MO-diagrams and energy-level diagrams.

It's very important to distinguish between MO-diagrams and diagrams showing the total energy levels of a molecule. The former have only meaning within MO theory (the MO-levels are solutions of the Fock equation), whereas the latter show solutions of the Schrödinger equation. In MO theory there is a relation between them, however. Each MO-diagram with electrons showing the occupation of the MO's corresponds uniquely to a Slater determinant, which corresponds to one energy level. So schematically we have a situation as in the following figure.

The left column shows the energy levels of a molecule. The boxes show the MO-levels with different occupations. Each box corresponds to one energy level, which is indicated by the arrows pointing to the left column.