Two particle realisation of the Poincare group with interaction
  A relative position 4-vector is constructed for two spin-zero particles. Some advantages of this relative position over Bakamjian-Thomas are pointed out. The centre-of-mass (CM) and relative positions and momenta are an explicit realisation of the so-called non-canonical covariant representation. The Hamiltonian including potential terms is factorised into CM and relative components, the latter is a Lorentz scalar readily evaluated in the CM rest frame when the relative position, momentum are canonical conjugates. 
Shaun N Mosley