatic compression has been extended to include uniaxial compressions in the < 100 >, < 010 >, < 001 >, < 110 > , < 101 > , < 011 > , and < 111 > crystallographic directions up to a compression ratio of V/ V-0 = 0.70. DFT predicts equilibrium properties such as lattice parameters and elastic constants, as well as the hydrostatic EOS, in agreement with available experimental data. Our results show a substantial anisotropy of various properties of PETN- I upon uniaxial compression. To characterize the anisotropic traits of PETN, different physical properties of the uniaxially compressed crystal such as the energy per atom, band gap, and stress tensor have been evaluated as a function of compression ratio. The maximum shear stresses were calculated and examined for a correlation with the anisotropy in shock- initiation sensitivity.