[gmx-users] gmx distance

[Justin Lemkul]

On 6/5/15 7:07 AM, Ming Tang wrote:
> Dear gromacs experts,
>
> I am stretching a triple helix along z direction by fixing one end and pull the other. The helix length changes from 35nm to 50nm during the MD simulation. However, after using
> gmx distance -n index.ndx -f traj.trr -s topol.tpr -select 'com of group start plus com of group end' -oav dist.xvg
> to get the distance between the two end groups,   I found that the distance varies from 2nm to 15nm during the simulation.
> I am quite confused about this, because the original length of the helix is around 35nm. The configuration looks correct....
> Am I wrong when using gmx distance?
>

Probably -pbc is relevant here.

  -[no]pbc        (yes)
      Use periodic boundary conditions for distance calculation

You're likely getting the inter-image distance rather than the one within the 
unit cell.

-Justin

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

Justin A. Lemkul, Ph.D.
Ruth L. Kirschstein NRSA Postdoctoral Fellow

Department of Pharmaceutical Sciences
School of Pharmacy
Health Sciences Facility II, Room 629
University of Maryland, Baltimore
20 Penn St.
Baltimore, MD 21201

jalemkul at outerbanks.umaryland.edu | (410) 706-7441
http://mackerell.umaryland.edu/~jalemkul

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