# AW: how to calculate the box size

From: Norman Geist (norman.geist_at_uni-greifswald.de)
Date: Wed Mar 07 2012 - 01:01:31 CST

I agree with Axel. Determining the box size even from a wrapped dcd by
measuring min-max isn’t correct as one don’t respect the cell center used to
create the data and will get a slightly dancing origin of the unit cell. By
the way this method is called bounding box. One can easily check that with
vmd when using the following command:

pbc box –center bb

and then play over the frames.

I would also advice to just use the box information namd wrote to the dcd,
what is actually really great as one have the whole history of the box size
over time and can even watch the box shrinking during pressure runs.
Actually this fulfills all dreams one can have about the box information.

Best wishes

Norman Geist.

Von: owner-namd-l_at_ks.uiuc.edu [mailto:owner-namd-l_at_ks.uiuc.edu] Im Auftrag
von Cristhian Boetsch
Gesendet: Dienstag, 6. März 2012 17:49
An: mjyang
Cc: namd-l_at_ks.uiuc.edu
Betreff: Re: namd-l: how to calculate the box size

Hi Mingjun:
there is a comand `xstFreq 1000`. If you set this comand together
with a frecuency same as dcdFreq you can output the vectors of the box in a
a xst file.
Cristhian

2012/3/6 mjyang <mjyang_at_hku.hk>:
> Dear NAMD users,
>
> I performed a simulation by NAMD for a protein system solvated by a
cubic water box. Now I extracted the snapshots from NAMD trajectory for
postpone analysis, in which the protein should be centered in the box. The
box size was calculated by (max_x-min_x, max_y-min_y, max_z-min_z), where
max_x,y,z and min_x, y, z are the maximum and minimum coordinate values of
the extracted frame. However, the box size calculated in this way is a
little larger than the real one, resulting in gaps in the recentered box. It
seems that the box size written out by NAMD in the *.xsc file is better than
what I calculated. But the NAMD output size (in *.xsc file) only corresponds
to the last frame of a segment of trajectory file and I need the size for
each frame in the traj. Could someone please tell me how NAMD calculate the
box along MD simulations?
>
>
> Many thanks.
>
>
> Mingjun
>

```--
Mic. Cristhian Boetsch