[ ( γ γ 1 ) μ μ ( p 1 − − A ~ ~ 1 ) μ μ + m 1 + S ~ ~ 1 ] Ψ Ψ = 0 , [ ( γ γ 2 ) μ μ ( p 2 − − A ~ ~ 2 ) μ μ + m 2 + S ~ ~ 2 ] Ψ Ψ = 0. where, in coordinate space, p is the 4-momentum, related to the 4-gradient by (the metric used here is η η μ μ ν ν = ( − − 1 , 1 , 1 , 1 ) ) p μ μ = − − i ∂ ∂ ∂ ∂ x μ μ and γ are the gamma matrices. The two-body Dirac equations (TBDE) have the property that if one of the masses becomes very large, say m 2 → → ∞ ∞ then the 16-component Dirac equation reduces to the 4-component one-body Dirac equation for particle one in an external potential.