The conformation and solid state structure of the two alpha and beta polymorphs of poly(butylene terephthalate) are here studied by means of state-of-the-art first principles calculations, carried out both for the crystals and the infinite one-dimensional chain models. Focusing in details on the debated beta form, induced by mechanical deformation, we verified the setting on of an all-trans conformation, as also supported by the simulation of the infrared spectra of the different polymorphs compared to the available experimental spectra. The transition from the alpha to the beta form is also simulated by applying increasing strains to the infinite polymer chain: a peculiar evolution of the intramolecular structure is indeed predicted, showing a transition from the GTG' conformation found for the alpha form to the TTT conformation of the strained beta form.