Parameterization universal force fields

The limitations of these methods should be understood. Their application requires the determination of the empirical parameters in eqns (5.2) and (5.3), as well as the partial atomic charges in the coulombic term. The former are usually parameterized from experimental solid state data such as vibrational frequencies or sublimation enthalpies, which in themselves contain some experimental uncertainties and variability from system to system. The partial atomic charges can and do vary with the choice of basis set for the calculations from which they are derived. The function chosen and the complete set of parameters are often collectively termed a force field . Ideally, one would like to develop a universal force field, but given the diversity... 

Universal. The parameters in the Universal force field (UFF) [14-16] are calculated using general rules based only upon the element, its hybridization, and its connectivity. For this reason, the UFF has broad applicability but is inherently less accurate than extensively parameterized force fields such as COMPASS. Bond-stretching terms in the UFF are either harmonic or Morse functions. The anglebending and torsion terms are described by a small cosine Fourier expansion. For nonbonded terms, the LJ 6-12 potential and Coulombic terms are used for steric and electrostatic terms, respectively. 

Table 2.3 gives a description of the functional form used in some of the common force fields. The torsional energy is written as a Fourier series, typically of order three, in all cases. Many of the force fields undergo developments, and the number of atom types increases as more and more systems become parameterized thus Table 2.3 may be considered as a snapshot of the situation when the data were collected. The universal type force fields, described in Section 2.3.3, are in principle capable of covering molecules composed of elements from the whole periodic table, these have been labelled as all elements . 

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