All-electron density-functional studies of hydrostatic compression of pentaerythritol tetranitrate C(CH2ONO2)(4)

All-electron calculations of the hydrostatic compression of pentaerythritol tetranitrate C(CH2ONO2)(4) (PETN) crystal have been performed using density-functional theory with the PBE functional in conjunction with the 6-31G** Gaussian basis set. Full optimizations of the atomic positions and ratio of lattice parameters c/a for the tetragonal crystal were performed for eight volume ratios 0.65less than or equal toV/V(0)less than or equal to1.00, where V-0 is the equilibrium volume at zero pressure. The pressure, linear compressibilities of lattice parameters a and c, and c/a ratio as functions of volume ratio are in good agreement with experiment. It is observed that c/a decreases monotonically with compression until V/V-0=0.8, and then increases monotonically for all higher levels of compression considered. Changes in intramolecular coordinates and close intermolecular contact distances were studied as a function of compression. The results indicate essentially rigid-molecule compression for V/V-0>0.8, with the onset of significant intramolecular distortion for higher compressions. Predictions of the bulk modulus B-0 and its pressure derivative B-0(') were obtained using various equation of state fitting forms. Values for these quantities are compared to experiment and to the results of a preceding molecular simulation study of PETN based on an empirical force field.