spin coupling with the orbital motion can lead to splitting of energy bands due to Dresselhaus or Rashba effects. In the macroscopic world of orbital mechanics, the term spin–orbit coupling is sometimes used in the same sense as spin–orbit resonance. LS coupling In light atoms (generally Z ≤ 30), electron spins si interact among themselves so they combine to form a total spin angular momentum S. The same happens with orbital angular momenta ℓi, forming a total orbital angular momentum L. The interaction between the quantum numbers L and S is called Russell–Saunders coupling (after Henry Norris Russell and Frederick Saunders) or LS coupling. Then S and L couple together and form a total angular momentum J: where L and S are the totals: This is an approximation which is good as long as any external magnetic fields are weak. In larger magnetic fields, these two momenta decouple, giving rise to a different splitting pattern in the energy levels (the Paschen–Back effect), and the size of LS coupling term becomes small. For an extensive example on how LS-coupling is practically applied, see the article on term symbols. jj coupling In heavier atoms the situation is different. In atoms with bigger nuclear charges, spin–orbit interactions are frequently as large as or larger than spin–spin interactions or orbit–orbit interactions. In this situation, each orbital angular momentum ℓi tends to combine with the corresponding individual spin angular momentum si, originating an individual total angular momentum ji. These then couple up to form the total angular momentum J This description, facilitating calculation of this kind of interaction, is known as jj coupling. Spin–spin coupling Spin–spin coupling is the coupling of the intrinsic angular momentum (spin) of different particles. J-coupling between pairs of nuclear spins is an important feature of nuclear magnetic resonance (NMR) spectroscopy as it can provide detailed information about the structure and conformation of molecules. Spin–spin coupling between nuclear spin and electronic spin is responsible for hyperfine structure in atomic spectra. Term symbols Term symbols are used to represent the states and spectral transitions of atoms, they are found from coupling of angular momenta mentioned above. When the state of an atom has been specified with a term symbol, the allowed transitions can be found through selection rules by considering which transitions would conserve angular momentum. A photon has spin 1, and when there is a transition with emission or absorption of