I wanted to know what qml.qchem.hf_state actually represents. Because I understand that it is supposed to represent a quantum state in terms of occupation of MOs, but is it ordered in energy ? Meaning that why would qml.qchem.hf_state(5,6) returns [1 1 1 1 1 0] and not say [1 1 0 1 1 1] ?

Because the actually ordering of the electrons depends on the energy of each orbitals, which isn’t indicated in qml.qchem.hf_state(electrons,orbitals).

Thank you for your answer!

Hi @Mohamed_SO,

Notice that the Hartree-Fock approximation is the configuration where the lowest-lying states are occupied. This means that if you have 6 orbitals [_ _ _ _ _ _], and 5 electrons, it will fill the first 5 orbitals and leave the last one empty. So getting [1 1 1 1 1 0] means that the first 5 orbitals are filled and the last one is empty.

This blog post about the Hartree-Fock method is really good at understanding this topic.

Please let me know if this helps!