Potential Models for Framework Modeling

Owing to the complexity of zeolitic systems, most computational studies are still performed with the help of classical models. These methods use a set of potential functions to describe the potential energy surface (PES) in a manydimensional space of geometrical parameters of the system. Although the PES can be tested in terms of observables such as equilibrium atom positions, vibrational frequencies, heats of formation, and other experimental information, the PES itself is not an observable quantity. Because of that, there is no unique representation of the PES, and several coordinate systems and parameteriza- 

An early potential model used for zeolite framework modeling corresponds to a central force field model and was developed by Demontis et al. In this model the framework potential energy U is expanded in a Taylor series with respect to interatomic distances 

Another early potential assumes that the character of the bonds in zeolite lattices is largely ionic and in line with a force field used for simulations of ionic solids, the potential energy is written in the form 

Equation [15] is often referred as a rigid ion potential model. 

In the first sum, indices i and denote shells and cations, the second term runs over all point charges in the system, and the third term accounts for interactions of the shells with their cores. The shell model takes into account the polarization of the anions by the crystal field of the solid, which is an important feature. To better reproduce the characteristics of systems with partly covalent bonds, such as zeolites, Eqs. [15] and [16] are supplemented with a term 

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Modeling of dynamics of zeolite frameworks has two main goals. The first is to test whether a particular force field is suitable for description of the dynamic characteristics of the lattice as well as to simulate the zeolite structure itself. Examples of such calculations were mentioned in the section on Potential Models for Framework Modeling. The second and most challenging goal is to provide a qualitative (quantative would be even more desirable) interpretation of experimental data, in particular, data from spectroscopic experiments. Mod-... 

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