Universal force fields

The combination of many atom types and the lack of a sufficient number of reference data have have prompted the development of force fields with reduced parameters sets, such as the Universal Force Field (UFF).46 The idea is to derive di-, tri- and tetra-atomic parameters (Estr, Ebend, Etors) from atomic constants (such as atom radii, ionization potentials, electronegativities, polarizabilities, etc.). Such force fields are in principle capable of describing molecules composed of elements from the whole periodic table, and these have been labelled as "all elements" in Table 2.4 below. They give less accurate results compared with conventional force fields, but geometries are often calculated qualitatively correctly. Relative energies, however, are much more difficult to obtain accurately, and conformational energies for organic molecules are generally quite poor. Another approach is to use simple valence bonding arguments (e.g. hybridization) to derive the functional form for the force field, as employed in the VALBOND approach.47

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