Documentation of orbital energies

[ktheis]

In the listing of orbital energies, what defines the zero reference point of these numbers? It would be nice to have an explanation right on the site. I tried to ask the community about this (https://chemistry.stackexchange.com/questions/128544/for-molecular-orbitals-how-is-an-energy-of-zero-set-or-defined), but I did not receive an answer that is "student-ready". It is very difficult to find MO-diagrams with an energy axis, eventhough the order of orbitals is discussed extensively in textbooks.