 be used
  in conjunction with "fix style bondtype" to make clusters of size 3
  entirely rigid (e.g. water)
the angletype option enables an additional check when SHAKE constraints
  are computed: if a cluster is of size 3 and both bonds in
  the cluster are of a bondtype specified by the 2nd parameter of
  angletype, then the cluster is SHAKEn with an additional angle
  constraint that makes it rigid, using the equilibrium angle appropriate
  to the specified angletype
IMPORTANT NOTE: the angletype option 