Where does the Universe object store the dimensions of the simulation cell?

[Satyen Dhamankar]

Hello,

I am learning the ropes of MDAnalysis so this is a very basic question. 

I am parsing a trajectory output by LAMMPS in the standard "atom" format. 

This is a snippet from the trajectory:

```
ITEM: TIMESTEP
0
ITEM: NUMBER OF ATOMS
42572
ITEM: BOX BOUNDS pp pp pp
0.0000000000000000e+00 7.5000000000000000e+01
0.0000000000000000e+00 7.5000000000000000e+01
0.0000000000000000e+00 7.5000000000000000e+01
ITEM: ATOMS id type x y z
1 1 53.1548 4.4557 0.70681
2 2 52.7814 5.36408 1.18418
3 1 51.9246 5.09719 1.8911
4 1 53.4932 5.97228 1.79943
5 3 52.4377 6.3564 0.0468971
6 1 52.2907 7.2622 0.634702
7 4 51.1551 5.8994 74.3893
8 5 50.9479 4.74092 74.0151
9 6 50.2509 6.89418 74.222
10 7 50.4368 7.7257 74.7573
```

I am analyzing the trajectory and calculating an RDF in the following manner:
```
import MDAnalysis as mda
from MDAnalysis.analysis import rdf
import matplotlib.pyplot as plt
import numpy as np

if __name__=="__main__":

    coords = mda.coordinates.LAMMPS.DumpReader("coords.npt.lammpsdump")
    u = mda.Universe("coords.npt.lammpsdump", atom_stype="id type x y z")

    # access atom types
    water_o = u.select_atoms('type 10')
    polymer_o = u.select_atoms('type 5')
    irdf = rdf.InterRDF(water_o, polymer_o, nbins=100, range=(0.0, 30.0), exclusion_block=(1,1))
    irdf.run()

    plt.plot(irdf.bins, irdf.rdf, c='steelblue')
    plt.savefig("oo_rdf", dpi=1200, bbox_inches="tight")

```

My question is, where does the Universe object store the dimensions of the simulation cell? The simulation is NPT so the volume of the box fluctuates, and I would like to understand how I can access the volume fluctuations via the Universe object.

[Roshan Shrestha]

Hi,

     You can access the dimensions at Universe.trajectory.dimensions

With best regards 

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