  Equal-time Green's function is used to derive a three-dimensional integral
equation from the Bethe-Salpeter equation. The resultant equation, in the
absence of anti-particles, is identical to the use of time-ordered diagrams,
and has been used within the framework of $\phi^2\sigma$ coupling to study the
role of energy dependence and non-locality when the two-body potential is the
sum of $\sigma$-exchange and crossed $\sigma$ exchange. The results show that
non-locality and energy dependence make a substantial contribution to both the
on-shell and off-shell amplitudes.
