The above set of equations follow from the constraints H i Ψ Ψ = 0 and the definition of the relative momenta given in Eqs.(1) and (2). If instead one chooses to define (for a more general choice see Horwitz), ε ε 1 = w 2 + m 1 2 − − m 2 2 2 w , ε ε 2 = w 2 + m 2 2 − − m 1 2 2 w , p = ε ε 2 p 1 − − ε ε 1 p 2 w , independent of the wave function, then