Electronic-structure theory failure

The failure is not caused by the assumptions of electronic structure theory but originates from the use of EH approximations. In this case, it is important to analyze the source of the failure from the viewpoint of the atomic parameters employed, modify the parameters appropriately, and repeat the calculations. This task is not difficult, if one becomes familiar with the concepts of perturbation and orbital interaction. In spirit, this process is not different from what one does with first-principles calculations. For example, when a chosen basis set or correlation level does not give correct results, one tries another basis set or correlation level. First-principles methods are based on rigorous theoretical and mathematical formulations, but their actual calculations do include approximations. 

Hartree-Fock (HF), molecular orbital theory satisfies most of the criteria, but qualitative failures and quantitative discrepancies with experiment often render it useless. Methods that systematically account for electron correlation, employed in pursuit of more accurate predictions, often lack a consistent, interpretive apparatus. Among these methods, electron propagator theory [1] is distinguished by its retention of many conceptual advantages that facilitate interpretation of molecular structure and spectra [2, 3, 4, 5, 6, 7, 8, 9]. 

Valence bond theory, in the terms defined by Pauling, is not able to account for the 4n+2 rule, and the properties of cyclobutadiene and cyclooctatetraene. It has been suggested that the problem with these molecules is the strain associated with the bond angles in the planar structures.10 However, this was shown to be incorrect by the observation that the addition of two electrons to cyclooctatetraene leads to the planar dianion. It is only recently that it has been recognized that cyclic permutations must be included in order to properly treat cyclic systems via valence bond theory.11 One of Pauling s few failures in structural theory is his nonrecognition of the problems associated with the 4n molecules. 

The failure of the free-electron model to reproduce even pronounced effects in ER is demonstrated in Figure 24. Here, the ER spectra for Cu(lll) on mica are shown for 5- and / -polarization at 45° angle of incidence (solid curves). The pronounced peaks in the experimental spectra at 2.2 eV, at the onset of the interband transition in Cu, cannot be reproduced by the free-electron model alone (dashed curves). A satisfactory agreement between theory and experiment—at least for the spectral shape—was, however, achieved when a shift of the interband transition energy with electrode potential of -0.2 eV/V was included in the calculation. Although the absolute value of this potential-induced band structure change in Cu may be somewhat in question, the result certainly indicates that the electrochemical double layer can influence the electronic structure of metal surfaces. 

Let us assume the availability of a useful body of quantitative data for rates of decay of excited states to give new species. How do we generalize this information in terms of chemical structure so as to gain some predictive insight For reasons explained earlier, I prefer to look to the theory of radiationless transitions, rather than to the theory of thermal rate processes, for inspiration. Radiationless decay has been discussed recently by a number of authors.16-22 In this volume, Jortner, Rice, and Hochstrasser 23 have presented a detailed theoretical analysis of the problem, with special attention to the consequences of the failure of the Born-Oppenheimer approximation. They arrive at a number of conclusions with which I concur. Perhaps the most important is, "... the theory of photochemical processes outlined is at a preliminary stage of development. Extension of that theory should be of both conceptual and practical value. The term electronic relaxation has been applied to the process of radiationless decay. 

This idea is a failure of simple resonance theory, not of VB theory. Taking into account the sign of the matrix element (overlap) between the five VB structures shows that singlet CsH5+ is Jahn—Teller unstable, and the ground state is in fact the triplet state. As shown later in Chapter 5, this is generally the case for all of the antiaromatic ionic species having 4n electrons over 4n + 1 or 4n - 1 centers (61). 

The molecular concept has become so central in chemistry that understanding of chemical events is commonly assumed to consist of relating experimental observations to micro events at the molecular level, which means changes in molecular structure. In this sense molecular structure is a fundamental theoretical concept in chemistry. As the micro changes are invariably triggered by electron transfer, the correct theory at the molecular level must be quantum mechanics. It is therefore surprising that a quantum theory of molecular structure has never developed. This failure stems from the fact that physics and chemistry operate at different levels and that grafting the models of physics onto chemistry produces an incomplete picture. 

An experimental study of diacetylene and HF in solid argon suggested both sorts of complexes (a and b in Fig. 6.6) were present and that they are of comparable stability. Correlated (MP2) calculations with a 6-31 -I- -l-G(d,p) basis set found the perpendicular complex (a in Fig. 6.6), wherein FH approaches one of the two triple bonds of diacetylene, is more stable than is complex b wherein C—H acts as proton donor. The electronic contributions to the binding energies of complexes a and b are calculated to be —3.8 and —2.6 kcal/rnol, respectively. However, these values are surely inflated by the failure to correct them for BSSE. One can conclude that the triple bond is a better proton acceptor than the alkynic C—H is a donor, at least when paired with the rather strong acid HF. The preference for this sort of geometry is confirmed by gas-phase measurements, and are valid also when HF is replaced by HCl . The importance of using a satisfactory level of theory for such complexes is reinforced by comparison with earlier SCF-level calculations which predicted a structure like b to be most stable. 

---