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

David van der Spoel spoel at xray.bmc.uu.se
Mon Oct 20 17:19:58 CEST 2008

```zcwang wrote:
> 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
do...

> 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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