Use Gromacs Utilities from Python

Gromacs has some neat command-line programs to interact with MD trajectories. A lot of them prompt for additional input when you’re running them. I’ll show you how to use Python’s subprocess module to deal with this.


In Python, you can call an external program with the subprocess module. For example

import subprocess
subprocess.check_call(['convert', 'image.jpg', 'image.png'])

will call ImageMagik’s convert utility. Note that the arguments need to be given as a list of strings (not one big string). You might be tempted to do

"convert image.jpg image.png".split()

to make it feel more natural. This might cause problems. You should use shlex.split instead.

Gromacs trjconv

Gromacs trjconv is useful. Despite being one of the primary maintainers of mdtraj, I still use it sometimes (blasphemy!). An example run looks like this:

$ gmx trjconv -f XTCs/Traj0/nug2-0.xtc -s nug2.pdb -o newtraj.xtc
GROMACS:    gmx trjconv, VERSION 5.0.7

[stuff removed here]

Library dir:  /home/harrigan/opt/gromacs/share/gromacs/top
Command line:
  gmx trjconv -f XTCs/Traj0/nug2-0.xtc -s nug2.pdb -o newtraj.xtc

  Will write xtc: Compressed trajectory (portable xdr format): xtc
  Select group for output
  Group     0 (         System) has   846 elements
  Group     1 (        Protein) has   846 elements
  Group     2 (      Protein-H) has   435 elements
  Group     3 (        C-alpha) has    56 elements
  Group     4 (       Backbone) has   168 elements
  Group     5 (      MainChain) has   225 elements
  Group     6 (   MainChain+Cb) has   278 elements
  Group     7 (    MainChain+H) has   283 elements
  Group     8 (      SideChain) has   563 elements
  Group     9 (    SideChain-H) has   210 elements
  Select a group:

You have to input numbers sometimes! We can do this with subprocess


You have to get down and dirty with the subprocess module to handle this. Instead of simply calling call (or its variants), we will open a process object and mess with it. The full details are in the python docs.

traj_fn = 'in.xtc'
out_fn = 'out.xtc'
top_fn = 'nug2.pdb'
p = subprocess.Popen(['gmx', 'trjconv', '-center',
                      '-f', traj_fn, '-s', top_fn,
                      '-o', out_fn],
p.communicate(b'1\n0\n') # Center on protein, output everything

We send a byte string (b"this is a string of bytes") to the process after it has been started with the communicate() method. We give subprocess advanced notice that we’re going to do this by telling it to read stdin=subprocess.PIPE. You can send whatever input you want here. Note that we use \n to send “enter”. You can use multiple calls to communicate if you want:

p = Popen(...)

Now you can script gromacs like a champ!

See also

  • Gromacs has some info about this problem (Thanks Nate!). Note that their examples use bash constructs, so you have to do"...", shell=True) for them to work. This is generally a bad idea.

  • GromacsWrapper is a Python package that makes this seamless. Thanks to Chris for pointing this out.