Small component reflection symmetry

We will choose the degenerate pairs as in 12.16 and 12.17 which reflect the symmetry properties of 12.14 and 12.15, respectively. In fact the HOMO and LUMO of the lower symmetry structures will look very much like these. The energies of the two functions 12.16 will differ by an amount which depends on the XY electronegativity difference alone, since these two orbitals are either completely X or completely Y located. The energies of 12.17 on the other hand are expected to be much like cyclobutadiene itself with a small gap between them. Both components... 

The Coulombic components alone give what is known as the polarization approximation [13], and can be obtained from ordinary RS perturbation theory for stationary states with the intermolecular potential V playing the role of a small perturbation. Not reflecting the full symmetry of the Hamiltonian if, the RS expansion converges very slowly, unlike expansions using antisymmetrized products, and an accurate energy cannot be obtained in finite order. There are two essential points in the RS expansion ... 

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