Intermolecular-potential-based field

In a classical simulation a force-field has to be provided. Experience with molecular liquids shows that surprisingly good results can be obtained with intermolecular potentials based on site-site short-range interactions and a number of charged sites... 

Jorgensen et al. has developed a series of united atom intermolecular potential functions based on multiple Monte Carlo simulations of small molecules [10-23]. Careful optimisation of these functions has been possible by fitting to the thermodynamic properties of the materials studied. Combining these OPLS functions (Optimised Potentials for Liquid Simulation) with the AMBER intramolecular force field provides a powerful united-atom force field [24] which has been used in bulk simulations of liquid crystals [25-27],... 

Among other approaches, a theory for intermolecular interactions in dilute block copolymer solutions was presented by Kimura and Kurata (1981). They considered the association of diblock and triblock copolymers in solvents of varying quality. The second and third virial coefficients were determined using a mean field potential based on the segmental distribution function for a polymer chain in solution. A model for micellization of block copolymers in solution, based on the thermodynamics of associating multicomponent mixtures, was presented by Gao and Eisenberg (1993). The polydispersity of the block copolymer and its influence on micellization was a particular focus of this work. For block copolymers below the cmc, a collapsed spherical conformation was assumed. Interactions of the collapsed spheres were then described by the Hamaker equation, with an interaction energy proportional to the radius of the spheres. 

Once the mathematical models (i.e., force fields) for the internal structure of each molecule (i.e., the intramolecular potential) and the interaction between molecules (i.e., the intermolecular potential) are known through classical statistical mechanics, one can predict the properties of a macroscopic sample of such molecules based on statistical averaging over the possible microscopic states of the system as it evolves under the rules of classical mechanics (Chandler 1987 McQuarrie 1976 Wilde and Singh 1998). Two of the most common techniques used are molecular dynamics (MD) (Alder and Wainwright 1957, 1959 McCammon et al. 1977 Rahman 1964 Stillinger and Rahman 1974) and Monte Carlo (MC) (Milik and Skolnick 1993 Ojeda et al. 2009) simulations. 

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. 

The Kelvin-equation-based methods were found to perform reasonably well for macro-porus and some mesoporous materials. However, it was foimd that the classical approach does not hold trae for micropores, in which case the intermolecular attractive forces between the sorbate and sorbent molecules predominate over bulk fluid forces such as surface tension. The potential energy fields of neighboring sorbent surfaces are known to overlap when the pores are only a few molecular dimensions wide. This results in a substantial increase in the interaction energy of an adsorbed molecule [12], which is not accounted for by simple classical thermodynamic models such as the Kelvin equation. 

Simple potentials based on equation (2) have been used to describe the interaction of adsorbed hydrocarbon molecules with zeolites see, for example, Kiselev et al. For all-silica zeolites the electrostatic interaction is frequently neglected in such simulations. Further simplification is achieved when the two-body terms are calculated for whole CH groups instead of for individual atoms. Many of these applications assume that both the zeolite and the molecules are rigid. Some allow at least for torsions about C-C bonds in saturated hydrocarbons. More advanced approaches combine the intermolecular potentials with some type of force fields for the adsorbed organic molecules and/or for the zeolite framework. ... 

The vast majority of CSP has been limited to using intermolecular potentials that lack explicit inclusion of polarization,although its importance has become a topic of interest.Nonpolarizable force fields, based on fixed partial charges or fixed atomic multipoles, must implicitly account for the 20-40% of the lattice energy attributable induction. " On the contrary, polarizable models such as the AMOEBA force field for organic molecules... 

The resonance-mediated coupling mechanisms described above involve subtle quantal intramolecular/intermolecular donor-acceptor effects that tend to be inadequately described by current-generation empirical potentials. Simulations based on these potentials are therefore likely to be inherently defective for describing realistic folding processes in proteins. However, approximations such as those illustrated in Example 5.8 may ultimately make it feasible to incorporate additional resonance-mediated effects into empirical force fields of tractable form. 

---