Buenker and Sigrid D.
Peyerimhoff and by Jerry L. Whitten , different ways of configuration selection discussed by Isaiah Shavitt and by Buenker and Peyerimhoff , and the choice of orbitals for CI, that is, natural orbitals Charles F. Ruedenberg presented advantages of the even-tempered orbital basis. Enrico Clementi showed that adjoined basis sets, used to evaluate less important integrals and matrix elements, could reduce the computation time for primitive Gaussians from 4 h to 35 min without loss of accuracy in the results.
The mixing of Rydberg states with valence states was discussed from experimental Camille Sandorfy and theoretical by Robert S. The role of negative ions as interstellar molecules Jurgen Barsuhn and responsible for Feshbach resonances Lefebvre-Brion was discussed, and suggestions were given for how to compute nonadiabatic couplings in predissociation processes due to avoided crossing of states Jean-Claude Lorquet.
Studies using equation of motion EOM methods for excited states and data on photoionization cross sections based on a discrete orbital basis Vincent McKoy were shown. Even first results for transition metal compounds Alain Veillard were presented.
This meeting was remembered by many of the participants for its scientific content, but also because of the beautiful site of this castle, which serves as the guest house of the University of Ulm. Scientist Awardee of the Alexander von Humboldt Stiftung. Trained as an experimental organic chemist, he had become aware of the great potential of computational quantum chemistry for the study of new chemical compounds and reactions.
Coming from the same background as other organic chemists, he spoke their language and - after a number of years 76 - was able to convince them of the practicability of this new theoretical tool. In particular, his work on organolithium compounds 77 and carbonium ions, carbanions, and reactive intermediates was essential in this respect. His work certainly had a great impact on the acceptance of computational chemistry within the community of German experimental organic chemists.
IBM in Germany organized a symposium on "Computational Methods in Chemistry" 78 at Bad Neuenahr in with the preface: "According to Graham Richards 79 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 Kammer 80 had made qualitatively correct predictions for the first cis 3 B 2 , 3 A 2 and trans 3 B u , 3 A u bent electronically excited states of this molecule. In Demoulin 81 had calculated the corresponding potential energy curves, and in Wetmore and SSchaefer 82 reliably determined the geometry of C 2 H 2 in these states. With the help of this guidance, Wendt, Hunziker, and Hippler 83 took up the search and succeeded in finding the theoretically predicted near infrared IR absorption for the cis conformer.
The measured spectrum confirmed all theoretical predictions quantitatively. This symposium showed very convincingly how theoretical methods had taken up a large variety of different problems and had influenced experimental studies. At the symposium, W. Meyer with co-workers P. Botschwina, P. Rosmus, and H. Werner reported on their work on molecular properties. This group discussed results on spectroscopic data R e , omega e , omega e kappa e , alpha e , ionization energies, electron and proton affinities, dipole moment functions, and static dipole polarizabilities, and even showed results on polarizability anisotropies.
Domcke, and J. Schirmer showed how the Greens function approach, 84,85 which computes energy differences directly, can be used to analyze vibrational structure and vibronic coupling effects in photoelectron spectra PES. They also discussed complications in inner-shell ionization spectra due to the breakdown of the one-particle picture. Inner-shell phenomena were also discussed by MR-Cl methods Buenker and Peyerimhoff , and it was shown that such methods can reliably predict details of molecular spectra in small polyatomic molecules including vibrational features and intensities.
The use of computer chemistry for the study of organic reactions in particular the Wolff rearrangement which involves isomerization of alpha-carbonyl carbenes into ketenes presented by the IBM crew was an excellent example of how quantum chemistry had joined experimental organic chemistry to study chemical reactions. First calculations on silicon clusters prepared the way to investigate problems in surface chemistry. This symposium not only treated quantum chemical methods, but demonstrated further uses of computers in chemistry; its title "Computational Methods in Chemistry" was thus fully justified.
Toward the end of the s, the challenge of Ch. In accurate quantum calculations, such a scheme excludes the description of branching ratios, dissociative processes, or heavy-light-heavy reactions associated with small skewing angles. The polar Delves or hyperspherical coordinates, on the other hand, allow the treatment of all such processes elastic, inelastic, reactive, and dissociative for all mass combinations.
Kuppermann and co-workers had thought very much along the same lines. Kutzelnigg 94 designed the IGLO individual gauge for localized orbitals method, and Schindler 95 presented the first systematic application of this method to compute 13 C chemical shifts of carbocations.
The computation of NMR chemical shifts 96 is an ideal link between theory and experiment because calculated shifts can be used in combination with the NMR measurements to differentiate between various structural possibilities of the species under investigation. Hence, IGLO calculations are an ideal tool to give fingerprints to identify transient species with unusual structures.
BEYOND The field of computational chemistry found itself in good times by the s, and many young students were fascinated by the combination of computer usage and chemical research. At the Theoretical Chemistry Symposium, about people participated. He stated that the Americans had been proud to have the center of gravity of quantum chemical or computational research after World War II.
He had to admit that this center had moved - seen from a geographical point of view - at least toward the middle of the Atlantic Ocean, and that German scientists had a heavy weight in this change. International cooperation had become the rule in universities and research institutions. 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 baud special telephone line. For these reasons the development of computational chemistry seems to have become very similar in various countries. Applications rather than method development became dominant and many inorganic and organic chemists as well as scientists in molecular physics and pharmaceutical research were no longer hesitant to use the new computational methods.
Computer programs for molecular modeling based on quantum chemistry, on classical mechanics, or on empirical force fields became available in international exchange. Monte Carlo simulations became feasable on vector computers and parallel machines, and access to large databases was made possible.
The quantum chemistry tools were extended to include relativistic effects, 97 which play an important role in transition metal and heavy-element chemistry. Such efforts were later on supported by a program from the European Science Foundation 98 which gave financial support to a number of European groups working in this field.
The possibility to include relativistic effects, either directly or by effective potentials 93 made quantum chemical calculations also interesting for many inorganic chemists and organometallic chemists. Today, theoretical and computational chemists participate in many of the collaborative research centers Sonderforschungsbereich at German universities, which are created to support interdisciplinary work in areas expected to have great impact for our future. At this point, a look at computational chemistry in German industry is appropriate. An evaluation is somewhat difficult, however, because generally only a few of the industrial computational chemists attend the annual chemistry conferences in Germany, and in addition, these chemists are generally reluctant to talk about details of their work.
The main topic in the era was presumably computer-aided synthesis. Pattern recognition was also an important tool to find structurally related compounds that show similar or better molecular properties. Computer programs for the automatic recognition of the maximal common substructures among drug molecules, or computerized systems with graphical and topological information for handling and analysis of large databases were topics at special conferences.
In the middle of the s, molecular modeling, molecular mechanics MM , molecular dynamics MD , and some ab initio quantum chemistry became important tools in industry to study quantitative structure-activity relationships QSAR. The necessary computer programs were purchased from academic or commercial institutions.
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A number of young German theoretical chemists accepted job offers from chemical industry in the s in the hope to build up a computational chemistry nucleus within the companies, doing applied but also some basic research. At a special symposium on "Scientific Computing and Modeling in Chemical Industry" at the annual meeting of the Physical Chemistry Society Bunsentagung , young computational chemists from seven chemical and pharmaceutical companies in Germany presented already 13 talks.
The main topics were QSAR, enzymes, polymers, and databases, and the studies were clearly dominated by applications. The restructuring of chemical companies that took place in the second half of the s under new managements left little room for the development of computational chemistry methods in industry. Invitations for consultants were seldom. The future will show whether the cooperation between German universities and industry in the area of computational chemistry will strengthen. Fast expansion of the German university system in the s had brought a considerable number of new positions in theoretical and computational chemistry to universities.
However, this positive side was turned around in the following 20 years. Financial restrictions lead to a decrease in budgets, and salary lines of postdoctoral positions at universities were often simply cut off. Since many of the professors, who came into office toward the end of the s, were quite young, there was essentially no university post open for young people until the mid s when retirement of this first generation of professors started.
In other words, the generation of young scientists who were all well trained in the field could not really use their talents for research at German universities or research centers. Many of those people went into computer-oriented industry or took their talents to other countries.
Looking back, I find it truly amazing with what intensity science in Germany recuperated after the total vacuum caused by the Nazi regime and World War II. If I ask myself what were the main influences on the positive development of computational chemistry in Germany, I see it in our "bottom-up" principle of support. Hence, a small number of young energetic people, in competition with each other, fascinated by new tools and methods, were the hard core to develop the field.
The first generation was generally trained in physics or mathematics; the second generation originated mostly from chemistry. The foresight of a few senior scientists that digital computers would become an extremely useful tool and should be made available to researchers recommended by a committee of the DFG and that it was worthwhile to bring the young German researchers together within a special priority program gave important support to the field.
The advance of quantum chemistry and computational chemistry and its introduction into the education of chemistry students occurred without the support of large government contracts from the ministry of education or ministry of research and technology and largely without the support of dedicated research institutions such as the Max Planck Institutes or industry. It is gratifying to observe that computational chemistry in Germany is again strongly visible internationally. This remarkable development should be kept in mind in the present tendency to favor the support of large scientific centers over modest proposals from young individuals.
In particular, I want to thank W. Kutzelnigg for information on the Theoretical Chemistry Symposia and for various articles in which he looks back on the history of quantum chemistry. I want to thank H. Kuhn for information on his work with the analog computer. References 1. Gavroglu and A. Heitler and F. Physik, 44, London, A , Correlation of Molecular and Atomic Electron States. Die Elektronenkonfiguration des Benzols und verwandter Verbindungen.
Schwarz, D. Andrae, S. Arnold, J. Heidberg, H. Hellmann Jr. Hinze, A.
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