The UV-Vis diffuse reflectance spectra were analyzed to determine the band gap of Npe:TCNB crystals, as described in Section 2.2.6. A direct band gap is determined as 2.45 eV. Our PBC DFT calculations determine the direct band gap to be 2.03 eV. The difficulties with which PBC DFT computes accurate band gaps are well documented, though hybrid functionals such as the one used in this study balance local and non-local exchange, resulting in computed band gaps that compare favorably with experimental values (see Ref. 32 and references within). We thus find reasonably good agreement between the PBC DFT and experimentally derived direct band gaps.