The graph above is a graph of the potential energy of Carbon Monosulfide at the three different basis sets used, 6-21G, 6-31G and DZV. The three basis sets are initially very similar as shown by the first segment of overlapping lines. However, 6-21G represented by the black line diverges from the other two basis sets and levels off at approximately -435.95 Hartrees. The 6-31G and DZV basis sets stay close together and both level off near -435.0 Hartrees. Despite their closeness, it is still evident that the DZV basis set is slightly lower in energy than the 6-31G basis set. The basis set with the lowest energy is the best because a lower energy means that the molecule is more stable. Therefore, the best basis set for Carbon Monosulfide is DZV.