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Basics of MO diagrams

In the simplest form an MO diagram shows just the energies of the orbitals of an atom, ion, or molecule. The energy increases from the bottom of the diagram upwards, and the energy levels are shown by horizontal lines. If there is degeneracy two or more lines are drawn next to each other: as many as there are orbitals with the same energy. Very often, but not always, the orbitals that are used to form a Slater determinant are indicated by drawing in electrons using arrows to indicate the spin. For example, the following MO diagram represents the O₂ molecule.

The MO's formed by the 1s orbitals have been left out. MO diagrams very often do not show all orbitals, but only the ones that are relevant for a particular aspect of the bonding. We can see the unpaired electron in the highest occupied MO. (This orbital is generally abbreviated to HOMO. The lowest unoccupied MO is called LUMO.) We also see that sometimes labels (s₃, s₄^*, ...) are used to indicate which orbital belong to a level. (The precise definition of these labels is outside the scope of this course. However, it should be clear that they can be used to distinguish s from p bonds. The * superscript indicates an antibonding orbital.) Occasionally values of the energies are written next to the levels.

MO diagrams