In
spectroscopy and
quantum chemistry, the
multiplicity of an
energy level is defined as
2S+1, where
S is the
total spin angular momentum. The multiplicity is often equal to the number of possible orientations of the total spin relative to the total
orbital angular momentum L, and therefore to the number of near–
degenerate levels that differ only in their
spin–orbit interaction energy. States with multiplicity 1, 2, 3, 4, 5 are respectively called
singlets,
doublets,
triplets, quartets and quintets.