# group2ndx command

# ndx2group command

## Syntax

    group2ndx file group-ID ...
    ndx2group file group-ID ...

-   file = name of index file to write out or read in
-   zero or more group IDs may be appended

## Examples

``` LAMMPS
group2ndx allindex.ndx
group2ndx someindex.ndx upper lower mobile
ndx2group someindex.ndx
ndx2group someindex.ndx mobile
```

## 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 [fix colvars](fix_colvars) input file. It can
also be used to save and restore group definitions for static groups.

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.

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.

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## Restrictions

This command requires that atoms have atom IDs, since this is the
information that is written to the index file.

These commands are part of the COLVARS package. They are only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

## Related commands

[group](group), [dump](dump), [fix colvars](fix_colvars)

## Default

none