The vibrational shifts calculated for HCN in several low-energy co-crystals phases are in excellent agreement (and as close as can be expected from 55 good DFT) with experiments (Tables ). However, analysis of vibrational shifts of ethane in our predicted structures is complicated by many Raman active bands, the possibility of overtones, and LO-TO splitting. Our approximate analysis of 60 hydrocarbon frequencies (Table ) is based on averaging over many Raman-active vibrational modes. The perturbations to the C-C stretching vibration are on average very small (0-2 cm -1 ), offering a possible explanation for the relatively 65 sharp band at 992-993 cm -1 in Fig. . The collection of C-H stretches in the region of ca. 2800 cm -1 are similarly predicted to both red-and blue-shift, limiting their utility for identification. The C-H bending modes near 1460 cm -1 are on average 70 predicted to blue-shift by 4-9 cm -1 in the lowest energy structures (Table ). However, because the signal affiliated with intercalated ethane is expected to be small relative to the solvent signal, this band (which is also broad) is not a useful indicator of solid-state mixing, or co-crystal formation.