The drug carbamazepine has been modeled, using ab initio density functional theory calculations [B3LYP/6-31G(d)], both as a single molecule and as a dimer. The predicted geometry of the single molecule calculation is compared with the crystallographic data on each of the polymorphs (carbamazepine forms I and III). From the predicted geometry it is possible to calculate the IR and Raman spectra; these predictions compare favorably with the observed spectra of both polymorphs of carbamazepine for most of the bands. The spectral differences between the polymorphs are more striking in the IR than the Raman spectra, with strong IR bands at 1688 and 1396 cm^-1 in form I shifting to 1678 and 1388 cm^-1 in form III. Analysis of the potential energy distributions for the calculated normal modes reveals that the vibrations are localized across different ring systems of the carbamazepine structure. Most notably, the polymorph-sensitive modes in the IR spectra are localized to the pendant CONH₂ group; it is these modes that show the greatest disparity from the calculated spectra, and it is this group that is perturbed in the polymorph crystal structures. The calculated dimer structure is similar to that of the single molecule, but the polymorph-sensitive IR modes are significantly better predicted by the dimer calculation.