[gmx-users] How do gromacs remove center of mass of a group?

Jiang ran ran_jiong_lang2 at hotmail.com
Tue Oct 21 15:12:06 CEST 2008

Fisrt one thing, In fact the C.O.M particle in the GROMACS software package for molecular simulation is indeed moving in absolute space. Best way therefore is to apply C.O.M particle as FREEZE group or just remove the motion of the box center. However, there might still be some drift in the non-real space-phase.
Hope this is any help,
Jiong Ran,
> Yeah, maybe my discription is a little bit complicated.
> Well, what I understand of the center of mass removal, e.g. linear
> removal, is that, the center of mass is not actually contrained or
> restrained, it's just that all velocities in the system are simply
> substracted by the velocities of the COM of the group every nstcomm
> steps.
> So in fact, the COM of the group is still moving in absolute space,
> right?
The COM of the system is stationary unless you have periodic boundary 
condition (in which it fluctuates).
The overall velocity is subtracted from all the atomic velocities.
> For example, if I add a force to a protein in a water enviroment while I
> set the COM of the protein to be removed linearly, then the protein will
> not actually stay stationarily in terms of COM as to the water
> environment, but it will be dragged in the water environment, feeling
> the friction of water mainly in the direction against the pulling
> direction.
You probably wat to give the protein and the water opposite force such 
that the net force is zero. Or else I don't understand what you want to 
> I wonder if my understanding of the COM removal algorithm is right.
> 在 2008-10-20一的 15:52 +0200,gmx-users-request at gromacs.org写道:
>> zcwang wrote:
>>> Hi all
>>> I would like to know how gromacs remove center of mass motion?
>> The COM velocity is the rate of change of the position of the COM,
>> i.e. 
>> the mass-weighted average velocity of the particles. If you subtract 
>> that from each particle velocity, the COM velocity becomes zero. 
>> Likewise for angular motion, if applicable.
>>> I thought it still allowed center of mass motion, but just did a
>>> transformation of the coordinates at a certain frequency.
>> I've never heard of GROMACS doing anything like that for this purpose.
>>> It's just like looking at the system in a noninertial system
>> attached to
>>> the COM of the group.
>>> Is that right?
>> I don't know what you mean.
>> Mark
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