On 12/20/12 2:47 PM, zugunder wrote:
Could it be, say, because of still big conformational changes of the protein?
The size of the system is not that small: the protein is hydrated with 10921
water molecules.


Generally restraints are applied to the solute during equilibration to avoid structural changes due to clashes with solvent. Pressure and density are related to box size, so any large change in the protein's conformation would probably be very obvious and potentially spurious.

What are the variations of pressure and density that could be tolerated at
this step?


Depends on the algorithm. Pressure is a fickle metric (see http://www.gromacs.org/Documentation/Terminology/Pressure and previous discussions on this list). It is generally advisable to run equilibration using weak coupling (i.e. Berendsen) methods, then switch to a more robust thermostat and barostat for further equilibration and data collection.

-Justin

--
========================================

Justin A. Lemkul, Ph.D.
Research Scientist
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

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