We have applied the momentum space version of the Density Matrix Renormalization Group method (k-DMRG) in quantum chemistry in order to study the accuracy of the algorithm in the new context. We have shown numerically that it is possible to determine the desired accuracy of the method in advance of the calculations by dynamically controlling the truncation error and the number of block states using a novel protocol which we dubbed Dynamical Block State Selection (DBSS). The relationship between the real error and truncation error has been studied as a function of the number of orbitals and the fraction of filled orbitals. We have calculated the ground state of the molecules CH2, H2O, and F2 as well as the first excited state of CH2. Our largest calculations were carried out with 57 orbitals, the largest number of block states was 1500-2000, and the largest dimensions of the Hilbert space of the superblock configuration was 800.000-1.200.000.

We have studied the ionic-neutral curve crossing between the two lowest 1 Σ + states of LiF in order to demonstrate the efficiency of the quantum chemistry version of the density matrix renormalization group method (QC-DMRG). We show that QC-DMRG is capable to calculate the ground and several low-lying excited state energies within the error margin set up in advance of the calculation, while with standard quantum chemical methods it is difficult to obtain a good approximation to Full CI property values at the point of the avoided crossing. We have calculated the dipole moment as a function of bond length, which in fact provides a smooth and continuous curve even close to the avoided crossing, in contrast to other standard numerical treatments.

Nitric oxide (NO) donors, which cause delayed headaches in migraineurs, have been shown to activate central trigeminal neurons with meningeal afferent input in animal experiments. Previous reports indicate that this response may be due to up-regulation of NO-producing cells in the trigeminal brainstem. To investigate this phenomenon further, we determined nitric oxide synthase (NOS)-containing neurons in the rat spinal trigeminal nucleus (STN), the projection site of nociceptive trigeminal afferents, following infusion of the NO donor sodium nitroprusside (SNP). Barbiturate anaesthetized rats were infused intravenously with SNP (50 microg/kg) or vehicle for 20 min or 2 h, and after periods of 3-8 h fixed by perfusion. Cryostat sections of the medulla oblongata containing the caudal STN were histochemically processed for detection of nicotineamide adenine dinucleotide phosphate (NADPH)-diaphorase or immunohistochemically stained for NOS isoforms and examined by light and fluorescence microscopy. The number of neurons positive for these markers was determined. Various forms of neurons positive for NADPH-diaphorase or immunoreactive to neuronal NOS (nNOS) were found in superficial and deep laminae of the STN caudalis and around the central canal. Neurons were not immunopositive for endothelial (eNOS) or inducible (iNOS) NOS isoforms. The number of NADPH-diaphorase-positive neurons increased time dependently after SNP infusion by a factor of more than two. Likewise, the number of nNOS-immunopositive neurons was increased after SNP compared with vehicle infusion. Around the central canal the number of NADPH-diaphorase-positive neurons was slightly increased and the number of nNOS+ neurons not changed after SNP treatment. NO donors increase the number of neurons that produce NO in the STN, possibly by induction of nNOS expression. Increased NO production may facilitate neurotransmitter release and promote nociceptive transmission in the STN. This mechanism may explain the delayed increase in neuronal activity and headache after infusion of NO donors.

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Zur Untersuchung von Cola‐Getränken sind bereits vielfach Experimente und Unterrichtseinheiten beschrieben worden. Dennoch hat die CHEMKON‐Redaktion sich entschlossen, dieses Thema noch einmal unter der Rubrik DAS EXPERIMENT aufzugreifen. Wir möchten diese Rubrik damit nicht nur zur Vorstellung neuer Experimente nutzen, sondern ebenso zur Optimierung und Ergänzung bereits vorhandener. In dem nachfolgenden Beitrag finden Sie daher unter (1) die (auffrischende) Darstellung eines wirksamen Einstiegsexperiments von Axel Schunk, unter (2) Auszüge aus einer sorgfältigen Prüfung und Optimierung von Versuchsvorschriften zur Untersuchung von Cola von Wolfgang Proske und Johannes Röder sowie unter (3) einen ergänzenden Versuch zur Betrachtung der Dichte im Anfangsunterricht von Walter Jansen und Renate Peper‐Bienzeisler. Weitere Informationen zu den genannten und weiteren Experimenten können von den Autoren oder der CHEMKON‐Redaktion bezogen werden!