Deformation of polyethylene terephthalate, PET, was simulated using atomistic molecular dynamics using a Parrinello-Rahman constant-stress constant-temperature algorithm under periodic boundary conditions. The evolution in trans and gauche populations is in good agreement with experimental data reported in the literature: In the isotropic model, approximately 15% trans angles are found, and the trans population increases up to a value of 50% at a draw ratio lambda of 3.2. There is a gradual increase in the orientation function of the aromatic cycles, (c), with lambda, but the values are higher than those reported in the literature, due to the absence of relaxation in the simulation. Moreover, the orientation of trans segments was found to be high, while the orientation of gauche segments was non negligible. A good agreement was found with the rubber elasticity model up to a lambda of 2.0, as was the case for previous polyvinyl phenol simulations. Simulated data fit well with the aggregate model in a broad range, for 1.0 lambda 5.0, contrarily to the case of poly(vinyl phenol), where the aggregate model predicted much higher orientation values that those obtained via molecular modelling simulations. This, difference is attributed to the different rigidities of the chain backbones. Simulations were found in good agreement with those performed by Zhou, Nicholson, Davies and Ward, using a different deformation simulation approach, both in terms of orientation of aromatic rings and of conformer evolution during deformation. (C) 2003 Elsevier Ltd. All rights reserved.