Substitution into the Schrödinger equation gives the Pauli equation. This Hamiltonian is similar to the classical Hamiltonian for a charged particle interacting with an electromagnetic field. See Lorentz force for details of this classical case. The kinetic energy term for a free particle in the absence of an electromagnetic field is just formula_10 where formula_11 is the "kinetic" momentum, while in the presence of an electromagnetic field it involves the minimal coupling formula_12, where now formula_13 is the kinetic momentum and formula_11 is the canonical momentum.