Energy transition in LS-coupling ! :) Hello ! I have a small question I hope someone can help me :) Lets take for example a 2 electron system with l1,s1 and l2,s2. One electron changes energy due to electric dipole. I understand the rules for electrons in electric dipole transitions: 1) Δl = +/- 1 (parity flips) 2) Δml = 0 or +/- 1 3) Δms = 0 But I dont get a selection rule for the terms ! One of the acceptable transitions is : ΔL = 0 But if (1) Δl cant be 0 and (2) Δml = 0 or +/- 1 how is this possible ? :) Because to my understanding L = l1 + l2 in vector form.