Both ground-and excited-state ab initio calculations are reported for four polycyclic aromatic hydrocarbons (PAHs) : pyrene, benzo(a)pyrene, fluoranthene, and benzo(b)fluoranthene. Ground-state geometries, IR frequencies, excitation energies, changes in excited-state electron distributions, excited-stale geometries, and excited-state frequencies are calculated and quantitatively compared to experimental data. Overall agreement with experiment is quite good, with the largest discrepancies occurring when predicting the excitation energies of the molecules. Changes in electron density correlate with changes in the excited-state geometry, with all PAHs lengthening along their axis of polarization upon excitation. These calculations are also used to examine characteristic differences between the alternant (pyrene and benzo(a)pyrene) and nonalternant (fluoranthene and benzo(b)fluoranthene) PAHs. Relative to their alternant isomers, nonalternant PAHs tend to possess higher ground-state energies, lower relative excitation energies, and greater changes in their excited-state electron densities and geometries.