The centroid-centroid distances between Npe and TCNB in the six isomers shown in Figure agree with the experimentally observed distances, ranging between 3.3 and 3.5 Å. The values quantifying degree of CT are very consistent for all isomers in the S0 and S1 states, varying between 0.01e and 0.02e in S0 and between 0.96e and 0.98e in S1, as shown in Figure . In all isomers found, the S1 state is a CT state characterized by the HOMO to LUMO transition. The computed oscillator strengths of these transitions, however, are quite different. Isomers a, d, and e exhibit S1 oscillator strengths with values of 0.021, 0.016, and 0.023 a.u., respectively, higher than the other conformers where the S1 state oscillator strength values range from 0.002 to 0.009 a.u. Of these, structure a (the lowest-energy isomer) is far closer geometrically to the experimental crystal structure than d or e, allowing structure a to emerge as the best model for Npe:TCNB, as predicted from energetic arguments alone in Section 3.1. PBC DFT calculations were carried out to study the effects of the extended solid on the geometry of the unit cell and found to be in agreement with the both the experimental crystal structure and MC model, shown in Figure . Due to the significant computational expense of PBC DFT calculations, we can only compute ground state geometries and energies. We thus focus our comparison between ground state PBC DFT and MC calculations. Two main structural differences are observed in the comparison between the PBC DFT structure and its MC counterpart. First is the alignment of the TCNB molecule relative to Npe. The MC structure exhibits a -15.50° rotation of the TNCB, while the PBC DFT structure exhibits a 2.0° rotation, and experimental crystal structure a 3.25° rotation. Another key difference is the planarity of Npe observed in the structures, calculated as the dihedral between the pyridine and naphthalene moieties. This dihedral angle is 2.8° in the PBC DFT structure and 13.3° in the MC structure, compared to the 2.0° in the experimental crystal structure. While we see larger differences between the MC geometry and experimental structure compared to the PBC DFT geometry, these geometric differences are relatively minor, with MC structures exhibiting the qualitative features of the crystal structure. Some geometric differences between PBC DFT and MC structures are to be expected, as the crystal structure constrains the structures of the D-A pairs. We thus turn to assess the importance of these structural differences via comparisons of the experimental UV-Vis spectra and theoretically predicted UV-Vis spectra of the dimer cluster.