Parallelization of the AV Routines

The AV routines are parallelized in a similar way to the other hydro routines. Using the gather-scatter approach, we add half of the interaction between particles in turn to the AV force for each, breaking up the work by having each processor loop over a subset of the particles. Note that the gather-scatter technique used here guarantees that AV-related forces conserve both momentum and angular momentum up to machine precision.

An important fact to note about using an AV prescription is that it can greatly reduce the size of the timestep when a shock forms. In extreme cases, the timestep can become so small that the code effectively freezes. Be on the lookout for this, and, if necessary, edit the timestep routine until acceptable stepsizes result, but beware spurious results.