A diagram that relates the energy levels of separate atoms to the energy levels of diatomic molecules and united atoms, the two limiting states being correlated with each other. Using this diagram, the types of molecular orbital possible for the molecules and their order as a function of increasing energy can be seen as a function of interatomic distance. Lines are drawn linking each united-atom orbital to a separated atom orbital. An important rule for determining which energy levels correlate is the noncrossing rule. This states that two curves of energy plotted against interatomic distance never cross if the invariant properties (e.g. parity, spin, etc.) are the same.