Evolutionary approach for determining first-principles hamiltonians

Modern condensed-matter theory from first principles is highly successful when applied to materials of given structure-type or restricted unit-cell size. But this approach is limited where large cells or searches over millions of structure types become necessary. To treat these with first-principles accuracy, one 'coarse-grains' the many-particle Schrodinger equation into 'model hamiltonians' whose variables are configurational order parameters (atomic positions, spin and so on), connected by a few 'interaction parameters' obtained from a microscopic theory. But to construct a truly quantitative model hamiltonian, one must know just which types of interaction parameters to use, from possibly 10(6)-10(8) alternative selections. Here we show how genetic algorithms, mimicking biological evolution ('survival of the fittest'), can be used to distil reliable model hamiltonian parameters from a database of first-principles calculations. We demonstrate this for a classic dilemma in solid-state physics, structural inorganic chemistry and metallurgy: how to predict the stable crystal structure of a compound given only its composition. The selection of leading parameters based on a genetic algorithm is general and easily applied to construct any other type of complex model hamiltonian from direct quantum-mechanical results.     

Diameter-dependent growth direction of epitaxial silicon nanowires

We found that silicon nanowires grown epitaxially on Si (100) via the vapor-liquid-solid growth mechanism change their growth direction from 111 to 110 at a crossover diameter of approximately 20 nm. A model is proposed for the explanation of this phenomenon. We suggest that the interplay of the liquid-solid interfacial energy with the silicon surface energy expressed in terms of an edge tension is responsible for the change of the growth direction. The value of the edge tension is estimated by the product of the interfacial thickness with the surface energy of silicon. For large diameters, the direction with the lowest interfacial energy is dominant, whereas for small diameters the surface energy of the silicon nanowire determines the preferential growth direction.     

Prolonged retention of methyl mercury by mallard drakes

Mallard drakes accumulated mercury rapidly from dietary dosage of methylmercury dicyandiamide and eliminated it slowly, retaining approximately one half at the end of 84 days; no measurable loss occurred between the end of the 7th and 56th days, but loss resumed concurrently with new feather growth, and continued through the 112th day, the close of the study.     

Quantitative methyl esterification of carboxyl end groups of polyethylene terephthalate

Summary For esterification of its carboxyl end groups in heterogeneous phase, polyethylene terephthalate was treated with boron trifluoride/methanol and with diazomethane in chlorobenzene, respectively. Whereas treatment with boron trifluoride/ methanol in some cases was accompanied by polymer degradation, reacting the polyester with diazomethane resulted in complete methyl esterification without any detectable side reaction.33. Communication in the series Synthetic fibers in the Wool Industry; 32. Communication see V. Rossbach, F. P.Schmitz u. J. Föhles, Angew. Makromol. Chem., submitted for publication     

Ein Rechenmodell für Zwei- und Vierwalzengerüste mit minimaler Auffederung

Erläuterung verschiedener Verfahren zur Berechnung der Bauteile von Walzwerken. Beschreibung eines Modells zur geschlossenen Berechnung von Zwei- und Vierwalzengerüsten mit minimaler Auffederung bei vorgegebenem Walzprogramm und Ständergewicht unter Berücksichtigung der zulässigen Spannungen in den einzelnen Bauteilen.     

Coffeinhaltige erfrischungsgetr?nke

Zusammenfassung Nach Überprüfung der z. Z. üblichen Verfahren zur Bestimmung von Coffein wird eine Methode bekanntgegeben, die es gestattet, bei Anwendung von nur 30 bis 40 ml des Erfrischungsgetränkes brauchbare Coffein-Werte zu erhalten und somit auch bei Doppelbestimmungen noch den Zuckergehalt, die Dichte, und gegebenenfalls den Phosphorsäuregehalt im Inhalt einer Flasche des Handels (200–250 ml) zu ermitteln.Zur Bestimmung der Phosphorsäure läßt sich das von O.Reichard für Wein und Most angegebene Verfahren auch auf coffeinhaltige Erfrischungsgetränke anwenden. Eine wesentliche Vereinfachung kann aber erzielt werden, wenn anstelle der umständlichen Veraschung eine Oxydation auf nassem Wege mit Kaliumpermanganat erfolgt. Die mit nur 10 ml Ausgangsmaterial durchgeführte Analyse liefert sowohl gravimetrisch, wie auch acidimetrisch gute Ergebnisse. Von 13 coffeinhaltigenErfrischungsgetränken des Handels werden Dichte ( 20°), p_H-Wert, Coffein und Phosphorsäuregehalt (letztere nur in 6 Proben enthalten) bekanntgegeben.     

Die Entwicklung der Medizingeschichte seit 1945

During the last decades, medical historiography has undergone considerable changes. This review attempts an outline of the developments since 1945. The first section sketches the institutional background of the discipline focusing on the characteristic features which emerged in different national traditions. The following sections—essentially restricted to the German speaking context—describe the development of the fields in research and teaching, ranging from the history of ideas to the social history of medicine, from philogical and editorial work to the philosophy and sociology of medical knowledge.