  The coagulation of dust aggregates occurs in various astrophysical
environments. Each one is characterized by different conditions that influence
the growth, e.g. relative velocities, composition, and size of the smallest
constituents (monomers). Here we study the microphysics of collisions of dust
aggregates in a four-dimensional parameter space. The parameters are the
collision energy, the initial compactness of agglomerates, the mass ratio of
collision partners, and the impact parameter. For this purpose we employ a
state of the art molecular dynamics type of model that has been extensively and
successfully tested against laboratory experiments. It simulates the motion of
individual monomers interacting dynamically via van der Waals surface forces.
The structure of aggregates is quantified by the filling factor that provides
information about the internal structure, the packing density of monomers, and
the projected surface area of aggregates. Our results show the importance of
the impact parameter that causes formation of elongated particles, due to
tensile forces acting in offset collisions. We also describe in detail the
internal structure of the resulting aggregates.
  Our findings are summarized in the form of a simple collision recipe. The
recipe specifies the outcome of a collision, averaged over the impact
parameter. It is provided in tabular form for a range of physical parameters
such as impact energy and pre-collision filling factor. The dependence on the
mass ratio of impactor and target is taken into account by providing both a
local and a global branch of the recipe.

Solar and Stellar Astrophysics