Theoretical values for the thermomechanical properties of poly(ethylene terephthalate) (PET) are determined self-consistently using the pcff force field to compute the potential energy and quasiharmonic lattice dynamics to determine the vibrational free energy. Complete sets of lattice constants, thermal expansion coefficients, elastic properties, and Gruneisen coefficients are reported between 0 and 400 K for the triclinic PET unit cell. Mean square displacement matrices for the constituent atoms of PET were determined, from which a theoretical B-factor for X-ray scattering of 4.0 Angstrom(2) at 300 K is estimated. The 50% probability ellipsoids for thermal vibration of all atoms in the asymmetric unit are computed. Calculated lattice parameters at 300 K agree with experimental data, to within the accuracy of the method. Calculated elastic constants for a transverse isotropic composite agree with data from X-ray and ultrasonic velocity measurements on highly oriented samples. The tensile elastic stiffness constants are temperature-dependent, while the shear stiffnesses are roughly constant in the range 0-400 K. Thermal contraction along the chain direction is observed in PET, consistent with results for other polymer crystals possessing chains in fully extended conformations. The driving force for contraction is entropic in origin, arising from negative gamma(3) and gamma(6) Gruneisen coefficients.