Abstract: It has been proposed that in LCAO-MO theory the contribution of the interaction between atom A and B to the total molecular energy, E_(AB), may be considered as a measure of the bonding situation. E_(AB) is known as atom-pair interaction energy. In this paper, the proposition is examined by MNDO calculation. The E_(AB) value of bonding atoms is negative (E_(AB)<-0.10 eV) and of non-bonding atoms is positive or small negative (about E_(AB)>-0.10 eV). When there is single, double, or triple bond between atoms of the second period, the value of E_(AB) is about -15, -25, -35 eV respectively. For the bond between hydrogen and second period atom E_(AB) is about -13 eV. The bond strength in organic molecules, the acid catalyzed dehydration of alcohol, the conjugation system and the freqency displacement of carbonyl group can be explained by the value of E_(AB). Correlations are obtained by comparing E_(CH) for C—H bond in many simple organic molecules with their isolated streching vibrational freqencies, ν_(CH), dissociation energies D°_(298), and in situ bond energes, E_b.

Key words: Bond strength, Atom-pair interaction energy