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I'm currently studying the EVB(Empirical Valence Bond) approach. While reading on this article (https://pubs.acs.org/doi/10.1021/ja00224a011), the author mentioned about the usage of SCF-MO formalism on describing bond-breaking/formation processes.

We could use an SCF-MO formalism to determine the energy of the system as a function of the coordinates of the X, Y, and C atoms over the course of the substitution reaction. However, there are bonds broken in the reaction, and it is well-known that the SCF-MO methods fail when treating bond-breaking processes.[38] Thus, we must use a formalism that will effectively treat bond breaking. Any method allowing for more than one Slater determinant (configuration) in the wave function will work. Some possible choices are the Cl, MCSCF, and VB methods, with all of these approaches being exactly equivalent in the limit of a complete basis set of configurations.

I first attempted to track the reference 38 to understand why SCF-MO methods fail when treating bond-breaking processes but it was a QC textbook written by Slater, and published a very past day (Slater, J. C. Quantum Theory of Molecules and Solids-, McGraw- Hill: New York, 1963; Vol. 1), which was inaccessible to me. Can I ask the reason why SCF-MO methods fail when treating bond-breaking processes? It seems like those are related to the number of Slater determinant used in computation but it would be highly appreciated if anyone could add some detailed explanation on here. Suggesting any reference that pointed out the failure of using SCF-MO methods on bond-breaking processes would be highly informative to me as well!

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