Excited States
Hund's rules work best for the determination of the ground state of an atom or molecule.
They are also fairly reliable (with occasional failures) for the determination of the lowest state of a given excited electronic configuration. Thus in the helium atom, the 1s2s triplet state (3S) is correctly predicted by Hund's first rule to be lower than the 1s2s singlet (1S). Similarly for organic molecules, the same rule predicts that the first triplet state (denoted by T1 in photochemistry) is lower than the first excited singlet state (S1), which is generally correct.
However Hund's rules should not be used to order states other than the lowest for a given configuration. For example, the titanium atom ground state configuration is ...3d2 for which a naïve application of Hund's rules would suggest the ordering 3F < 3P < 1G < 1D < 1S. In reality, however, 1D lies below 1G.
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