The formula the total energy of a covalent system is easily derived on such assumption that the overlap integrals between non-bonded orbitals are neglected.
This total energy of the system can be divided into three parts with the use of the mulliken's approximation of the moleclsr integrals. The first is the sum of the energies of the atomic valence states, the second is the sum of the interaction energies between the bonded atoms, and the third is the sum of the interaction energies between the non-bonded atoms. The first two parts give the bond energies. The last part gives the small contribution of the environment to the bond energies and may be neglected (This adds 0.32 eV. to the C-C bond energy in dismond.) We may prove the additivity rule of the binding energies of the saturated hydrocarbon and their related molecules on the above described assumptions.
Using the Slater's atomic wave functions, we have obtained 3.67 eV. as the value of the C-C bond energy, which is very closed to the observed one (3.713 eV. in diamond), although we have neglected the contribution from the exchange integrals between non-bonded orbitals.
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