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

All-electron calculations of the hydrostatic compression of pentaerythritol tetranitrate $\mathrm{C}({\mathrm{CH}}_2{\mathrm{ONO}}_2{)}_4$ (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 $0.65<~{V/V}_0<~1.00,$ 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 $B_0^{\prime }$ 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.