22. Can I model inorganic/organometallic compounds with HyperChem?

With HyperChem, one can model inorganic and organometallic compounds to some extent using molecular mechanics and the MM+ force field. In general, one may need to optimise the given parameters or to add new ones, since the force field was originally optimised for organic structures. MM+ can handle any element in the periodic table, and its default scheme for parameter generation can give at least a reasonable starting point for your work. One can use structural restraints to mimic the effects of pi coordination and the electronic interactions which affect geometries.

One can also model these compounds with the ZINDO/1 semi-empirical method -- it generally does a good job with second-row transition metals but often does poorly with first-row transition metals. With HyperChem release 4.5, organometallics can be modelled by ab initio methods -- though for most of these compounds, such calculations are pretty big calculations and can take quite a while to run. Keep in mind, too, that Hartree-Fock is a poor approximation for systems with small or non-existent HOMO-LUMO gaps -- a situation that arises more often in inorganic chemistry than in organic. If you are wondering about modelling a specific structure or series of structures, please ask us. If we do not know how well HyperChem can model your structure, we may be able to try it to find out.

The Crystal Builder module of ChemPlus can help one to construct a 3D lattice of an inorganic compound.

In general, there are not any good general-use structure-modelling tools for inorganic chemists. The basic problem is that the possible atomic combinations of organic-type elements are relatively limited, so it is possible to come up with a not-too-large set of general molecular mechanics parameters that will do a reasonable job of describing the systems. But for inorganic compounds, there are many more possible combinations of atom types; a general parameter set would have to be much larger. The computational problem of creating and optimising such a parameter set is a couple of orders of magnitude larger than the corresponding problem of generating parameters for organic compounds alone.

Also, standard molecular mechanics methods do not include the electronic effects that critically affect the geometries of inorganic compounds.

When it comes to quantum mechanics calculations, the organic-type elements are all at the low-atomic-number end of things, and the calculations are much simpler than for the high-end atoms. With the heavier elements, the simplifications and approximations that are necessary to do the calculations at all make them less accurate than they are for the light elements.

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