Spin multiplicity

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In quantum mechanics, multiplicity is a property of a system due to the spin, or angular momentum, of its component particles. These are normally electrons in most applications of the term.

The number of states with a given resultant angular momentum is 2S+1 where S is the total spin. SO states with S=0 are singlets, S=1/2 are doublets, S=1 are triplets, and so on.

In chemistry, multiplicity is usually associated with free radicals, which have unpaired electrons and therefore could exist in a state other than a singlet. In particular, a diradical can be a singlet or a triplet; normally the triplet is the ground state.