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

Justin A. Lemkul jalemkul at vt.edu
Tue Apr 27 16:37:32 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?

Interfacial systems such as membranes can translate independently of surrounding 
aqueous solvent.  Thus, the lipids could move one way, the water can move in the 
other way, but overall, the net COM motion is zero.  If you specify two groups, 
as Xavier suggested, you treat the COM motion more appropriately.  So, more 

comm-grps = DPPC_CHOL_MOL SOL

...replacing, of course, whatever your small molecule name where I have "MOL." 
You will need a custom index group created by make_ndx to generate this first group.


> Sorry that I didn't understand you explanation.
> Emma
> ------------------------------------------------------------------------
> *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.
>> Emma
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Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080


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