CompuServe Messages

Stan, The MM+ force field, which as you say is the one to use for most non-biomolecules, is closely the MM2(1977) force field of N. L. Allinger. Probably the biggest single weakness of this force field (as well as all others in which bond characteristics are assigned solely on the basis of the atom types involved in that bond, which includes many different force fields), is just the one you point out. Bond lengths are assigned on the basis of atom types, and consequently you get an inaccurate treatment of conjugated (non-aromatic) bonds. There are some ways around this in the development of force fields, such as adding bond order to the information used to define bond characteristics, or add some non-molecular-mechanics feature to the algorithm, such as a quantum-mechanical description of the pi electrons. We are currently looking at alternative ways of improving this force field compatible with our intent of using only published, public-domain science for reasons of reproducibility. Depending on your application, I can suggest a couple of things: 1). The model builder gives pretty good structures for these systems. If all you are interested in is structure (not energetics) then selecting and buildig the conjugated part would give you reasonable bond lengths. 2). The semi-empirical methods do, as you say, give good structures and energetics for conjugated systems, as well as most others. They take longer, of course, but for smallish systems may provide what you want. The stereochemistry of conjugated systems is pretty well-defined, so that inaccuracies in these bond lengths are very unlikely to have global effects on the structure of your molecule, of course. On your other point, the support mechanism for HyperChem is the same as for most other Autodesk products: the dealers (value-added resellers) who sell the product have responsibility for support. The dealers in turn get support from Hypercube. Hope this helps, Tom Slee, Hypercube.