update group2ndx/ndx2group docs

doc/src/group2ndx.rst

@@ -34,21 +34,54 @@ Description
 Write or read a Gromacs style index file in text format that associates
 atom IDs with the corresponding group definitions. This index file can be
 used with in combination with Gromacs analysis tools or to import group
-definitions into the :doc:`fix colvars <fix_colvars>` input file. It can
-also be used to save and restore group definitions for static groups.
+definitions into the :doc:`fix colvars <fix_colvars>` input file.
+
+It can also be used to save and restore group definitions for static groups
+using the individual atom IDs. This may be important if the original
+group definition depends on a region or otherwise on the geometry and thus
+cannot be easily recreated.
+
+Another application would be to import atom groups defined for Gromacs
+simulation into LAMMPS.  When translating Gromacs topology and geometry
+data to LAMMPS.
 
 The *group2ndx* command will write group definitions to an index file.
-Without specifying any group IDs, all groups will be written to the index
-file. When specifying group IDs, only those groups will be written to the
-index file. In order to follow the Gromacs conventions, the group *all*
-will be renamed to *System* in the index file.
+Without specifying any group IDs, all groups will be written to the
+index file.  When specifying group IDs, only those groups will be
+written to the index file.  In order to follow the Gromacs conventions,
+the group *all* will be renamed to *System* in the index file.
 
-The *ndx2group* command will create of update group definitions from those
-stored in an index file. Without specifying any group IDs, all groups except
-*System* will be read from the index file and the corresponding groups
-recreated. If a group of the same name already exists, it will be completely
-reset. When specifying group IDs, those groups, if present, will be read
-from the index file and restored.
+The *ndx2group* command will create of update group definitions from
+those stored in an index file.  Without specifying any group IDs, all
+groups except *System* will be read from the index file and the
+corresponding groups recreated.  If a group of the same name already
+exists, it will be completely reset.  When specifying group IDs, those
+groups, if present, will be read from the index file and restored.
+
+File Format
+"""""""""""
+
+The file format is equivalent and compatible with what is produced by
+the `Gromacs make_ndx command <https://manual.gromacs.org/current/onlinehelp/gmx-make_ndx.html>`_.
+and follows the `Gromacs definition of an ndx file <https://manual.gromacs.org/current/reference-manual/file-formats.html#ndx>`_
+
+Each group definition begins with the group name in square brackets with
+blanks, e.g. \[ water \] and is then followed by the list of atom
+indices, which may be spread over multiple lines.  Here is a small
+example file:
+
+.. code-block:: ini
+
+   [ Oxygen ]
+   1  4  7
+   [ Hydrogen ]
+   2  3  5  6
+   8  9
+   [ Water ]
+   1 2 3 4 5 6 7 8 9
+
+The index file defines 3 groups: Oxygen, Hydrogen, and Water and the
+latter happens to be the union of the first two.
 
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