The relative fraction of correlation energy recovered using AB2, Sano, FNO or other choices for antibonding orbitals to complete an active space can help us discern which ones are most appropriate to use for configuration interaction with fixed orbitals, as well as for a CASSCF initial guess. As a simple example, we stretch the C-C bond in C 2 H 4 while keeping the geometry of the methylene groups fixed at those of the equilibrium ground state geometry of ethene. Looking at Fig. , we see that canonical virtual orbitals capture less and less correlation as the def2 basis set is improved from SVP to TZVPP to QZVPP. We also observe that the gap between Sano and AB2 orbitals increases with increasing the size of the basis set. Finally, we can see that FNO and AB2 orbitals perform almost identically and are the Since the AB2 and FNO orbitals seem to capture quite a lot of the static correlation, we sought to compare them to CASSCF orbitals. In Fig. we are comparing the smallest singular value of the overlap matrix between the CASSCF orbitals and those of canonical, Sano, AB2, and FNO, at the optimized geometry of C 2 H 4 . Once again, the canonical orbitals become dramatically worse with increasing the basis set size. Sano and FNO both become very slightly worse with increasing the size of the basis set, namely by increasing zeta, while AB2 seems to be nearly basis set-independent.