Donor-acceptor theory of hybridization in ionic bonding

4 Donor-acceptor theory of hybridization in ionic bonding 

Of the 17 kcal mol-1 total error, about half is estimated to arise from the single hp— h[ j sigma-type interaction shown in Fig. 2.8, while the remainder arises from weaker pi-type interactions (2-3 kcal mol-1 each). For example, we can carry out a partially localized variational calculation, similar to that described above but with only h prevented from delocalizing into tip (hisleads to a stabilization energy (at 7 = 1.6 A) 

The corresponding estimate from NBO second-order perturbative analysis is 

For the present case, the leading donor-acceptor correction Lda(2), Eq. (1.24), can be rewritten as 

If we denote the right-hand side of (2.9) as (X), we can variationally minimize X) with respect to X by setting 

---