Qualitative models, electronic structure calculations

There are (at least) two major opportunities for research by those interested in this topic. On the computational side, there is definite room for improvement in simulation methods. Right now none of the simulation approaches has the user friendliness that has brought electronic-structure calculation into the realm of routine applicability by nonspecialists. Nor has the field seen the development of the qualitative or semiquantitative models that did so much to make the results of molecular orbital calculations useful to organic chemists. On the experimental side, it will be obvious to the reader that the techniques for detecting the effects of nonstatistical dynamics are still very rudimentary and indirect. There is clearly room for creative scientists to come up with techniques whose results can give us more direct insight into these issues. 

Perhaps more valuable over time than the quantitative predictions of spectra, structural parameters, and relative enthalpies and entropies of RIs, which can be obtained from electronic structure calculations, are the qualitative models of the electronic structures and reactivities of RIs that emerge from the computational results. Any model, to be successful, must do two things. 

The CF picture gives a useful understanding of coordination chemistry and one that is widely used for qualitative discussions. A more sophisticated version of this approach, LF theory see Ligand Field Theory Spectra), which is really a conventional molecular orbital (or MO) picture, can be used for more accurate electronic structure calculations. In the CF model (Figure 5) we consider the s, the three p, and the five d orbitals of the valence shell of the isolated ion as well... 

It is important to emphasize that this comparison of molecular orbital calculations and experimental spectra is qualitative, as the limitation to a single molecule and the intrinsic precision of the computational approach used do not allow a quantitative comparison. Nevertheless, the characteristics of the orbitals, such as the electron density distribution, are very similar for different density functional and semiempirical calculations, indicating that the calculations are useful for a qualitative interpretation of the spectra. Detailed electronic structure calculations have been recently applied to the understanding of intermolecular magnetic interaction pathways in nitroxide radicals [43], underlining the importance of comparisons between computational data and spectroscopic results which provide a quantitative test for the theoretical models. 

Although a separation of electronic and nuclear motion provides an important simplification and appealing qualitative model for chemistry, the electronic Sclirodinger equation is still fomiidable. Efforts to solve it approximately and apply these solutions to the study of spectroscopy, stmcture and chemical reactions fonn the subject of what is usually called electronic structure theory or quantum chemistry. The starting point for most calculations and the foundation of molecular orbital theory is the independent-particle approximation. 

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