Russell-Saunders (RS) terms classify the states of the many-electron atom according to their energy. That is, each RS term corresponds to an energy level. The RS terms have the following form: \begin{equation} ^{2s+1}L \end{equation} where L represents the orbital angular momentum expressed by the letter corresponding to its value (S,P,D,F.. ..), and 2S+1 is the multiplicity of the term. If 2S+1=1, we speak of a spin singlet; 2S+1=2, is a spin doublet; 2S+1=3, spin triplet; 2S+1=4, spin quartet, etc. 3P, read triplet P; $^{1}D$, read singlet D.