Lattice models force field parameters

Bjornsson and Biihl have proposed an approach to modeling moleeular crystals such that crystal effects on the equilibrium geometry of the moleeule can be captured. In this approach, the molecule of interest is embedded in the presence of point charges and Lennard Jones potentials representing its neighbors in the crystal lattice, which are updated in self-consistent fashion. The novelty here is that the medium effects no longer require a previously established set of force field parameters as these are determined at the same time and as needed in the process. These have been applied to explain the gas-to-solid bond contraction of HCN-BF3 and solid state V NMR chemical shifts in VOCI3 and a vanadium catechol complex. 

If generic properties of polymers need to be determined, it is often sufficient to rely on lattice models. For comparison with experiments of particular melts and blends, more sophisticated off-lattice models are typically applied. These models are described by force fields that determine the interactions between atoms or groups of atoms, and the quality of the modeling is essential for the predictive quality of the simulations. Force field parameters can be derived from direct comparison with experimental data, from quantum mechanical calculations, or both. In the first part of this section, we present generic polymer models that are commonly used in molecular simulations without focussing on any particular substance. Emphasis is placed on lattice and simple off-lattice models that will also be discussed in the next three sections. Section 1.5 is dedicated to chemically realistic descriptions. 

The simulations of paraffin crystals was also carried out with a full-atom model in which hydrogens are explicitly included, making the size of a crystal about 30,000 atoms [18]. As expected from the proper rotational potentials used in the united atom model, the gauche concentration and lattice parameter distribution are not significantly different when the H-atoms are included explicitly. The observed crystal structure was, surprisingly, as mentioned above, also not close to experimental equilibrium paraffin structure, an indication that further work is needed to find the subtle adjustment needed in the intermolecular force field that governs the stable crystal structure, or develop simulations of... 

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