High-spin molecules theoretical approaches

All molecules will show an MCD spectrum. The spectrum of many types of molecule can now be simulated, often through more than one theoretical approach. Several areas need further study however. It would be useful if all types of MCD spectrum could be calculated at a range of levels of theory so that a scientist interested in predicting a spectrum would be able to choose an approach that suitably fits their needs. More benchmark calculations are needed to improve our understanding of the limitations of the present methods. These calculations would be aided by the availability of more high-quality spectra of small molecules that would be suitable test cases. While the MCD spectra of many molecules can now be calculated, one area that probably still cannot be studied with confidence is paramagnetic molecules that include very heavy elements. In this case spin-orbit effects will be very large and perturbational treatments like those applied thus far are unlikely to provide accurate results. 

Clearly, the treatment of a solid involving of the order of 10 3 electrons is even a more complicated matter than that of an isolated molecule or complex in spite of the simplifications introduced by symmetry, and the use of effective potentials, and thus of a band theoretical approach, is probably not adequate in the discussion of wave function sensitive parameters such as spin distributions. But many important properties of solids reflect the electronic energy levels, rather than the finer details of the electronic distributions, and in spite of the fact that band calculations are rarely carried through to self consistency, band structures and energies of simple compounds may be determined sufficiently well to provide a good comparison with experimental data. The main effort has been directed to metals, where the valence electrons are weakly bound, and to simple compounds of high symmetry with the sodium chloride or diamond-like structure. In the latter case this effort also reflects the importance of these compounds in solid state physics and electronics and the elucidation of the band structure was essential for an understanding of many of the important properties of these materials. 

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