Generic force field

Equation (9.53) for the desired molecular field is nonlinear, typically solved iteratively. For this molecular-field approach to become practical, an alternative to this nonlinear iterative calculation is required. A natural idea is that a useful approximation to this molecular field might be extracted from simulations with available generic force fields. Then with a satisfactory molecular field in hand, the more ambitious quasichemical evaluation of the free energy can be addressed, presumably treating the actual binding interactions with chemical methods specifically. This is work currently in progress. 

A number of functions with multiple minima have been proposed to model the valence angles around metals. The molecular mechanics program DREIDING, based on a generic force field, uses a harmonic cosine function (Eq. 2.16)1321. 

The parameterization of an interaction is dependent not only on the atoms involved but also on their environment. With the exception of generic force fields, each chemically unique interaction type has to be parameterized independently. For both generic and conventional force fields, the atom types need to be specified in the input file. 

Metal-phosphine bonds can generally be modeled in much the same way as any other metal-heteroatom bond. The fact that phosphines participate in x-backbonding (filled dn (metal) -> empty d or a (phosphorus) interaction) is only of importance for generic force field parameterization schemes, and half-integer bond orders have been used to describe the effect of x-back-donation[ 153). In the usually adopted empirical force field formalism, x-bonding effects, like most of the other structural/elec-tronic effects, are accommodated by the general parameter-fitting procedure (see Parts I and III). 

Structure optimization of main group molecules with generic force fields has the same advantages and problems as molecular mechanics calculations of organic and organometallic compounds with similar approaches on one hand there is no need to fit a specialist force field, on the other hand is the expectation of lower accuracy1279,2801. The structural results are especially poor for molecules where electronic effects are important, e.g., those with hypervalent or dative bonds. 

Molecular mechanics, generic force field (for applications in inorganic chemistry... 

The zeolite framework was described by a specific force field developed by van Santen et al. [11] while the hydrocarbon molecules and their interaction among themselves and with the zeolite lattice were described by the generic force field Drdding n [12]. All the internal coordinates of the alkane molecules were allowed to fully relax. The nonbonded interactions (electrostatic and van der Waals) were computed for aU atoms within a cutoff-radius of 12A. Periodic boundary conditions were imposed along the three axes of the zeolite model to simulate an infinite crystal. 

For the molecules investigated, the MD and MC methods furnish similar adsorption energies although the MD results are slightly better when compared to the experiments. Since the MC and MD simulations have been performed in different ensembles but with the same force field, the difference in the results of the two simulations may be partly due to finite size effects. However, temperature fluctuations during the MD simulations in the NVE ensemble may also contribute to this difference. For the linear alkanes the MM results are qualitatively correct but only when a specific force field is used to describe the zeolite. However, for the branched alkane the MM results are comparable to the MD and MC ones even when a generic force field, such as Dreiding n, is used to represent the zeolite. 

S.L. Mayo, B.D. Olafson and W.A. Goddard III, DREIDING A Generic Force Field for Molecular Simulations. J. Phys. Chem., 94 (1990)8897. 

Mayo SL, Olafson BD, Goddard WA III. DREIDING A generic force field for molecular simulations. J Phys Chem 1990 94 8897-8909. 

Once a crystal structure has been obtained, the choice of force-field and partial atomic charges can significantly influence the quality of the morphology simulation. Depending on the molecular complexity of the system in question, a generic force field may be used (e.g., Clydesdale et al. 1996), or for molecular inorganic species, intermolecular force fields must initially be derived for the system in question, before any morphology simulation may be made (e.g., Telfer et al. 1998). 

These limits can be pushed back by the extension of existing force fields and the development of new ones the refinement of generic force fields (see Section 3.3) quantum-mechanically driven molecular mechanics (e.g., for transition states see Section 3.3) the development of tools that refine parameter sets based on data banks, including genetic algorithms and neural networks or more conventional techniques (see Section 3.3 and 16.3). 

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