by Tarek Musslimani

Created on: October 21, 2010

Molecular orbital theory is a theoretical method in chemistry that tries to give an explanation to the chemical bond in terms of molecular orbitals formation that are formed by the addition and subtraction of two atomic orbital functions.  This method tries to rationalize the formation of the chemical bond, based on the fact that the formed molecular orbital from the linear combination of the two atomic orbitals is energetically lower than each of the original atomic orbitals.  This is due to the delocalization of the two electrons that are involved in the bond between the two positively charged nuclei of the two atoms that make the bond.  

This method is able to explain chemical bonding and rationalize it in many cases where other methods fail to explain the same phenomenon.  For example, the molecule of oxygen or O2 where there is a triplet state of two unshared electrons.  This paramagnetic structure of the oxygen molecule could not be explained by other chemical theories but the molecular orbitals theory succeeded to explain the existence of the triplet state of oxygen based on the construction of a molecular orbitals diagram that shows that there are two degenerate orbitals in the molecule that each accomodates one electron with the same spin of the other electron.  This explanation was a triumph for the theory of molecular orbitals theory that succeeded in this case where other theories failed. 

The method of constructing molecular orbitals from linear combination of atomic orbitals is a feature of a calculation method in theoretical chemistry that is called the Hartree-Fock method or the self consistent  field.  By choosing a basic set of atomic orbitals, and making a linear combination of them as a potential molecular orbital function, this function is then used to find the optimal energies and wave functions that are called molecular orbitals.  This is done by using the variation method in order to minimize the coefficients in the linear combination. 

This method of calculation (the Hartree-Fock method) is considered now an obsolete method and has been replaced by more accurate methods that take into consideration electron-electron interaction which is not considered in the Hartree-Fock method.  In the Hartree-Fock method the obtained orbitals are obtained from the solution of the Schroedinger equation of multi-electronic atoms by assuming

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