Conformations in chains of poly(ethylene terephthalate) (PET) have been studied by using the molecular orbital package, Mopac, and the molecular modelling package, Polygraf. Both methods reveal two stable structures, namely one where the molecule is planar, and the other in which the CO-O-C-C dihedrals are rotated to similar to +/- 80 degrees. These correspond to the well known triclinic crystal structure of PET and a new observed mesophase. Various methods were used to predict the moduli of such chains, both as isolated chains and in a crystal structure. A qualitative agreement with the experimental moduli was found.