The increasing role of general relativity in the dynamics of stellar
systems with central massive black holes, in the generation of extreme
massratio inspirals, and in the evolution of hierarchical triple
systems inspires a close examination of how postNewtonian effects are
incorporated into Nbody dynamics. Most approaches incorporate
relativity by adding to the Newtonian Nbody equations the standard
twobody postNewtonian terms for a given star around the black hole
or for the close binary in a triple system. We argue that, for
calculating the evolution of such systems over timescales comparable
to the relativistic pericenter advance timescale, it is essential to
include ``cross terms" in the equations of motion. These are
postNewtonian terms that represent a coupling between the potential
of the central black hole or the inner binary system and the potential
due to the other star(s) in the system. Over pericenter precession
timescales, the effects of such terms can actually be ``boosted" to
amplitudes of Newtonian order. We display the postNewtonian Nbody
equations of motion including a central black hole in a truncated form
that includes all the relevant cross terms, in a format ready to use
for numerical implementation. For hierarchical triple systems, we
show explicitly the effects of cross terms on the orbitaveraged
equations for the orbit elements of the inner binary for the special
case where the third body is on a circular orbit, and present the
results of numerical integrations of those equations.
