  In this paper we test the performance of explicit simulations of groups of
galaxies, (i.e. simulations in which each galaxy is treated as a mass point and
the physics of the interactions is modelled by specific analand the physics of
the interactions is modelled by specific analytical prescriptions for merging
conditions), by comparing them with fully self-consistent simulations starting
from identical initial conditions. The quality of the explicit simulations is
very unequal. For somThe quality of the explicit simulations is very unequal.
For some prescriptions the results are complete disagreement with the
self-consistent simulations. The inclusion of other dynamical effects like
dynamical friction gives, in some cases, better agreeement.like dynamical
friction gives, in some cases, better agreeement. We also propose a new merging
criterion, which, combined with dynamical friction, gives much better agreement
with self-consistent simulations in a variety of initial conditions, but even
this criterion has a limited ranof initial conditions, but even this criterion
has a limited range of applicability.
