[gmx-users] Energy Minimization problem

George Abadir georgea at ece.ubc.ca
Thu Apr 19 02:24:38 CEST 2007

     I am doing an energy minimization for a carbon nanotube (end capped 
with hydrogen atoms) in a box of water. I get the following message:
" Stepsize too small, or no change in energy.
Converged to machine precision,
but not to the requested precision  Fmax < 10

Double precision normally gives you higher accuracy.
You might need to increase your constraint accuracy, or turn off 
constraints altogether (set constraints = none in mdp file)

Steepest Descents converged to machine precision in 1836 steps, but did 
not reach the requested Fmax < 10.
Potential Energy = -2.4277438+06
Maximum force = 5.9858018+03  on atom 13
Norm of force =  5.1364398+04"

The potential energy is extremely low and the maximum force is 
considerably large. I turned off the constrained and tried other energy 
minimization methods but still got the same error (with different 
magnitudes of the force and energy of course). Is it likely that double 
precision simulation can make such a big difference? If not (which I 
think) is there any other way to solve the problem?

Thank you very much in advance,
George Abadir
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