Geometry optimizations and normal-mode analyses of the pentaerythritol tetranitrate (PETN) conformer belonging to the S-4 molecular point group and comprising the crystalline solid were performed using density functional theory methods (B3LYP and B3PW91). The basis sets used in this study were 6-31G(d) and 6-311+G(d,p). The structural results are in good agreement with experimental X-ray diffraction data. The predicted bond lengths and bond angles are accurate to within similar to2.5% and similar to1.2%, respectively. Raman and infrared spectra of crystalline PETN were measured and compared with the calculated spectra. The calculated and measured spectra agree very well in the spectral region below 1100 cm(-1); the agreement is satisfactory for frequencies higher than 1100 cm(-1). On the basis of the calculations and analyses, normal mode assignments were made and mode symmetries determined.