GROMACS is a molecular dynamics package mainly designed for simulations of proteins, lipids, and nucleic acids. It was originally developed in the Biophysical Chemistry department of University of Groningen, and is now maintained by contributors in universities and research centers worldwide. GROMACS is one of the fastest and most popular software packages available, and can run on central processing units (CPUs) and graphics processing units (GPUs). It is free, open-source software released under the GNU General Public License (GPL), and starting with version 4.6, the GNU Lesser General Public License (LGPL). GROMACS (http://www.gromacs.org) is one of the major software packages for the simulation of biological macromolecules. It is aimed at performing the simulation of large, biologically relevant systems, with a focus on both being efficient and flexible to allow the research of a number of different systems . The program has been used by research groups all around the globe, with several hundred publications being based directly or indirectly on it published during the last few years.