  The non-equilibrium early time evolution of an ultra-relativistic heavy ion
collision is often described by classical lattice Yang-Mills theory, starting
from the colour glass condensate (CGC) effective theory with an anisotropic
energy momentum tensor as initial condition. In this work we investigate the
systematics associated with such studies and their dependence on various model
parameters (IR, UV cutoffs and the amplitude of quantum fluctuations) which are
not yet fixed by experiment. We perform calculations for SU(2) and SU(3), both
in a static box and in an expanding geometry. Generally, the dependence on
model parameters is found to be much larger than that on technical parameters
like the number of colours, boundary conditions or the lattice spacing. In a
static box, all setups lead to isotropisation through chromo-Weibel
instabilities, which is illustrated by the accompanying filamentation of the
energy density. However, the associated time scale depends strongly on the
model parameters and in all cases is longer than the phenomenologically
expected one. In the expanding system, no isotropisation is observed for any
parameter choice. We show how investigations at fixed initial energy density
can be used to better constrain some of the model parameters.
