This was inspired by a recent question How is the zero energy defined for molecular orbitals? which made me pause to think if I understood clearly or not about the zero of energy of an MO.
For example, I know you cannot compare the total electronic energy of one molecule to another (as computed by any conventional HF or DFT code for example) since the zero is different - since they have different numbers/types of nuclei.
But is the same true of a MO energy, for example HOMO energy? That post I linked above had an answer describing how the zero of MO energy is an infinitely diffuse orbital of a single electron, which I take to mean irrespective of how many nuclei the system has or where they are. Consequently - is it possible to compare for example the HOMO energy of one molecule to the HOMO energy of some other molecule of different size, constituents, etc? Whether that comparison is useful is another question, of course!
(Energy differences like HOMO-LUMO gaps can be compared safely, that at least seems intuitive to me)