Inspecting the σ C-C bond in C 2 H 4 in Fig. shows that the shape of the occupied Pipek-Mezey and converged CCVB bonding orbitals both do not change much upon increasing the size of the basis set. By contrast, when looking at σ * in Fig. we see even poorer behaviour of the Sano C-C antibonding orbital as a function of basis set size than we did for H 2 . This is confirmed in Fig. where we compare the orbital energy and the orbital variance of the bonding and the antibonding C-C σ orbital in C 2 H 4 . The Sano σ * orbital does not converge with the size of the basis set, with the variance decreasing, and the energy increasing. By contrast, the AB2 σ * orbital converges rapidly both in terms of energy and variance for similar reasons to before. In Fig. we compare the orbital energy and the orbital variance of the bonding and the antibonding orbitals for the C-C π in C 2 H 4 . The shortcomings of Sano seem to be much less severe in π * orbitals. We believe this is due to the diffuse nature of the π orbitals making the maximum exchange, thus spatial locality, sufficient to describe the π * . However, we can still see that the orbital energy and variance do not converge for Sano while they do for AB2, and converge to drastically different orbital energy and orbital variance. The quantitative advantage of the AB2 antibonding orbitals relative to the Sano orbitals seen so far can also become qualitative advantages in systems with more complex electronic structure. One such example is Cu 2 , which, considering that the valence state of Cu can be taken as 3d 10 4s 1 , is isoelectronic to H 2 . The σ orbital (HOMO) of Cu 2 is shown in the upper panel of Fig. , along with the optimized correlating orbital from CCVB, as well as the Sano and AB2 antibonding orbitals. Maximizing exchange results in a Sano antibonding orbital that resembles an empty π-bond between the two metals. By contrast, the AB2 and CCVB orbitals look qualitatively identical.