The 2-spinor ψ + corresponds to a particle with 4-momentum (E, p) and charge q and two spin states (σ = ± ⁠ 1 / 2 ⁠, as before). The other 2-spinor ψ − corresponds to a similar particle with the same mass and spin states, but negative 4-momentum −(E, p) and negative charge − q, that is, negative energy states, time-reversed momentum, and negated charge. This was the first interpretation and prediction of a particle and corresponding antiparticle. See Dirac spinor and bispinor for further description of these spinors. In the non-relativistic limit the Dirac equation reduces to the Pauli equation (see Dirac equation for how). When applied a one-electron atom or ion, setting A = 0 and ϕ to the appropriate electrostatic potential, additional relativistic terms include the spin–orbit interaction, electron gyromagnetic ratio, and Darwin term. In ordinary QM these terms have to be put in by hand and treated using perturbation theory. The positive energies do account accurately for the fine structure.