Force Fields for Solid-state Systems

Zeolites are materials generally composed of silicon, aluminium, oxygen and a metal cation or proton. They have a multitude of commercial uses including catalysis and separation (e.g. they are used in oil refining to separate linear and branched alkanes). Many of these important properties are a consequence of the presence within the zeolite of channels of molecular dimensions. It is therefore natural that molecular modelling techniques should be used to investigate the intrinsic properties of such materials and the way in which they interact with adsorbates. 

The size of many zeolite systems means that considerable computational resources may be required for the calculation. In some cases therefore, such as the study of adsorption 

The covalent approach is rarely appropriate for ionic and polar solids such as oxides and halides. The usual starting point for studying such systems is to write the potential as a series expansion of pairwise, three-body, etc., terms  

One of the oldest of such models is due to Bom [Born 1920], who restricted the series to pairwise terms, which were in turn divided into long-range Coulomb interactions and short-range repulsive forces. If an inverse power law is used for the repulsive term the potential energy is thus  

The Born model with integral or partial charges assumes that the ions have zero polaris-ability. This is reasonable for small cations such as Li or Mg + but can introduce significant errors for other systems. One pxropjerty that clearly demonstrates this is the high-frequency dielectric constant. At a suitably high frequency only the electrons can keep up with the external field and the dielectric constant is given by the Clausius-Mosotti relationship  

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