# XTC (GROMACS compressed trajectory file)¶

 Coordinate reader MDAnalysis.coordinates.XTC.XTCReader Coordinate writer MDAnalysis.coordinates.XTC.XTCWriter

The GROMACS XTC trajectory compresses data with reduced precision (3 decimal places by default). MDAnalysis can only read coordinates from these files. See TRR (GROMACS lossless trajectory file) for uncompressed files that provide velocity and force information.

MDAnalysis uses XDR based readers for GROMACS formats, which store offsets on the disk. The offsets are used to enable access to random frames efficiently. These offsets will be generated automatically the first time the trajectory is opened, and offsets are generally stored in hidden *_offsets.npz files. 1
Trajectories split across multiple files can be read continuously into MDAnalysis with continuous=True, in the style of gmx trjcat.
Occasionally, MDAnalysis fails to read XDR offsets, resulting in an error. The workaround for this is to create the Universe with regenerated offsets by using the keyword argument refresh_offsets=True, as documented in Issue 1893.