1. Angular Momentum in Multielectron Molecules (in Analogy to LS Coupling)

2. Notation in Multielectron Molecules
The ground state is denoted X.
Excited states with the same multiplicity as the ground state are denoted with capital letters A,B,C,D,... in order of their energy.
Excited states that have a different multiplicity are denoted with lower-case letters (a,b,c,d,...) in order of their energy. Unfortunately, the literature is full of inconsistent orderings, since the identification of excited states for many molecules has “historically evolved”.
Polyatomic molecules often have a tilde above the term symbol to avoid confusion with group theory labels.
Term symbols can be determined using a microstate analysis, but recall that the symbol used is that of  = |ML|, not L.
Note that  is used in a confusing manner: as the quantum number for the spin projection and as a designation for a =0 state