Which Treatment Is Better—MO or VB

21 The Resonance and Molecular-Orbital Methods and Their Applications. Pericyclic Reactions 

We can say that, as calculated by the MO method, the molecule does not dissociate properly.  

Why do these calculations yield results so far from the ub initio curve There are two reasons. First, atomic orbitals are used that are appropriate for isolated atoms, but are hardly expected to be the best orbitals for the electrons when two or more atoms are in close proximity. It is convenient to use atomic orbitals in simple calculations because they are mathematically simple, but more complicated orbitals are known to give better results. Second, neither treatment properly takes into account electron-electron repulsions. For two electrons, a term of the form e2lr2n (in which e is the electronic charge and r12 is the distance between the electrons) is required to describe the repulsion between electrons. The exact calculations avoid both difficulties but are so complex mathematically as to be devoid of any capability for providing qualitative understanding. 

Why does an electron-pair bond calculated by the MO method not dissociate properly We have seen that half of the time both electrons in the low-energy molecular orbital are in the vicinity of just one of the nuclei. But as the nuclei move far apart, this corresponds to a far greater energy than having only one electron in the vicinity of each nucleus, as the VB method suggests. 

There is no unequivocal answer to the question as to which is the better method. Calculations by the VB method are likely to be more reliable than those by the MO method, but in practice are much more difficult to carry out. For many-electron molecules the MO procedure is simpler to visualize because we combine atomic orbitals into molecular orbitals and then populate the lower-energy orbitals with electrons. In the VB method, atomic orbitals are occupied, but the electrons of different atoms are paired to form bonds, a process that requires explicit consideration of many-electron wave functions. To put it another way, it is easier to visualize a system of molecular orbitals containing N electrons than it is to visualize a hybrid wave function of N electrons. 

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