OpenMD is an open source molecular dynamics engine which is capable of efficiently simulating liquids, proteins, nanoparticles, interfaces, and other complex systems using atom types with orientational degrees of freedom (e.g. “sticky” atoms, point dipoles, and coarse-grained assemblies). Proteins, zeolites, lipids, transition metals (bulk, flat interfaces, and nanoparticles) have all been simulated using force fields included with the code. OpenMD works on parallel computers using the Message Passing Interface (MPI), and comes with a number of analysis and utility programs that are easy to use and modify. An OpenMD simulation is specified using a very simple meta-data language that is easy to learn.