We performed quantum mechanical calculations using density functional theory (DFT) to evaluate the energies of an isolated molecule 20 in the vacuum and its stereoisomers with different center and staircase chirality. It is observed that in the crystal structure, the -NH moiety on the amide group is involved in the intermolecular hydrogen bonds across neighboring cells, which is absent in the calculations performed here for an isolated molecule in the vacuum. For this reason, the -NH moiety on the amide group was replaced with the -NCH3 moiety to simplify the energy comparison among different stereoisomers. Following the generation of the initial structures of different isomers, their geometries were optimized using DFT with the B3LYP functional, the def2-SVP basis set and the D3 dispersion correction (B3LYP-D3/def2-SVP). At the optimized geometries, the energies were re-evaluated using the def2-TZVP basis set plus D3 dispersion correction (B3LYP-D3/def2-TZVP). The normal mode analysis and zero-point energy (ZPE) corrections were performed on these minima at the B3LYP-D3/def2-SVP level of theory. The final energy reported for each optimized structure was the B3LYP-D3/def2-TZVP energy plus the ZPE correction.