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Atomic theory says singlet and triplet energies for the same orbital differ because of exchange interaction. However, molecular orbital (MO) theory describes the energy between the HOMO (highest occupied MO) and LUMO (lowest unoccupied MO) as the same regardless of whether triplets or singlets are being considered. Why is this the case?

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  • $\begingroup$ Not quite sure what you mean by "singlet and triplet energies for the same orbital". When you have two electrons in the same orbital, they can only be in a singlet state due to the Pauli principle. $\endgroup$
    – Roman
    Oct 31 at 20:50
  • $\begingroup$ I worded that wrong, I meant that when an electron is excited to a new orbital, energy differs depending on the state being a triplet or a singlet (despite the electron being promoted to the same orbital). And I;m confused about this being the case when it comes to molecular orbitals $\endgroup$
    – siete
    Nov 2 at 7:46

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