Cross Terms Class 1,2 and 3 Force Fields

Coupling between the stretdiing of the bonds as an angle closes. 

The stretching of the two bonds adjoining an angle could be modelled using an equation of the following form (as in MM2, MM3 and MM4)  

In a Urey-Bradley force field, angle bending is achieved using 1,3 non-bonded interactions rather than an explicit angle-bending potential. The stretch-bond term in such a force field would be modelled by a harmonic function of the distance between the 1,3 atoms- 

A stretch-torsion cross term can be used to model the stretching of a bond that occurs in an eclipsed conformation. Two possible functional forms are  

It has been suggested that the presence of cross terms (together with some other features) can provide a general way to classify force fields [Hwang et at. 1994], A class I force field was considered one which is restricted to harmonic terms (e.g. for bond stretching and 

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