Computer simulations at the atomic scale play a fundamental and ever-increasing role in molecular biology, chemistry, and condensed-matter physics. However, accurate simulation techniques typically have a large computational cost, which strongly reduces their scope in the study of rare events like, e.g., protein folding, drug binding, chemical reactions, and phase transitions. To enlarge the timescales which can be investigated by atomistic simulations, several powerful enhanced-sampling and free-energy calculation methods have been developed in recent years. Different approaches have been implemented on a variety of existing computer simulation codes, and it often occurs that a given technique is re-implemented multiple times in the same program by different researchers.
PLUMED (see http://www.plumed-code.org) is a freely-available series of routines which can be automatically interfaced with the most common molecular dynamics programs (Gromacs, Namd, Dl_poly, Amber, etc.) and which enables to perform free-energy calculations by using state-of-the-art enhanced sampling techniques, i.e. metadynamics (in all its variants), umbrella sampling, and Jarzynski method. This facilitates the comparison among different enhanced sampling techniques and permits to choose freely the simulation program most appropriate for each specific application and computer resources available. PLUMED is developed and maintained by a team of computational scientists including the developers of some of the latter techniques, and it benefits by the daily feedback from a rapidly growing community of users (>320 independent downloads as of Jan 2010).