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I am learning about MO theory in my advanced inorganic chemistry course and am starting to realize that it is truly the most accurate representation of how molecular orbitals look like, where they are located in the molecule, and their relative energies to each other and the original atomic orbitals from which they are composed of. We are using Symmetry Adapted Linear Combinations as the approximation method and so far this method has successfully explained all chemical/magnetic/electronic properties for any molecule investigated thus far.

My question is: Is MO theory perfect? Or does it have a flaw somewhere like every other bonding theory (Lewis, VB, hybridization) I have learned about so far in my chemistry career?

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    $\begingroup$ Everything is flawed because we can't solve the Schrodinger eqn. By MO theory I'd assume you are referring to a Hartree-Fock wavefunction, which gets electron correlation pretty wrong. $\endgroup$
    – orthocresol
    Commented Mar 29, 2016 at 22:47
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    $\begingroup$ Thank you for orthocresol for the great answer. No thank you to the other two for making me feel stupid. I struggle enough with chemistry, no need to rub it in. Just trying to learn. $\endgroup$
    – Nova
    Commented Apr 2, 2016 at 1:31
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    $\begingroup$ Nova, I don't think that anyone wanted to make you feel stupid. I believe the key problem is - and I came across that many times - that you are learning quantum chemistry from an inorganics point of view. The theory you learn there is usually a short version of the whole deal. It is good that you question the concept, stay curious. I think (hope) that @Karl wanted to express that to gain deeper insight into the matter it would have been better if you were introduced properly to quantum chemistry first. $\endgroup$
    – Martin - マーチン ♦
    Commented Apr 6, 2016 at 5:52
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    $\begingroup$ The problem with MO theory are... the orbitals themselves! Orbitals are, by definition, single particle wavefunctions and they have no real physical meaning since you are dealing with a many-body system. For this reason MO theory is far from perfect! $\endgroup$
    – user23061
    Commented Apr 11, 2016 at 12:46
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    $\begingroup$ MO is a way to approach FCI by using orthogonal one-electron basis. Its limitation is quantum mechanics itself. Since FCI can reach the exact solution, even it typically takes infinity long expansion. $\endgroup$
    – Rodriguez
    Commented Jun 3, 2016 at 19:56

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MO theory fails pretty fantastically in a wide-variety of cases.

While it works well near the equilibrium geometry of a molecule, it cannot dissociate even the simplest bonds correctly.

H2 dissociation curves, image taken from: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/21%3A_Resonance_and_Molecular_Orbital_Methods/21.07%3A_Which_Is_Better-_MO_or_VB

It is well-known by computational chemists that any quantitative predictions of MO theory (ionization energies, HOMO-LUMO gaps, etc.) cannot be trusted, even for the simplest molecules.

In more complex molecules, such as those with transition metal centers, calculations can even fail to qualitatively describe the electronic structure correctly, even giving ground states with incorrect symmetries.

In my opinion, inorganic chemists like MO theory so much because it's is the simplest theory of molecular bonding and geometrically its results are quite intuitive. It is an excellent conceptual tool for understanding quantum chemistry, but a horrible one for actual predictions of chemical properties.

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    $\begingroup$ Depends on what exactly you call "MO theory". If you're talking about only Hartree-Fock, then you're right, but it's also a bit of a moot point because noone in their right mind uses HF as a standalone method. If you extend it to include post-HF methods that are based on those MOs, then your answer is not accurate. $\endgroup$
    – Antimon
    Commented Jan 10, 2022 at 18:55
  • $\begingroup$ Post-HF or Multi-Reference methods, I should have said. - Also, what happened to UHF? That one describes the dissociation of e.g. H2 just fine. $\endgroup$
    – Antimon
    Commented Jan 10, 2022 at 19:09
  • $\begingroup$ UHF has its own set of problems with spin-contamination. Also, UHF theory emphasizes spin-orbitals, most inorganic classes focus entirely on spatial orbitals. Of course post-HF methods can be used to improve HF results, but post-HF methods are no longer really orbital theories. For example it no longer makes sense to talk about a HOMO/LUMO, because even in the ground-state virtual orbitals can be partially occupied. $\endgroup$
    – user120468
    Commented Jan 10, 2022 at 23:10
  • $\begingroup$ More concretely, you give a graph showing how far off things are, but for what molecule? (I'm guessing H2.) And what reference? $\endgroup$
    – Oscar Lanzi
    Commented Feb 12, 2023 at 10:10

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