Force Fields and Molecular Mechanics
A force field is a parameterized expression for molecular potential energy that lets molecular mechanics compute structures and energies of very large systems without quantum calculations.
Definition
An empirical potential-energy function, together with its parameters, that approximates the energy of a molecular system as a function of atomic coordinates using classical mechanics.
Scope
Covers the functional form of classical force fields, bonded terms for bonds, angles, and torsions, non-bonded electrostatic and van der Waals terms, atom typing and parameterization strategies, and the major biomolecular force-field families. Also addresses polarizable and reactive force fields and the inherent transferability limits of fixed parameters.
Core questions
- What physical interactions do the terms of a force field represent?
- How are force-field parameters derived and how transferable are they?
- How do the major biomolecular force fields differ in philosophy and target systems?
- What can fixed, non-reactive force fields not describe?
Key theories
- Additive potential-energy decomposition
- Total energy is written as a sum of independent bonded and non-bonded contributions, an approximation that makes evaluation fast and parameterization modular.
- Empirical parameterization
- Force-field parameters are fitted to reproduce experimental data and quantum-chemical calculations for representative compounds, with the assumption that parameters transfer to similar chemical environments.
Clinical relevance
Force fields are the foundation of all classical biomolecular and materials simulation; their quality sets an upper bound on the realism of molecular dynamics and free-energy predictions used in drug and materials design.
History
From Allinger's MM2/MM3 force fields for organic molecules, the field expanded into biomolecular force fields such as AMBER, CHARMM, OPLS, and GROMOS in the 1980s and 1990s, with ongoing development of polarizable and machine-learned potentials.
Key figures
- Peter Kollman
- Alexander MacKerell
- William Jorgensen
- Norman Allinger
Related topics
Seminal works
- cornell1995
- mackerell1998
Frequently asked questions
- Why can't a force field describe chemical reactions?
- Standard force fields hold bonds as fixed springs and cannot break or form them; describing reactions requires reactive force fields or a quantum treatment of the reactive region.
- Are all force fields interchangeable?
- No; different families are parameterized for different systems and with different conventions, so results can vary, and mixing parameters across force fields is generally invalid.