Density functional theory at the B3LYP level, using the 6-31G(d) basis set, was employed to calculate structural properties (e.g., bond distances, bond angles, charge distributions, and Raman vibrational frequencies) for isolated single molecules of the cyanine dye 3,3'-diethyl-5,5'-dichloro-9-benzothiacarbocyanine (hereinafter referred to as DDPT). Calculations indicate that there are two favorable ground-state equilibrium structures: the more-stable structure is near planar, with a 14.4degrees twist between the two benzothiazole moieties, whereas the other structure lies 107 cm(-1) above it and is planar. For the two structures, calculations indicate that the methyl groups of the ethyl substituents lie on the same side of the structural surface of the macrocycle, opposite to the phenyl ring on the methane bridge, and the phenyl ring makes an angle of 83.4degrees and 89.7degrees with the near-planar and planar conformers, respectively. We further have observed that, with nonresonant excitation for aggregated DDPT, changes in the Raman band intensity indicate that aggregation results in enhanced scattering for bands composed of vibrational modes that contain major contributions from in-plane skeletal deformations of the benzothiazole moieties, as well as vibrations involving the phenyl ring and the trimethine bridge. With resonant excitation of the aggregate, two new Raman bands in the low-frequency region (at 142 and 161 cm(-1)) appear, as well as other bands that are associated with out-of-plane distortions of the benzothiazole moieties and the phenyl ring. Our findings are discussed in terms of Albrecht's A and B terms, and we utilize information concerning active vibrational motions to aid in deciphering how monomers are arranged within the aggregate structure.