We start by looking at the σ * orbital of H 2 , as shown in Fig. , evaluated by the Sano procedure, the AB2 approach, and CAS(2,2) (performed as 1-pair perfect pairing). It is visually clear that the Sano σ * orbital is contracting as the basis set is improved. Fig. displays the orbital energy (diagonal matrix element of the Fock operator) and the variance ( r 2 -r 2 ) of the σ bonding orbital, and the Sano and AB2 models of the antibonding orbital. The variance confirms that the size of the Sano σ * -orbital contracts with basis size, while its orbital energy increases (reflecting increasing electron confinement) unsatisfactorily.