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# 5.4. CRD structure files in MDAnalysis — `MDAnalysis.coordinates.CRD`

Read and write coordinates in CHARMM CARD coordinate format (suffix “crd”). The CHARMM “extended format” is handled automatically.

class `MDAnalysis.coordinates.CRD.``CRDReader`(crdfilename, convert_units=None, **kwargs)

  CRD reader that implements the standard and extended CRD coordinate formats

  `Writer`(filename, **kwargs)

    Returns a CRDWriter for filename.

    Arguments : | filename filename of the output CRD file
    Returns : | `CRDWriter`

class `MDAnalysis.coordinates.CRD.``CRDWriter`(filename, **kwargs)

  CRD writer that implements the CHARMM CRD coordinate format.

  It automatically writes the CHARMM EXT extended format if there are more than 99,999 atoms.

  `write`(selection, frame=None)

    Write selection at current trajectory frame to file.

    write(selection,frame=FRAME)

    selection MDAnalysis AtomGroup frame optionally move to frame FRAME

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