In
computational chemistry, a forcefield is simply a set of rules to describe the physics of molecular interactions. These are useful in
molecular dynamics simulations, molecular
docking experiments and all kinds of other techniques. The force field contains terms that describe the elasticity of bonds between atoms, angular strain caused by rotating molecules around a bond, through space interactions with
van der Waals (
dispersion) and
electrostatic energy components, and other terms. The major challenge is to build a force field that accurately models the real behavior of molecules. Some approaches work on
parameterizing a force field by running a simulation, testing the results to see if they are life-like, and then readjusting the parameters until they work. While moderately successful, this approach results in a forcefield that is hard to physically justify. In addition, if a new system which behaves in a novel fashion is modelled, the forcefield may not accurately describe its behavior. The other type of forcefield, is an
ab initio one, based solely on basic principles. The
AMBER forcefield developed in
Peter Kollman's lab at the
Univerisity of California, San Francisco is a good example of a forcefield of this type that seems to be successful. The most commonly used forcefield in protein molecular dynamics is the
CHARMm forcefield, first assembled in the
Karplus lab. CHARMm is highly parameterized, but has been (so far) reasonably successful at modeling many kinds of physical processes at the molecular scale. Other commonly used forcefields include
OPALS and
CVFF (the constant valence force field).
There are other challenges to molecular modelling besides accurate force fields, including how to deal with
solvent. Developing new
solvation models is an intense area of research.
Quantum chemical modelling does not use forcefields, but instead depends on
basis sets of functions which are used to build the
wavefunction of a molecule.