Progress in Quantum Chemical Methods

Even though computers were an essential tool in quantum chemical calculations, the main challenge was the further development of methods and concepts to describe even more facets of chemistry and with higher accuracy. Methods that account for electron correlation were extended to be able to describe energy surfaces more reliably. Several variants of the CEPA Ansatz (CEPA-1, CEPA-2) were developed as well as the method of self-consistent electron pairs (SCEP). Formulations using canonical or localized orbitals (e.g., pair natural orbitals, PNO, as a kind of optimized virtual orbitals) were put forth. These methods were extensively used for two decades, primarily in Germany, until coupled cluster formulations became more popular.  

In the middle of the 1970s, experimentalists realized that theoretical treatments had made great progress and renewed their interest in cooperation or in challenging the theoreticians. At the 1976 Theoretical Chemistry Symposium, for example, Christoph Schlier (Freiburg), an expert on molecular beam experiments, presented his talk on Scattering Collision Experiments—And What We Always Would Have Liked To Know About It from Theoretical Chemistry. Similarly, Peter Toennies (Gottingen) had approached theoretical chemists on this subject before. 

IBM in Germany organized a symposium on Computational Methods in Chemistry at Bad Neuenahr in 1979 with the preface According to Graham Richards the Third Age of Quantum Chemistry has started, where the results of quantum chemical calculation can guide the experimentalists in their search for the unknown. One of the examples chosen to underline this statement was the acetylene molecule. In 1970 Kammer had made qualitatively correct predictions for the first cis ( Bu and trans ( Bu A ) bent 

Work on clusters also had its origin in this decade. Initial work led in following years to the production of the well-known Stuttgart pseudopoten-tials, which enables the realistic calculation of systems containing heavier elements. It also prepared the route to many studies on magic numbers in cluster chemistry and eventually to fullerenes and nanotubes. 

International cooperation had become the rule in universities and research institutions. Computers became cheaper so that it became possible to purchase minicomputers such as VAX 11/780 (Digital Equipment Corporation), Perkin-Elmer 8/32, or Convex C220 for dedicated purposes. For a number of theoretical chemistry groups, this helped them to become independent of the long queue of users at their university central computer. In addition, access over a network to machines at a remote site became realistic, even if it was only via a 1200-baud special telephone line. For these reasons the 

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We should mention the recent progress in Au(I) and Au(III) catalyzed organic reactions [24, 297]. A number of groups of theoretical chemists are currently investigating the reaction mechanisms using quantum chemical methods [298-301]. Li and Mia published DFT calculations on AU5H5X hydrometal pentagons with Dsh planar pentacoordinate nonmetal centers (X = Si, Ge, P, S) [302]. The introduction of the nonmetal centers X introduces p aromaticity to MHX complexes. 

The use of zeolite clusters in quantum chemical calculations has now progressed to quite a sophisticated level. Elementary steps of reaction mechanisms can now be characterized and the results used to distinguish which steps are the most plausible. Computational power is such that clusters and methods can avoid obvious pitfalls (too small a cluster, basis set, etc.). Several key concepts that have arisen from theoretical studies are illustrated in the preceding discussion. These include the following carbo-cations exist as parts of transition state structures, rather than as stable intermediates, and their stabilization is controlled by the zeolite lattice. The transition states are very different from the ground states to either side of them, and each different reaction has been shown to proceed via a different transition state. 

Recent progress in computational hardware and the development of efficient algorithms have assisted the routine development of molecular quantum-mechanical calculations. New semiempirical methods calculate realistic quantum-chemical molecular quantities in a relatively short computational time frame. Quantum-chemical calculations are thus an attractive source for molecular descriptors that can express all of the electronic and geometric properties of molecules and their interactions. Quantum-chemical methods can be applied to QSARs by direct derivation of electronic descriptors from the molecular wave function. 

Despite the tremendous progress made in this field, there is still a severe drawback. The quantum chemistry developed by theoretical chemists tools are primarily suited for isolated molecules in vacuum or in a dilute gas, where intermolecular interactions are negligible. Another class of quantum codes that has been developed mainly by solid-state physicists is suitable for crystalline systems, taking advantage of the periodic boundary conditions. However, most industrially relevant chemical processes, and almost all of biochemistry do not happen in the gas phase or in crystals, but mainly in a liquid phase or sometimes in an amorphous solid phase, where the quantum chemical methods are not suitable. On the one hand, the weak intermolecular forces,... 

The enormous progress in accessible computational power over the past decade has allowed for increased application of high-level ab initio quantum-chemical methods to questions of structure and reactivity, and this trend has been reflected in studies on pyrans and derivatives. As is the case with many computational studies, there has been substantial effort directed toward the comparison of data obtained by various computational methods with empirical data. Table 1 provides a compilation of studies involving the applications of theoretical methods to pyrans and related molecules. 

Great progress has been made during the last decade in theoretical treatments of solvent effects by various quantum-chemical methods and computational strategies. When indicated, relevant references are given to the respective solution reactions or absorptions. However, a critical evaluation of all the theoretical models and methods used to calculate the differential solvation of educts, activated complexes, products, ground and excited states, is outside the expertise of the present author. Thus, a book on all kinds of theoretical calculations of solvent influences on chemical reactions and physical absorptions has still to be written by someone else. 

The periodic approach is not the only one available for atomistic simulations of these materials and we should first mention that much progress has been made in the application of molecular quantum chemical methods using cluster representations of the local structure of oxide materials [1, 2], More recently, this has given way to mixed quantum mechanics/molecular mechanics (QM/MM) calculations. In QM/MM simulations the important region, the active site for catalysis, is represented at a quantum chemical level while the influence of its environment, the extended solid, is represented using the computationally less-demanding atomistic force field approach. This allows complex structures such as metal particles supported on oxides to be tackled [3]. 

A scientific research group, concentrated mainly at Gainesville, Florida, carried out a detailed study on various quantum chemical methods, incorporating explicitly correlated Gaussian geminals into the basis set [75, 76, 77, 78, 79], The research in this direction is still in progress [80, 81, 82, 83, 84]. 

Reliable potential energy surfaces (PES s) are prerequisite to obtaining accurate rate constants for chemical reactions. With the significant progress made in the field of quantum chemical methods in recent years as alluded to in the introduction, scientists not only can reproduce experimental thermochemical data but also can make accurate predictions on rate constants for which experimental data are unknown or uncertain. The Gaussian-X (GX) (X = 1, 2, 3) series of methods and the most recent new family of G3 methods, referred to as G3X, developed Pople et al. [24] and... 

It was at about this time that our own research group became involved in this issue. We hoped to first address the question as to whether a CH- - -O interaction constituted a tme H-bond. In order to accomplish this task, we planned to make use of the quantum chemical methods that had become progressively more available to researchers in the years leading up to the end of the 20th century. Calculations had established... 

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