First-principles theoretical methods were used to investigate the vibrational properties of pentaerythritol tetranitrate (PETN) under hydrostatic compression up to 4 GPa. Bond lengths and bond angles for the PETN molecules and the pressure-volume relation for the crystal under high pressure were calculated and compared with previous calculations and with experimental results. Based on the calculated optimized geometries at ambient conditions and under hydrostatic pressure, the frequencies of the internal vibrational modes of the PETN molecules in the crystal and pressure-induced frequency shifts of these vibrational modes were obtained. The calculated vibrational frequencies are compared with previous single molecule calculations and with experimental data. (c) 2006 Elsevier B.V. All rights reserved.