Chemisorption energy and charge transfer

If a single electron is in AO a), the unperturbed ground-state energy of the non-interacting adatom and substrate is 

As in (1.92), the chemisorption energy is the difference between the electronic energy of the adatom-substrate system before and after the interaction occurs, i.e., 

Since (naCT) is given by (4.76), the problem of finding AE reduces to that of calculating AEa. 

In order to perform this task, we introduce the function (Newns 1967) 

Inserting (4.98) in (4.97), we see that the logarithmic functions cancel out, so that 

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