Pure central force field model

Computer programs for empirical force-field calculations that use other concepts have been tested, and some of these will be discussed elsewhere in this book. Among these approaches are one based on a pure central force-field model, used for simple organic compounds [208], an equipotential surface force-field model, used for carbonyl cluster complexes [99,103], and a program that includes ligand field terms, developed for transition metal complexes in the LFMM model (ligand field molecular mechanics) [21, 105, 108]. 

Careful determination of the internal state distributions of one of the photoproducts can lead to an understanding of the nature of the forces that act upon the molecule as it breaks apart. One of the nicest examples of this kind of work is water photolysis through the A state. Photolysis via the A state produces little rotational excitation as has been observed by LIF of the OH product [19,20], Ab initio studies of the A state have shown that it is purely repulsive, that is, a direct dissociation similar to methyl iodide in Figure 1(a). There is calculated to be little or no variation in the PES as a function of the HOH bending angle [21]. This result means that the OH recoils from the dissociation as if from a central force field and little angular momentum is imparted. This example of photodissociation is an example that nearly perfectly follows the simple one-dimensional model. 

Adoption of Central Field Model at Equilibrium.—To make progress, we now adopt the central field model, for which, for neutral molecules, we saw that the chemical potential n was zero. Secondly, we established the existence of an equilibrium bond length at Re, given by equation (89), where the virial theorem for equilibrium under purely Coulomb forces takes the usual form... 

Proper parametrization of proteins requires the selection of appropriate model compounds for which adequate target data exist. As the peptide backbone C, O, N, H and C atoms are common to all amino acids selection of the appropriate model compounds for optimization of the peptide backbone parameters is central to the success of any protein force field. The most often used model compounds are NMA and ALAD, shown in Figure 1. Both structures contain the peptide bond capped by methyl groups. Earlier studies often employed formamide or acetamide as model compounds however, the free amino or aldehyde groups make them poor models for the peptide bond in proteins. Data available on NMA range from structural and vibrational data in both the gas and conden.sed pha.ses to crystal structures, pure solvent properties and heats... 

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