Often it is necessary to run MD simulations of crystal structures, for example to determine the melting line. In that case, it is nice to have a CIF file that describes the exact crystal structure, and somehow convert it to a file that Gromacs or LAMMPS can read.
I've written a nice little Python script called "read_cif.py" that does exactly that: read_cif.py (tar/gzip file, 24 July 2016 version). This latest version of the script uses the PyCifRW package to parse CIF files, which is available at: https://bitbucket.org/jamesrhester/pycifrw/. For convience, a minimal version of PyCifRW is included in the tgz file.
Some nice CIF files are those for water (ice), for silica (SiO2), and a few others. A large database of CIF files can be found at http://rruff.geo.arizona.edu/AMS/amcsd.php
Here is a little tutorial on how to use the read_cif.py script. To take a look at the crystal in quartz-alpha.cif, one can use for example:
./read_cif.py -i quartz-alpha.cif -o quartz-222.cif -b 2 2 2
and then open the quartz-222.cif file with Jmol. The result should look like: