I am reading Albright's Orbital Interactions in chemistry. The qualitatively perturbational treatment of molecular orbitals seem to be a central tool used throughout the book. I've referred to the Appendix for a thorough derivation of the theory. However, I got stuck at equation 22(a). I don't understand how it follows from everything that is spelled out before it.
Hopefully, somebody who has read the same book or is knowledgeable about this topic can help me out. Thank you!
Remark: I know how to derive the nondegenerate perturbational theory in QM but it seems that with MO theory the derivation is more complex, since we are combining AOs to form MOs rather than working with totally independent, orthonormal eigenstates as in QM.