ations
for shake style, only atoms in small clusters can be constrained -
  e.g. water molecules, CH3 groups, but not the C backbone of a
  long polymer chain - a cluster is defined as a central atom
  connected to others in the cluster by constrained bonds connected
  together by constrained bonds - the max size of such a cluster is
  4 atoms to enable easier parallelization
for shake style, the max iteration count need not be large (e.g. 3) since
  iterations are only done within a cluster and converge qu