[gmx-users] Could not converge NPT constraints

Justin Lemkul jalemkul at vt.edu
Tue Oct 9 20:05:43 CEST 2012

On 10/9/12 11:31 AM, juan-manuel.castillo at mv.uni-kl.de wrote:
> I have not much experience with Gromacs, although I have worked for years on
> molecular simulations. I am trying to run a NPT simulation of 1000 SPCE water
> molecules at 293 K with the opls force field. I equilibrated the system first
> during 1 ns using a velocity rescale thermostat, a Berendsen barostat, and a
> leap-frog integration method. Then I run a simulation using the md-vv-avek
> integration method, a Nose-Hoover thermostat, and a MTTK barostat. I have tried
> to change different parameters of the simulation, but after some steps I always
> obtain the following error:
> Fatal error:
> Could not converge NPT constraints
> Anybody can help to explain what happens? I can provide input files if needed.

An .mdp file would be helpful.

The failure is coming from the done_iterating() function in 
src/mdlib/iteratedconstraints.c, and there are actually quite extensive comments 
in the code about what the function is doing.  Perhaps you can deduce some 
useful information from the description there.



Justin A. Lemkul, Ph.D.
Research Scientist
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080


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