TRR files contain information on the particle positions, velocities and forces, as well as on the size of the simulation box with high precision. The exact precision depends on the way Gromacs was compiled and usually 4 byte floating point numbers are considered precise enough. XTC files are more compressed than TRR files and omit the information on velocites and forces. They achieve a high compression by storing only the required number of significant digits as integer values. The number of significant digits stored can be set by the user. Their smaller size makes XTC files ideal for long term storage of simulation data and transfer between different computers. Moreover, their precision is usually high enough for most analysis tasks. To reduce the file size even further it is possible to restrict the content of XTC files to only those molecules that are of interest for the analysis, for example only protein atoms.