Using these types of models allows for large scale calculations in Molecular Dynamics (MD) using classical physics (solving Newton's equations of motion), which otherwise would simply not be feasible with nowadays computers if one uses the above mentioned more fundamental methods derived from quantum mechanics. Examples of such force fields include OPLS , CHARMM , UFF , AMBER , MMFF94 , among many others. The force field models can be parametrized in different ways, e.g., MMFF94 uses a combination of both experimental and computational data from the Cambridge Structural Database (CSD) and from MP2/6-31G* calculations . Alternatives to the r -12 term in the Lennard-Jones styled part of the potential have been explored, for example by replacing it with an exp(-Cr) term (Buckingham potential) .