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I'm currently working on quantum chemistry with quantum machine learning and one of the operations considered for the calculation is the excitation (and desexcitation) of an electron from an orbital to the other (which corresponds mathematically to a Givens rotation).

So far, these excitations have been taken as spin-preserving, meaning that : say I have initially two electrons in the ground state orbital, if I excite one of them to the excited state, its spin won't be change. Apparently this is something common in quantum chemistry and I wanted to know why.

An argument I thought of was that the necessary energy to flip the spin of the electron is way higher than the typical electronic energy scale of the atom/molecule, hence making it impossible to flip.

Thank you for your response!

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    $\begingroup$ The electron (spin) angular momentum has to change to make spins 'parallel' and in a molecule this can be achieved via coupling of spin and orbital angular momentum, i.e. by spin-orbit coupling. This is described in many quantum and spectroscopy text books. An example is that S.O. coupling allows triplet states to form from singlet excited states and hence phosphorescence to be observed as well as fluorescence. $\endgroup$
    – porphyrin
    Dec 28, 2022 at 13:09

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