To probe the smoothness in the potential energy surface from DFTB/ChIMES, we have also computed the potential energy profile for rotation around the dihedral angles in alka-nes. The torsional profile for n-butane is shown in Figure of the metastable minimum (at ±70 • ) in good agreement with wB97X reference data with deviations of less than 0.5 kcal/mol. DFTB, however, underestimates the torsional barriers by 0.9 and 1.7 kcal/mol for the lower-energy (at ±120 • ) and main barrier (at 0 • ), respectively. DFTB/ChIMES is more accurate overall, with deviations of less than 0.5 kcal/mol for predicting all energy barriers discussed here.