Intermolecular-potential-based field parameters

Other potential functions differ in the way the potential is partitioned into various contributions representing intra and intermolecular interactions. Cornell and collaborators 300 proposed an approach to determine the force field parameters based as much as possible on ab initio calculations. In this work each biomolecular system is divided into small residua, for which geometry optimization can be performed by an ab initio method. Ab initio calculations give partial atomic charges on atoms and the equilibrium geometry, i.e. the equilibrium values for the bond lengths, and planar and dihedral angles. 

The Sanchez-Lacombe model [48-50] is a lattice fluid model in which each component is divided into parts that are placed in a lattice. The different parts are allowed to interact with a mean-field intermolecular potential. By introducing an appropriate number of vacant sites (holes) in the lattice, the correct solution density can be obtained. SAFT [51-53] is based on the perturbation theory. The principle of perturbation theory is that first a model is derived for some idealized fluid with accurately known properties, called the reference fluid . Subsequently, the properties of this model are related to those of a real dense fluid. By expanding this reference fluid into power series over a specified parameter, the power terms can be regarded as corrections or "perturbations for the reference fluid as compared to reality. Obviously, the more the reference model approaches reality, the smaller the corrections are. Therefore, the key issue for applying perturbation theory is deriving the most suitable reference fluid. 

Based on DFT calculations on chlorophylls and, additionally, on ubiquinone and the RC main detergent, lauryl dimethylamine oxide or LDAO, we have then developed a force field for their classical modelization. Our approach to this undertaking was straightforward. We initially use the DFT optimized structures and the vibrational analysis to determine the bonded part of the potential parameters described by the AMBER potential function. Then, atomic ab initio partial charges on the chromophore are used to account for electrostatic effects. At a later stage, experimental data from X-ray crystallography are used to check the structural properties of the molecule in the condensed state and to refine the intermolecular Lennard-Jones parameters. 

In real systems, nonrandom mixing effects, potentially caused by local polymer architecture and interchain forces, can have profound consequences on how intermolecular attractive potentials influence miscibility. Such nonideal effects can lead to large corrections, of both excess entropic and enthalpic origin, to the mean-field Flory-Huggins theory. As discussed in Section IV, for flexible chain blends of prime experimental interest the excess entropic contribution seems very small. Thus, attractive interactions, or enthalpy of mixing effects, are expected to often play a dominant role in determining blend miscibility. In this section we examine these enthalpic effects within the context of thermodynamic pertubation theory for atomistic, semiflexible, and Gaussian thread models. In addition, the validity of a Hildebrand-like molecular solubility parameter approach based on pure component properties is examined. 

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