GROMACS is extremely fast due to algorithmic and processor-specific optimization, typically running 3-10 times faster than many simulation programs. GROMACS is operated via the command-line, and can use files for input and output. GROMACS provides calculation progress and ETA feedback, a trajectory viewer, and an extensive library for trajectory analysis.[2] In addition, support for different force fields makes GROMACS very flexible. It can be executed in parallel, using MPI or threads. GROMACS contains a script to convert molecular coordinates from a PDB file into the formats it uses internally. Once a configuration file for the simulation of several molecules (possibly including solvent) has been created, the actual simulation run (which can be time consuming) produces a trajectory file, describing the movements of the atoms over time. This trajectory file can then be analyzed or visualized with a number of supplied tools.