Organic force fields

MMl, MM2, MM3, and MM4 are general-purpose organic force fields. There have been many variants of the original methods, particularly MM2. MMl is seldom used since the newer versions show measurable improvements. The MM3 method is probably one of the most accurate ways of modeling hydrocarbons. At the time of this book s publication, the MM4 method was still too new to allow any broad generalization about the results. However, the initial published results are encouraging. These are some of the most widely used force fields due to the accuracy of representation of organic molecules. MMX and MM+ are variations on MM2. These force fields use five to six valence terms, one of which is an electrostatic term and one to nine cross terms. 

MMn with n = 1-4 is the series of subsequently developed general-purpose organic force fields [189-191], All methods of the MMn family use the Buchingham potential for the van der Waals forces. The specific of MMn is occasional employing of the bond dipole based electrostatic energy contribution instead of the charge-based models (see below). 

For organic force fields, as well as their extensions to transition metal compounds [180], ab-initio calculations are more and more often used. The validity of the quantum-mechanical method for the classes of compounds in question must be validated by comparison to experimental data. Calculated data are used as consistent reference data, or where experimental data are not available [181,182], as well as in the tuning of hybrid QM/MM approaches [183, 184]. 

For the organic parts of the ligand unaffected by coordination, it is generally adequate to take force constants and undeformed angle values from appropriate organic force fields. For ligating and adjacent atoms, the effects of coordination need... 

When organic force fields are developed, it is common practice to assign only like center based potential parameters for nonbonded interactions. To generate parameters for heterogeneous interactions between species i and / (as opposed to homogeneous, i-i or /-/ interactions) combining rules can be specified to approximate the needed i-/ parameters. Thus, for example, Lorentz-Bertholet mixing rules yield Lennard-Jones a and e parameters as follows ... 

Also frequently employed for organic force fields are geometric mean combining rules,4 as well as more sophisticated mixing schemesA° >i3i... 

First, most organic force fields are molecular mechanics force fields, which describe molecules in terms of bonds, bond angles, torsions, and the cross-terms between them. In addition they commonly include a van der Waals... 

Atoms with unusual hybridizations can be particularly difficult to include. Most organic force fields describe atoms with hybridizations whose bond angles are all equivalent (i.e., sp, sp, and sp hybridizations with bond angles of 180, 120, and 109.5°, respectively). In contrast to this, a square planar atom will have some bond angles of 90° and some angles of 180°. In this case, it may be necessary to define the bond and angle terms manually, modify the software, or hold the bond angles fixed in the calculation. 

---