Stanislav Bobritsky wrote:

     > Well, but (as I know from Manual, Ch. 6, p. 120) there is
    "approach to do targeted MD" in GMX.

    Did you add position restraint to your protein
    topology?


No, I did not.
What group must be restrained therefore? Whole protein or whole protein except loop?

From "man grompp",

When using position restraints a file with restraint coordinates can be
supplied with -r, otherwise restraining will be done with respect to the
conformation from the -c option. For free energy calculation the the
coordinates for the B topology can be supplied with -rb, otherwise they will
be equal to those of the A topology.

Thus I expect your earlier grompp command line didn't do the job you thought it did.

You could restrain just the loop if you knew the protein was stable, but easiest is to present conformation A as -c and B as -r. pdb2gmx will have supplied you with a posre.itp file, and you need -DPOSRES in the include .mdp directive so that the former works.

Your .mdp file as supplied earlier should not T-couple only your solute, and neither should it couple your counterions separately from the solvent. See http://wiki.gromacs.org/index.php/Thermostats

Based on the foregoing, I'd guess that you're fairly new to MD and/or GROMACS. I'd strongly recommend doing some (more) tutorials and background reading so that you have more idea what "normal" MD is, and the consequences of changes you might make. Otherwise the odds are excellent you're just using an complicated quasi-random number generator.

Mark
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