Perturbation of the Valence Region

We shall now proceed with Eq. (4.42) and show the importance of this approximation. 

The formula for the ground-state energy of a molecule follows from Eq. (4.42)  

Consider also the valence region energy of the isolated ground-state atoms k,l, 

The total energy of an isolated ground-state atom k is 

Equation (4.14) thus tells us that the LHS of (4.44) is A , the energy of atomization of the molecule. So we write 

---