CHARMM22

CHARMM22 is an all-atom empirical force field within the CHARMM family used for molecular dynamics simulations of proteins. It was developed by Martin J. MacKerell Jr. and colleagues and published in the late 1990s. The parameter set provides bonded terms (bonds, angles, dihedrals) and nonbonded interactions (electrostatics with fixed partial charges, van der Waals via Lennard-Jones) for atom types defined in the CHARMM22 topology. The protein-focused parameterization aimed to reproduce quantum mechanical data and experimental observables such as conformational preferences, thermodynamic properties, and NMR data. It emphasizes backbone and side-chain torsion potentials, enabling realistic sampling of secondary structures in MD simulations.

To address known imbalances in the phi-psi energy landscape, the CHARMM22 force field was later augmented with a two-dimensional correction map (CMAP) that modifies the backbone dihedral energy as a function of phi and psi. The resulting package is commonly referred to as CHARMM22/CMAP and has been influential in improving protein conformational accuracy. Subsequent force fields, such as CHARMM27 and CHARMM36, built on the CHARMM philosophy and refined parameters, but the CHARMM22 lineage remains a reference point for comparing parameterization strategies.

Early studies reported that CHARMM22 could over-stabilize certain secondary structures in some contexts, prompting further revisions and the development of alternative balancing techniques such as different torsional terms or CMAP refinements.

CHARMM22 and its CMAP variant have been widely used in biomolecular simulations, structure prediction, and NMR data interpretation. They underpin numerous studies and remain part of the historical development of modern protein force fields within CHARMM.