Editing CFF91 force field

CFF91 is useful for hydrocarbons, proteins, protein-ligand interactions. For small models it can be used to predict: gas-phase geometries, vibrational frequencies, conformational energies, torsion barriers, crystal structures; for liquids: cohesive energy densities; for crystals: lattice parameters, rms atomic coordinates, sublimation energies; for macromolecules: protein crystal structures. It has been parameterized explicitly (based on quantum mechanics calculations and molecular simulations) for acetals, acids, alcohols, alkanes, alkenes, amides, amines, aromatics, esters, and ethers. CFF91 has parameters for functional groups that consist of H, Na, Ca, C, Si, N, P, O, S, F, Cl, Br, I, and/or Ar.