The ionization potential (IP) and electron affinity (EA) of the isolated single dye molecule and a hypothetical isolated J-aggregated dimer are calculated as an energy difference between separately minimized ground and ionized states. Three quantum methods are employed: density functional theory (DFT) Gaussian03 B3LYP/6-311G** (++G**); DFT using Dmol3; and a modification of CNDO/S, called CNDO/S-Δζ, which is developed for rapid calculation of the IP and EA. Results indicate that for the monomer, 1,1′-dimethyl-2,2′carbocyanine chloride, the vertical IP and EA are 6.2 ± 0.1 and 1.90 ± 0.05 eV, respectively. This is consistent with the threshold IP and EA predicted by the Yianoulis and Nelson “Statistical Model” of spectral sensitization. For the isolated J-aggregated dimer, whose configuration is consistent with being adsorbed on a dielectric substrate, the calculations predict a value of 5.2 ± 0.2 and 2.35 ± 0.05 eV for the IP and EA, respectively. Significant charge density is removed from the halide anion in the ionization process. The HOMO of the dye molecule is an MO associated with the halide anion. Calculation of the isolated entities is a necessary preliminary step in the study of the IP and EA of the adsorbed dye monomer and aggregate.