The constraints in the potential of mean force algorithm have not
converged in the permitted number of cycles. (The SHAKE algorithm for
PMF constraints is iterative.) Possible causes
include: a bad starting configuration; too large a time step used;
incorrect force field specification; too high a temperature;
inconsistent constraints involving shared atoms etc.
Action:
Corrective action depends on
the cause. It is unlikely that simply increasing the iteration number
will cure the problem, but you can try: follow standard user response
to increase the parameter mxshak. But the trouble is much more
likely to be cured by careful consideration of the physical system
being simulated. For example, is the system stressed in some way? Too
far from equilibrium?