Quantum chemically derived potentials

It is instructive to compare the performance of empirically derived force fields with those potentials derived from quantum mechanical calculations. Sierka and Sauer <> compared results from Jackson and Catlow s empirical shell model potential with Schroder and Sauer s Hartree-Fock based and their own density functional based shell model potentials. The mean deviation between computed and observed unit cell parameters was found to be 0.7%, 1.9%, and 1.4%, respectively, for the three potentials. This means that the empirical shell model potential is twice as accurate as the best quantum chemically derived force field for unit cell predictions. However, the calculated vibrational spectra of silicalite are in good agreement with experiment for both of the quantum chemically derived potentials,whereas agreement is not as satisfactory for the empirical force field. ... 

Papers dealing with this topic are exhaustively reviewed in Comprehensive Heterocyclic Chemistry I (84CHEC-I(6)235) and II (96CHEC-II(3)373). Nevertheless, little information is available on the 5-oxides. Recently, the heteroaromaticity of thiazole compared with isothiazole and thiadiazole 5,5-dioxide systems was studied (97MI1). Quantum-chemical calculations and X-ray studies were performed on 3,3 -di[l,3-thiazolidin-4-one] derivatives (95JCC(25)589) studied for their potential biological activity (97FA(52)43). 

Feng and others (1986) have performed quantum chemical calculations of aromatic nitration. The results they obtained were in good accord with the ionization potentials of N02 and benzene and its derivatives. The radical pair recombination mechanism is favored for nitration whenever the ionization potential of the aromatic is much less than that of N02. According to the calculations, nitration of toluene and xylene with NO) most probably proceeds according to the ion radical mechanism. Nitration of nitrobenzene and other benzene derivatives with electroacceptor substituents can proceed through the classical polar mechanism only. As for benzene itself, both mechanisms (ion radical and polar) are possible. 

A molecule contains a nuclear distribution and an electronic distribution there is nothing else in a molecule. The nuclear arrangement is fully reflected in the electronic density distribution, consequently, the electronic density and its changes are sufficient to derive all information on all molecular properties. Molecular bodies are the fuzzy bodies of electronic charge density distributions consequently, the shape and shape changes of these fuzzy bodies potentially describe all molecular properties. Modern computational methods of quantum chemistry provide practical means to describe molecular electron distributions, and sufficiently accurate quantum chemical representations of the fuzzy molecular bodies are of importance for many reasons. A detailed analysis and understanding of "static" molecular properties such as "equilibrium" structure, and the more important dynamic properties such as vibrations, conformational changes and chemical reactions are hardly possible without a description of the molecule itself that implies a description of molecular bodies. 

Quantum chemical calculations provide the values of a multi-dimensional potential at the mesh points of a grid. Several coordinates are varied step by step and to each set of all these coordinates there is a corresponding number. However, we need a potential energy function which is analytic if possible, continuous and differentiable in any order. At the mesh points of the grid the values of this function are to be as close as possible to the computed ones. In addition, the values of the derivatives of this function with respect to any coordinate must be physically possible on the limiting contour surrounding the region of the potential surface studied. This is very im-... 

---