  In dense molecular clouds collisions between dust grains alter the ISM-dust
size distribution. We study this process by inserting the results from detailed
numerical simulations of two colliding dust aggregates into a coagulation model
that computes the dust size distribution with time. All collisional outcomes --
sticking, fragmentation (shattering, breakage, and erosion) -- are included and
the effects on the internal structure of the aggregates are also tabulated. The
dust aggregate evolution model is applied to an homogeneous and static cloud of
temperature 10 K and gas densities between 10^3 and 10^7 cm^-3. The coagulation
is followed locally on timescales of ~10^7 yr. We find that the growth can be
divided into two stages: a growth dominated phase and a fragmentation dominated
phase. Initially, the mass distribution is relatively narrow and shifts to
larger sizes with time. At a certain point, dependent on the material
properties of the grains as well as on the gas density, collision velocities
will become sufficiently energetic to fragment particles, halting the growth
and replenishing particles of lower mass. Eventually, a steady state is
reached, where the mass distribution is characterized by a mass spectrum of
approximately equal amount of mass per logarithmic size bin. The amount of
growth that is achieved depends on the cloud's lifetime. If clouds exist on
free-fall timescales the effects of coagulation on the dust size distribution
are very minor. On the other hand, if clouds have long-term support mechanisms,
the impact of coagulation is important, resulting in a significant decrease of
the opacity on timescales longer than the initial collision timescale between
big grains.
