How well do STARLAB and NBODY4 compare? I: Simple models
  N-body simulations are widely used to simulate the dynamical evolution of a
variety of systems, among them star clusters. Much of our understanding of
their evolution rests on the results of such direct N-body simulations. They
provide insight in the structural evolution of star clusters, as well as into
the occurrence of stellar exotica. Although the major pure N-body codes
STARLAB/KIRA and NBODY4 are widely used for a range of applications, there is
no thorough comparison study yet. Here we thoroughly compare basic quantities
as derived from simulations performed either with STARLAB/KIRA or NBODY4.
  We construct a large number of star cluster models for various stellar mass
function settings (but without stellar/binary evolution, primordial binaries,
external tidal fields etc), evolve them in parallel with STARLAB/KIRA and
NBODY4, analyse them in a consistent way and compare the averaged results
quantitatively. For this quantitative comparison we develop a bootstrap
algorithm for functional dependencies.
  We find an overall excellent agreement between the codes, both for the
clusters' structural and energy parameters as well as for the properties of the
dynamically created binaries. However, we identify small differences, like in
the energy conservation before core collapse and the energies of escaping
stars, which deserve further studies. Our results reassure the comparability
and the possibility to combine results from these two major N-body codes, at
least for the purely dynamical models (i.e. without stellar/binary evolution)
we performed. (abridged)
