Help on SHAKE constraints

 Molecular Dynamics Experts:
 I wish to ask about the use of constraints like SHAKE in molecular
 dynamics.  What are the reasons that one would use SHAKE?  What are
 the deficiencies of SHAKE?  When SHAKE (or the like) is used and you
 experience problems: 1)What are the symptoms of "failure"?, 2)Under
 what conditions does this failure occure? and 3)What is the workaround
 for this failure?
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