(This does not require that [ P , λ λ i ] = 0 since the H i Ψ Ψ = 0 .) Thus, the total momentum P is a constant of motion and Ψ Ψ is an eigenstate state characterized by a total momentum P ′ . In the c.m. system P ′ = ( w , 0 → → ) , with w the invariant center of momentum (c.m.) energy. Thus