Original Parameterization

Understanding how the force field was originally parameterized will aid in knowing how to create new parameters consistent with that force field. The original parameterization of a force field is, in essence, a massive curve fit of many parameters from different compounds in order to obtain the lowest standard deviation between computed and experimental results for the entire set of molecules. In some simple cases, this is done by using the average of the values from the experimental results. More often, this is a very complex iterative process. 

The first step in creating a force field is to decide which energy terms will be used. This determines, to some extent, the ability of the force field to predict various types of chemistry. This also determines how dilficult the parameterization will be. For example, more information is needed to parameterize an-harmonic bond-stretching terms than to parameterize harmonic terms. 

The parameters in the original parameterization are adjusted in order to reproduce the correct results. These results are generally molecular geometries and energy differences. They may be obtained from various types of experimental results or ah initio calculations. The sources of these correct results can also be a source of error. Ah initio results are only correct to some degree of accuracy. Likewise, crystal structures are influenced by crystal-packing forces. 

---

MOMEC is a force field for describing transition metal coordination compounds. It was originally parameterized to use four valence terms, but not an electrostatic term. The metal-ligand interactions consist of a bond-stretch term only. The coordination sphere is maintained by nonbond interactions between ligands. MOMEC generally works reasonably well for octahedrally coordinated compounds. 

The off diagonal tenn does not vanish here as it does in the original parameterization.)... 

Although ZINDO was originally parameterized to give good spectroscopic results, it had also been used in studies of the energetics and structures of transition metal-based catalytic systems (a) GL Estiu, MC Zerner. J Phys Chem 97 13720-13729, 1993. (b) GL Estiu, MC Zemer. Int J Quantum Chem 26 587, 1992. 

The most rigorous dielectric continuum methods employ numerical solutions to the Poisson-Boltzmann equation [55]. As these methods are computationally quite expensive, in particular in connection with calculations of derivatives, much work has been concentrated on the development of computationally less expensive approximate continuum models of sufficient accuracy. One of the most widely used of these is the Generalized Born Solvent Accessible Surface Area (GB/SA) model developed by Still and coworkers [56,57]. The model is implemented in the MacroModel program [17,28] and parameterized for water and chloroform. It may be used in conjunction with the force fields available in MacroModel, e.g., AMBER, MM2, MM3, MMFF, OPTS. It should be noted that the original parameterization of the GB/SA model is based on the OPLS force field. 

Because many effective pairwise additive potentials do not include the self-energy of the dipoles in their original parameterizations, their reported enthalpy values require corrections to make a proper comparison with experi-... 

The early papers " on the effect of polarization on water-water interactions most often used existing water potentials suitably modified to permit the use of a grafted polarization correction. It is most desirable to employ polarizable potentials in the original parameterization and construction of the model. 

The original parameterization was done using a rigid (R) or flexible (F) model. 

For many materials, the simple spectral model gives reasonable estimates of the Hamaker constant when the microwave term is omitted and a single oscillator term is used. An example for a material with a more complex spectral response is water. The original parameterization of Ninham with a microwave term and just one oscillator term for IR and UVeach [23] was improved in several steps to a model including a relaxation term, five IR, and six UV oscillator terms [26, 27]. Two alternative models closer to a full spectral analysis for water have recently been proposed by Dagastine et al. [28] and Fernandez-Varea and Garcia-Molina [29]. 

---