The relative computational ease of doing calculations with non-overlapping orbitals in MOT is said to have contributed to its popularity. In addition, the successful explanation of π-systems, pericyclic reactions, and extended solids further cemented MOT as the preeminent approach. Despite this, the two theories are just two different ways of representing the same wavefunction. As shown below, at the same level of theory, the two methods lead to the same results. H2 - Molecular Orbital vs Valence Bond Theory The relationship between MOT and VBT can be made more clear by directly comparing the results of the two theories for the hydrogen molecule, H2. Using MOT, the same basis orbitals (a and b) can be used to describe the bonding. Combining them in a constructive and destructive manner gives two spin-orbitals The ground state wavefunction of H2 would be that where the σ orbital is doubly occupied, which is expressed as the following Slater determinant (as required by MOT) This expression for the wavefunction can be shown to be equivalent to the following wavefunction which is now expressed in terms of VB determinants. This transformation does not alter the wavefunction in any way, only the way that the wavefunction is represented. This process of going from an MO description to a VB description can be referred to as ‘mapping MO wavefunctions onto VB wavefunctions’, and is fundamentally the same process as that used to generate localized molecular orbitals. Rewriting the VB wavefunction derived above, we can clearly see the relationship between MOT and VBT Thus, at its simplest level, MOT is just VBT, where the covalent and ionic contributions (the first and second terms, respectively) are equal. This is the basis of the claim that MOT does not correctly predict the dissociation of molecules. When MOT includes configuration interaction (MO-CI), this allows the relative contributions of the covalent and ionic contributions to be altered. This leads to the same description of bonding for both VBT and MO-CI. In conclusion, the two theories, when brought to a high enough level of theory, will converge. Their distinction is in the way they are built up to that description. Note that in all of the aforementioned discussions, as with the derivation of H2 for VBT, normalization constants were ignored for simplicity. 'Failures' of Valence Bond Theory When describing the relationship between MOT and VBT, there are a few examples that are commonly cited