Density functional studies have been performed for two crystalline analogs of Bisphenol A polycarbonate (BPA-PC) and for the isolated structural unit. The calculations are free of adjustable parameters and yield equilibrium structures that agree well with available data. Vibrational frequencies have been calculated for the molecular unit. For all structures we have calculated the energy barriers for rotation of segments of the molecule, and we have compared the results with experiment. All phenylene groups rotate freely in the isolated molecular unit, but the corresponding pi-flips in the crystalline phases are hindered by the neighboring molecules and reflect the chain packing. The minimum energy barrier for the rotation in the crystal (7.7 kcal/mol) is associated with the rotation of the inner phenylene ring in the molecular unit. The activation energy for the rotation of the methyl groups is 3.4 kcal/mol, independent of the chain packing, and the barriers for rotation of the carbonyl group are 8.7 and 19 kcal/mol in the molecule and crystal phase, respectively.