2S+1
Multiplicity; from the resulting total spin S of each electron spin Example:

+,−
Symmetry of the wavefunction with respect to reflection on a plane which contains both nuclei. not: For Π, Δ, Φ ... always both + and − exist. Thus, only &Sigma states are classified. (The  sign is only possible for a combination of π, δ,... orbitals.) 
2S+1 

+,− 
^{Ω(g,u)} 
Λ
Projection of the orbital angular momentum (of the electrons) on the internuclear axis; classified according to Greek letters: 0 ≡ Σ, 1 ≡ Π, 2 ≡ Δ, 3 ≡ Φ,... A π resp. a δ electron (projection λ _{π} = ± 1, resp. λ_{d} = ± 2) combine to

Ω
Ω = Λ+Σ ^{2}Π state (Λ =1) and spin S=½ yields Λ = 1+½ =^{3}/_{2} and Λ = 1½ =^{1}/_{2} g,u Symmetry of the wavefunction with respect to inversion. Only possible for homonuclear molecules : g gerade, because sign remains unchanged : u ungerade, because sign changes 
Examples:
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