If S denotes the resultant of the individual electron spins, s ( s + 1 ) ℏ ℏ 2 are the eigenvalues of S and as in the case of atoms, each electronic term of the molecule is also characterised by the value of S. If spin-orbit coupling is neglected, there is a degeneracy of order 2 s + 1 associated with each s for a given Λ Λ . Just as for atoms, the quantity 2 s + 1 is called the multiplicity of the term and.is written as a (left) superscript, so that the term symbol is written as 2 s + 1 Λ Λ . For example, the symbol 3 Π Π denotes a term such that Λ Λ = 1 and s = 1 . It is worth noting that the ground state (often labelled by the symbol X ) of most diatomic molecules is such that s = 0 and exhibits maximum symmetry. Thus, in most cases it is a 1 Σ Σ + state (written as X 1 Σ Σ + , excited states are written with A , B , C , . . . in front) for a heteronuclear molecule and a 1 Σ Σ g + state (written as X 1 Σ Σ g + ) for a homonuclear molecule.