[gmx-users] Constrained simulations crash when bilayer moves in the z direction in the box

ERIKSSON, EMMA E.ERIKSSON1 at nuigalway.ie
Tue Apr 27 16:32:53 CEST 2010

Hi again,
Thanks Xavier for your reply. I didn't know that this mdp option existed. However, I read the manual and also checked the mdout.mdp files for my previous simulations, and I understood it as if those are the default settings even if you don't specify any of them in the md.mdp file. The default comm_groups is the whole system so I guess if I'm not writing anything there it will take the whole system. In that case I think that in my previous simulations the translational motion should have been removed for the whole system, but since it's obviously not remove something is wrong. Or did I misunderstand everything? My system consists of DPPC lipids, cholesterol, water and one small molecule. Should I specify comm_groups as only the lipids? In that case I get a warning from grompp. You wrote comm-grps = membrane solvent + ions. Only the water then?
Sorry that I didn't understand you explanation.


Från: gmx-users-bounces at gromacs.org [gmx-users-bounces at gromacs.org] för XAvier Periole [x.periole at rug.nl]
Skickat: den 27 april 2010 12:41
Till: Discussion list for GROMACS users
Ämne: Re: [gmx-users] Constrained simulations crash when bilayer moves in the z direction in the box

The crash seems to be expected as by crossing the pbc the distance will change significantly
and in way the algorithm can not handle.

Note that the overall translational motion of your system should always be removed.

The removal of the COM motion of your bilayer should be sufficient to prevent the overall
motion of the bilayer. have a look at the following option in the mdp:
; mode for center of mass motion removal
comm-mode                = Linear
; number of steps for center of mass motion removal
nstcomm                  = 1
; group(s) for center of mass motion removal
comm-grps                = membrane solvent+ions

On Apr 27, 2010, at 12:07 PM, ERIKSSON, EMMA wrote:

Hi all!

I have a problem regarding lipid bilayer simulations in Gromacs 4. During some of my simulations the whole system is moving in the z direction in the box, meaning that after some time the lipids are going out in the bottom of the box and coming in in the top of the box, since I'm using periodic boundary conditions. This doesn't matter (I think) when running non-constrained simulations, however when I'm constraining the distance between the lipids (pull_geometry=cylinder) and a molecule in the system the system explodes and the simulations crash when the lipids are starting to cross over to the other side. The fact that the system is moving in the box must be the problem since the system explodes exactly when the first lipid passes over to the other side and nothing like this ever happens when the bilayer is not moving in the box. Is there any way to freeze the cylindrical COM of the lipids or something like that so that they stay more or less in the middle of the box all the time? I don't want to use freezegrps and freezedim = N N Y as this freezes the lipids completely in the z direction, and that's not what I want, I want them to be free to move as before but I want to stop the whole system from moving too much in the z direction. Anyone that has experienced a similar problem or know how to go about with this? I would really appreciate any help I can get. Thanks.

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