Density functional studies have been performed for the crystalline forms of the cyclic dimer and tetramer of bisphenol A polycarbonate (BPA-PC), as well as for the isolated structural units and their fragments (including carbonic acid, benzene, phenol, monophenyl carbonate, and BPA). There are no adjustable parameters in the calculation, and the optimized structures agree well with the results of X-ray diffraction analyses where available. Calculated vibration frequencies are compared with experimental data and with earlier calculations. Ring-opening of the cyclic oligomers is an important polymerization technique, and the cyclic tetramer provides an interesting model for future calculations of reactions between polymer segments and additional molecules.