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HI
I have a doubt:

Reading some textboos i see thisc hemical bond energy for HF :

H-F: 5.8 eV

My question is: how can i determine this energy with chemical quantum mechanics??
My book for example, find using the variational method two MO for the HF molecule:

1)psi+ =0.97 (1s_H) -0.24(2pz_F)
  E+    = -13.4V

2) psi- = 0.24 (1s_H) +0.97(2pz_F)
    E-   =-17.6 eV

N.B:  the term "S" (overlap integral)in this calculus is set equal to Zero (is an approximation)

Where psi+ is the antibonding MO while psi- the bonding one.
So why i haven't got the value of energy corrisponding to the bond H-F ,whose sperimental value is  5.8 eV??


In a LCAO-MO/variational calculus the value of energy for a  certain (single,localized) chemical bond (for example H-F bond) at which MO founded  corresposponds?? at a single MO bonding?? or this bond energy   is the sum of all MO(founded,for example,using the variaitonal method)  of the molecule??


Thanks!!!!