Secondly, the mathematical consistency condition also eliminates the relative energy in the c.m. frame. It does this by imposing on each Dirac operator a structure such that in a particular combination they lead to this interaction independent form, eliminating in a covariant way the relative energy. P ⋅ ⋅ p Ψ Ψ = ( − − P 0 p 0 + P → → ⋅ ⋅ p ) Ψ Ψ = 0.