FATO molecular mechanics of oxygen atom. Model water molecule

FATO molecular mechanics of oxygen atom. Model water molecule.4 

4Reprinted from A.L. Tchougreeff. J. Mol. Struct. (THEOCHEM), 632, 91, 2003 with permission of Elsevier. 

Inserting eq. (3.67) into the expression for resonance energy we find that the minimum of the resonance part alone is reached when  

The form of the bonding HOs may be, nevertheless, specified on the basis of the orthonormality relations for the HOs, which are consequences of the group 50(4) structure of the hybridization manifold. According to eq. (3.61) the interhybrid angle for the bonding HOs is given by  

For symmetry reasons, the py- AO of the oxygen atom does not contribute to the lone pair of HOs and thus its weight is equally distributed between the bonding HOs  

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