Universal Force Field UFF

In the following equations, all parameters have their usual meanings, unless otherwise noted  

This expression is used in all cases where atom i is connected to exactly three other atoms, /. k and l. Here, Kiju is the force constant, ( ijki is the angle between the i-l axis and the i-j-k plane and the expansion coefficients depend on the central atom type  

As in the case of DREIDING, the underlying philosophy of UFF does not allow for parameters to be developed for specific molecular systems (e.g. amino, nitro or nitroso). Rather, these are extracted from atomic parameters using a set of combination rales. Several calculations on amino and nitro compounds were performed as part of the validation of UFF58,59. In the following we refer to the structural results and save the discussion of the energetic ones to Section II.D. 

TABLE 21. Selected structural parameters for dimethylamine, trimethylamine, 1,3,5,9-tetraazacyclohexadecane (7), azetidine (3) and nittomethane as calculated by UFF and MM2 and obtained from experiment58,59 

Energetic Comparison Between MM2, MM3, AMBER, Tripos 5.2, DREIDING and UFF 

Those for dimethylamine and trimethylamine were calculated for this work with MM2-91 and those for nitromethane were taken from Reference 43. 

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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. 

Additionally, computational simulations of the enantioselection process using Rappe s universal force field (UFF) [43-45] were carried out which clearly indicated that the /R/-LSB complex complexes better as a pro-(R) adduct than as pro-(iS/ adduct (AE=4.9 kcal/mol) [41]. The proposed catalytic cycle is shown in Scheme 14. Thus, the basic LSB complex also acts as a Lewis acid, controlling the orientation of the carbonyl function and so activating the enone to attack. It ap-... 

All resulting e and r) parameters should be checked through their consistency with inorganic periodic principles. A universal force field (UFF) has been presented for atom types throughout the entire periodic table [62], and includes values for xj( = 2rf) and D ( = e ). The UFF values were derived from atomic properties, observed and calculated, with considerable assumption and uncertainty. Nevertheless there is general agreement between the UFF values for uncharged elements and other literature values, and these UFF... 

Atom-atom two- or three-body interactions of these sorts are incorporated within available forcefields that have been derived to fit empirical data. Commonly used forcefields include the universal force field (UFF) and the constant valence force field (CVFF). ... 

The parameters utilized for this approach are from Breck [57] for the gases He, H2, CO2, O2, N2 and CH4, Poling [58] for the gases CO, Ar, C2Huniversal force field (UFF) values [59] are used for the surface atoms, as summarized in Table 5.2. This potential difference has been termed the suction energy since a positive W translates to a suction force of the molecule from the outside to the inside of the pore, while a negative W translates to a repulsive force directing the molecule away from the pore [23]. From this, a new transport mechanism is proposed in [23] as suction diffusion , where enhanced velocities are predicted as the gas molecules are sucked into the pore. 

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. 

The alternative approach is the generic FF in which some or all the parameters are derived from atom-based properties. The DREIDING FF and the universal force field (UFF) are examples of this approach. They avoid the necessity of developing new parameters every time a new type of bond is encountered but can suffer from reduced accuracy relative to a specialist FF. The VALBOND FF takes elements from both approaches and uses a conventional para-metrization for all terms except the angle bend, where general but fitted atom-pair-based parameters are used. ... 

For example, the Universal Force Field (UFF), i which is a molecular mechanics force field, uses the sum of atom-type-specific, single-bond radii with some corrections for bond order and electronegativity as the reference bond length and derives the force constant for this bond based on a generalization of Badger s rules.Bond angle distortions are described by means of a cosine Fourier expansion ... 

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