Using ab initio DFT (Density Functional Theory) investigation, an attempt was made to gain insight into some physical properties of polyethylene terephthalate (PET) including polymerization (repeating monomer units) and cis-trans isomerization at 298 K. DFT calculations were performed at Becke's three parameter functional and Lee–Yang–Parr functional (B3LYP) level of calculation with 6-31+G(d,p) basis set. Geometry optimization of a series of conformations was employed to greatly increase the global energy minimum for each molecule. Molecular structure calculations yielded several thermochemical parameters, including the magnitude and direction of the dipole moment, electron densities, total electronic energy at 0 K, enthalpy, Gibbs free energy at 298 K of the investigated polymer, and its conformers. Both the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) energy gaps were also represented.