United Atom Force Fields and Reduced Representations

15 United Atom Force Fields and Reduced Representations 

In a united atom force field the van der Waals centre of the united atom is usually associated with the position of the heavy (i.e. non-hydrogen) atom. Thus, for a united CH3 or CH2 group the van der Waals centre would be located at the carbon atom. It would be more accurate to associate the van der Waals centre with a position that was offset slightly from the carbon position, in order to reflect the presence of the hydrogen atoms. Toxvaerd has developed such a model that gives superior performance for alkanes than do the simple united atom models, particularly for simulations at high pressures [Toxvaerd 1990]. In 

In some force field models, even simpler representations are used than the united atom approach, with entire groups of atoms being modelled as single interaction points. For example, a benzene ring might be modelled as a single site with appropriately chosen parameters. 

45 The Gay-Beme modelpr liquid crystal systems and some typical arrangements. 

X is the shape anisotropy parameter, it is zero for spherical particles and is equal to 1 for infinitely long rods and -1 for infinitely thin discs erg is typically set equal to ag. 

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