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# 6.3. Gromacs selections

Write `MDAnalysis.core.AtomGroup.AtomGroup` selection to a ndx file that defines a Gromacs index group. To be used in Gromacs like this:

    <GROMACS_COMMAND> -n macro.ndx

The index groups are named mdanalysis001, mdanalysis002, etc.

class `MDAnalysis.selections.gromacs.``SelectionWriter`(filename, mode='wa', numterms=None, preamble=None, **kwargs)

  Set up for writing to filename.

  Arguments : | filename output file mode overwrite (“w”) for every write, append (“a”) to existing file, or overwrite an existing file and then append (“wa”) [“wa”] numterms number of individual index numbers per line for output formats that write multiple entries in one line. If set to 0 or `False` then no special formatting is done [8] preamble string that is written as a comment at the top of the file [] kwargs use as defaults for `write()`

  `comment`(s)

    Return string s interpolated into the comment format string.

    If no `SelectionWriter.commentfmt` is defined (None) then the empty string is returned because presumably there is no way to enter comments into the file.

    A newline is appended to non-empty strings.

  `write`(selection, number=None, name=None, frame=None, mode=None)

    Write selection to the output file.

    Arguments : | selection a `MDAnalysis.core.AtomGroup.AtomGroup` number selection will be named “mdanalysis<number>” (`None` auto increments between writes; useful when appending) [`None`] name selection will be named name (instead of numbered) [`None`] frame write selection of this frame (or the current one if `None` [`None`]

  `write_preamble`()

    Write a header, depending on the file format.

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