Ligand nonbonded interaction

Recent SCF—XaSW calculations indicate that the above geometry is stabilized by attractive ligand-ligand nonbonded interactions ... 

MOMEC is a force field for describing transition metal coordination compounds. It was originally parameterized to use four valence terms, but not an electrostatic term. The metal-ligand interactions consist of a bond-stretch term only. The coordination sphere is maintained by nonbond interactions between ligands. MOMEC generally works reasonably well for octahedrally coordinated compounds. 

The success of the ligand-ligand repulsion model prompted its adoption as an element of a molecular mechanics program. In the resulting approach the valence angles around the metal ion are modeled solely by nonbonded interactions, using the usual van der Waals potential (for example, Eq. 2.9 kg = 0 in Eq. 2.7 Urey-Bradley approach)136. 6 Again, the fact that the electronic effects responsible for the directionality of bonds are not explicitly modeled here may seem questionable but extensive tests have shown the model to be reliable 371. An explanation for this apparent contra-... 

Table 2.5. The influence of 1,3-nonbonded interactions on the ideal metal-ligand bond distance of chro-mium(III), cobalt(III), and nickel(II) hexaamines.

The s and f block elements present a particular challenge in the molecular mechanics field because the metal-ligand interactions in both cases are principally electrostatic. Thus, the most appropriate way to model the M-L bonds is with a combination of electrostatic and van der Waals nonbonded interactions. Indeed, most reported studies of modeling alkali metal, alkaline earth metal and rare earth complexes have used such an approach. 

For MM+ (energy calculations of small biomolecules or ligands) Choose either Bond dipoles or Atomic charges (assigned via Builds Set charge) for use in the calculations of nonbounded Electrostatic interactions. Select None (calculate all nonbonded interactions recommended for small molecules), Switched or Shifted for Cutoffs (for large molecules). 

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