# [gmx-users] obtaining .gro timeframe

Chetan Mahajan chetanvm10 at gmail.com
Wed Jul 2 05:28:35 CEST 2014

```Thanks, Justin. *Until now, I was thinking that xtc coordinates are
wrapped. It's good to know that coordinates from xtc files are unwrapped
(please comment if not correct, since we talked only with respect to gro
file). *But I am confused at another thing. I have known at the origin of
the box is at one of its vertices ( is that not true?). *Then why are some
coordinates in .gro file (the one generated at the end of gromacs MD run)
negative, if coordinates are wrapped in the box?*

Is my original command using trjconv okay?

Whereas I do get a sample x-coordinate in .gro file, generated at the end
of MD run (let's call it gro1) from that in .gro file, generated using
trjconv on xtc (let's call it gro2) by adding a box length to concerned
coordinate in gro2, (as shown in excerpt 1) , it's only by subtracting a
boxlength that we interconvert between similar values, as shown in excerpt
2 of the SAME file. *Why is there no consistency of conversion between SAME
two .gro files for all the coordinates? How do I mathematically obtain file
2 from file 1 with some consistent measure?  *

- : for .gro file at the end of gromacs MD run (gro1)
+ : for .gro timeframe file obtained from .xtc trajectory ( using trjconv)
(gro2)

*EXCERT 1*

-   11SOL    HW1 2188   3.244   3.071  10.934
-   11SOL    HW2 2189   3.211   2.940  10.842
+   11SOL    HW1 2188   0.023   3.071  10.934
+   11SOL    HW2 2189  -0.011   2.940  10.842

*EXCERT 2*
-   56SOL    HW1 2323  -0.031   1.076   1.130
-   56SOL    HW2 2324  -0.040   1.096   1.292
+   56SOL    HW1 2323   3.191   1.076   1.130
+   56SOL    HW2 2324   3.182   1.096   1.292

Thanks
Chetan

On Tue, Jul 1, 2014 at 8:10 PM, Justin Lemkul <jalemkul at vt.edu> wrote:

>
>
> On 7/1/14, 9:08 PM, Chetan Mahajan wrote:
>
>> Thanks, Justin. Are you saying, the coordinates in .gro  file extracted
>> from xtc trajectory are not wrapped, whereas those in the .gro file
>> generated at the end of gromacs MD run are wrapped in the box?
>>
>>
> Yes, unless you've manipulated the trajectory in some way with trjconv
> beforehand.
>
>
>  Now I have a question:  the negative coordinates that are present in the
>> following text are from .gro file (generated at the end of MD run),
>> whereas
>> positive ones are from .gro file obtained using trjconv on xtc.
>>
>>
> Either way, it's a periodicity effect.  Molecules are made whole.
>
>
> -Justin
>
> --
> ==================================================
>
> Justin A. Lemkul, Ph.D.
> Ruth L. Kirschstein NRSA Postdoctoral Fellow
>
> Department of Pharmaceutical Sciences
> School of Pharmacy
> Health Sciences Facility II, Room 601
> 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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```