refer to each value: Λ = 0, 1, 2, 3... are coded as Σ, Π, Δ, Φ... respectively (analogous to S, P, D, F for atomic states). The molecular term symbol is then defined as 2S+1Λ and the number of electron degenerate states (under the absence of spin-orbit coupling) corresponding to this term symbol is given by: (2S+1)×2 if Λ is not 0 (2S+1) if Λ is 0. Ω and spin–orbit coupling Spin–orbit coupling lifts the degeneracy of the electronic states. This is because the z-component of spin interacts with the z-component of the orbital angular momentum, generating a total electronic angular momentum along the molecule axis Jz. This is characterized by the MJ quantum number, where MJ = MS + ML. Again, positive and negative values of MJ are degenerate, so the pairs (ML, MS) and (−ML, −MS) are degenerate: {(1, 1/2), (−1, −1/2)}, and {(1, −1/2), (−1, 1/2)} represent two different degenerate states. These pairs are grouped together with the quantum number Ω, which is defined as the sum of the pair of values (ML, MS) for which ML is positive. Sometimes the equation Ω = Λ + MS is used (often Σ is used instead of MS). Note that although this gives correct values for Ω it could be misleading, as obtained values do not correspond to states indicated by a given pair of values (ML, MS). For example, a state with (−1, −1/2) would give an Ω value of Ω = |−1| + (−1/2) = 1/2, which is wrong. Choosing the pair of values with ML positive will give a Ω = 3/2 for that state. With this, a level is given by Note that Ω can have negative values and subscripts r and i represent regular (normal) and inverted multiplets, respectively. For a 4Π term there are four degenerate (ML, MS) pairs: {(1, 3/2), (−1, −3/2)}, {(1, 1/2), (−1, −1/2)}, {(1, −1/2), (−1, 1/2)}, {(1, −3/2), (−1, 3/2)}. These correspond to Ω values of 5/2, 3/2, 1/2 and −1/2, respectively. Approximating the spin–orbit Hamiltonian to first order perturbation theory, the energy level is given by E = A ML MS where A is the spin–orbit constant. For 4Π the Ω values 5/2, 3/2, 1/2 and −1/2 correspond to energies of 3A/2, A/2, −A/2 and −3A/2. Despite of having the same magnitude, levels of Ω = ±1/2 have different energies associated, so they are not degenerate.