Universal Force Fields

The Universal Force Field, UFF, is one of the so-called whole periodic table force fields. It was developed by A. Rappe, W Goddard III, and others. It is a set of simple functional forms and parameters used to model the structure, movement, and interaction of molecules containing any combination of elements in the periodic table. The parameters are defined empirically or by combining atomic parameters based on certain rules. Force constants and geometry parameters depend on hybridization considerations rather than individual values for every combination of atoms in a bond, angle, or dihedral. The equilibrium bond lengths were derived from a combination of atomic radii. The parameters [22, 23], including metal ions [24], were published in several papers. 

Rappe A K, K S Colwell and C J Casewit 1993. Application of a Universal Force Field to Metal Complexes. Inorganic Chemistry 32 3438-3450. 

UFF (universal force field) a molecular mechanics force field unrestricted (spin unrestricted) calculation in which particles of different spins are described by different spatial functions VTST (variational transition state theory) method for predicting rate constants... 

From this discussion, the limitations of the force field should have become clear. There is no such thing as a universal force field which describes every system in every condition. The force field is a function with few adjustable parameters and can, therefore, not be expected to reproduce all properties of all chemical species under all circumstances. This means, for example, that an OH group in an aliphatic alcohol will have to be treated differently from a phenolic OH or from the OH of a carboxylic acid group. Similarly, the density and temperature window of a force field is often limited [22]. 

MM, UFF, molecular mechanics, universal force field. Ref. (14). Charge transfer reactive force field. Ref. (92). 

By design, no conformational engine was implemented in DISCO, based on the assumption that at the time, no universal force fields and methods suitable for all types of compounds were available [53]. However, the commercial distributor Tripos provides access to 3D converters and conformational search engines such as Concord and Confort via the Sybyl interface. These algorithms will not be reviewed here as strictly seen they are not part of any pharmacophore identification program. The distance geometry approach has been used... 

The EHM will require far fewer parameters. This is easy to see, because each atom requires just one parameter for each valence atomic orbital. For C, for example, we need an ionization energy for the 2s, and one for the 2p orbitals, just two parameters (strictly, valence state ionization energies, VSIEs - see Harder Question 9).3 Each H needs only one parameter, for its Is orbital. So for an EHM program that will handle hydrocarbons in general we need only three parameters (as in Hoffmann s pioneering paper on hydrocarbons [1]). In contrast, an early but viable molecular mechanics forcefield limited to alkanes had 26 parameters [2]. The Universal Force Field, which sacrifices accuracy for wide applicability, has about 800 parameters, and the accurate and quite broadly applicable Merck Molecular Force Field 1994 (MMFF94) has about 9,000 parameters [3]. 

At the heart of molecular mechanics is a force field [89] and the ultimate force field should be fully transferable between all types of molecule. However, progress towards comprehensive force fields, such as the Universal Force Field of Casewit and Rappe [90], is invariably accompanied by an almost exponential increase in the number of parameters. The effort to reformulate molecular mechanics in terms of valence-bond concepts [91] reduces the number of formal parameters, but at the expense of almost the same number of hybridization parameters, which the authors [92] describe as follows ... 

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... 

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