use_collective

In VASP.4.5, the MPI version of VASP avoids collective communication, since they are very ineffciently implemented in the public domain MPI packages, such as LAM or MPICH. On the SGI Origins and on the T3E, on the other hand the collective MPI routines are highly optimised. Hence use_collective should be specified on these platforms, and whenever the collective MPI routines were optimised for the architecture.