where p μ μ is the four-momentum operator, A μ μ is the electromagnetic four-potential, a is proportional to the anomalous magnetic dipole moment, F μ μ ν ν = ∂ ∂ μ μ A ν ν − − ∂ ∂ ν ν A μ μ is the electromagnetic tensor, and σ σ μ μ ν ν = i 2 [ γ γ μ μ , γ γ ν ν ] are the Lorentzian spin matrices and the commutator of the gamma matrices γ γ μ μ . In the context of non-relativistic quantum mechanics, instead of working with the Schrödinger equation, Pauli coupling is equivalent to using the Pauli equation (or postulating Zeeman energy) for an arbitrary g -factor.