Deformation energy nonbonded interactions

The first three terms describe potential energy variations due to bond b), valence angle (0), and bond torsion angle (j ) deformations. The remaining (nonbonding) terms are Lennard-Jones and Coulomb interactions between interaction sites separated by a distance ry = r, — i j. Nonbonded interactions usually exclude pairs of sites belonging to the same bond or valence... 

An alternative approach to modeling the L-M-L angles is to set the force constants to zero and include nonbonded 1,3-interactions between the ligand atoms. In most force fields, 1,3-interactions are not explicitly included for any atoms, instead they are taken up in the force constants for the valence angle terms. This is an approximation because the 1,3-interactions are most often repulsive and thus the function used to calculate the strain energy arising from valence angle deformation should be asymmetric. It was shown that the nonbonded 1,3-interactions around the metal atom are in many cases a major determinant of the coordination... 

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