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On a NIST (National Institute for Standards and Technology) page on the molecular details of water and it's various isotopologues, I came across this:
Electronic energy: -76.4089547246 hartree
There is, really, no other context on the page....
I found the page after typing in 'ionization energies of water'....
I put this here on Chemistry SE rather than Physics SE, I hope that is OK....
I DID look around to try to find an answer elsewhere, before coming here and posting...
The energy is the potential energy of all electrons in the molecule. Sometimes the term also includes the potential energy of the nuclei but in most cases it doesn't.
As reference or zero energy you can imagine the case where the electrons are non interacting or infinitely far apart from each other. In this sense it tells you the energy required to "remove" all electrons, i.e. pulling them away from each other so that they no longer interact with each other.
It can be interpreted as ionization energy where you remove all electrons in one go form the system but that is an theoretical process. The nuclei would push each other apart before you could ionize water to that degree. Normally the ionization energy refers to the removal of one electron. And the second ionization energy normally refers to the removal of a second electron after the system relaxed from the first ionization. And so on for higher ionization. This consecutive ionization process is different from removing all electrons in one go and would yield a different energy than the electronic energy given here.
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