We now turn to studying the electronic structure, crystal structure, and UV-Vis spectra of Npe:TCNQ. We performed scans of the PES of the Npe:TCNQ dimer to identify structures of the model cluster that best reproduce the crystal structure. Figure shows the optimized minimum-energy structures located on the Npe:TCNQ dimer PES. Four low-energy dimer structures were identified with a 0.029 eV energy gap between the lowest and the highest energy structure. The four conformers differ from each other by the rotation of the TCNQ above the phenyl rings with significant rotations of the pyridine ring in conformers b and c, labeled in Figure . An intermolecular distance of 3.4 Å is found in all structures except b, with a shorter inter-ring distance of 3.2 Å. This shorter distance can be explained by the alignment of the TCNQ molecule above the Npe molecule that results in strong hydrogen bonding between the N atoms of the TCNQ and the H atoms of the Npe pyridine ring.