Chemistry at Harvard Macromolecular Mechanics (CHARMM) is the name of a widely used set of force fields for molecular dynamics, and the name for the molecular dynamics simulation and analysis computer software package associated with them. The CHARMM Development Project involves a worldwide network of developers working with Martin Karplus and his group at Harvard to develop and maintain the CHARMM program. Licenses for this software are available, for a fee, to people and groups working in academia. Force fields The CHARMM force fields for proteins include: united-atom (sometimes termed extended atom) CHARMM19, all-atom CHARMM22 and its dihedral potential corrected variant CHARMM22/CMAP, as well as later versions CHARMM27 and CHARMM36 and various modifications such as CHARMM36m and CHARMM36IDPSFF. In the CHARMM22 protein force field, the atomic partial charges were derived from quantum chemical calculations of the interactions between model compounds and water. Furthermore, CHARMM22 is parametrized for the TIP3P explicit water model. Nevertheless, it is often used with implicit solvents. In 2006, a special version of CHARMM22/CMAP was reparametrized for consistent use with implicit solvent GBSW. The CHARMM22 force field has the following potential energy function: The bond, angle, dihedral, and nonbonded terms are similar to those found in other force fields such as AMBER. The CHARMM force field also includes an improper term accounting for out-of-plane bending (which applies to any set of four atoms that are not successively bonded), where is the force constant and is the out-of-plane angle. The Urey-Bradley term is a cross-term that accounts for 1,3 nonbonded interactions not accounted for by the bond and angle terms; is the force constant and is the distance between the 1,3 atoms. For DNA, RNA, and lipids, CHARMM27 is used. Some force fields may be combined, for example CHARMM22 and CHARMM27 for the simulation of protein-DNA binding. Also, parameters for NAD+, sugars, fluorinated compounds, etc., may be downloaded. These force field version numbers refer to the CHARMM version where they first appeared, but may of course be used with subsequent versions of the CHARMM executable program. Likewise, these force fields may be used within other molecular dynamics programs that support them. In 2009, a general force field for drug-like molecules (CGenFF) was introduced. It "covers a wide range of chemical groups present in biomolecules and drug-like molecules, including a large number of heterocyclic scaffolds". The general force field is designed to cover any combination of chemical groups. This