A plausible structure for the quasipotential Φ Φ can be found by observing that the one-body Klein–Gordon equation ( p 2 + m 2 ) ψ ψ = ( p → → 2 − − ε ε 2 + m 2 ) ψ ψ = 0 takes the form ( p → → 2 − − ε ε 2 + m 2 + 2 m S + S 2 + 2 ε ε A − − A 2 ) ψ ψ = 0 when one introduces a scalar interaction and timelike vector interaction via m → → m + S and ε ε → → ε ε − − A . In the two-body case, separate classical and quantum field theory arguments show that when one includes world scalar and vector interactions then Φ Φ depends on two underlying invariant functions S ( r ) and A ( r ) through the two-body Klein–Gordon-like potential form with the same general structure, that is Φ Φ = 2 m w S + S 2 + 2 ε ε w A − − A 2 . Those field theories further yield the c.m. energy dependent forms m w = m 1 m 2 / w , and ε ε w = ( w 2 − − m 1 2 − − m 2 2 ) / 2 w , ones that Tododov introduced as the relativistic reduced mass and effective particle energy for a two-body system. Similar to what happens in the nonrelativistic two-body problem, in the relativistic case we have the motion of this effective particle taking place as if it were in an external field (here generated by S and A ). The two kinematical variables m w and ε ε w are related to one another by the Einstein condition ε ε w 2 − − m w 2 = b 2 ( w ) , If one introduces the four-vectors, including a vector interaction A μ μ p = ε ε w P ^ ^ + p , A μ μ = P ^ ^ μ μ A ( r ) r = x ⊥ ⊥ 2 , and scalar interaction S ( r ) , then the following classical minimal constraint form H = ( p − − A ) 2 + ( m w + S ) 2 ≈ ≈ 0 , reproduces