Non self-consistent field theory — A new approach in quantum mechanical calculations

An R ^o-independent electronic repulsion matrix is constructed, replacing the R ^o-dependent Hamiltonian matrix (R ^o is the density matrix). A non-SCF theory is developed to solve the eigenequation without using an iterative procedure. Three methods are proposed to solve for the eigenvectors and eigenvalues. Illustrative calculations are reported comparing the non-SCF and SCF theories. The calculated results are as expected: the ground state energies are nearly unchanged while the orbital energies are nearer to the experimental results. Other physical properties and spectral quantities are also compared. It is found that the ZDO assumption is applicable in the non-SCF theory if it is applicable in SCF theory.

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Zusammenfassung Eine R ^o-unabhängige Elektronenabstoßungsmatrix wird eingeführt, die die R ^o-abhängige Hamiltonmatrix ersetzt (R ^o ist die Dichtematrix). Zur Lösung der Eigenwertgleichung ohne iterative Prozeduren wird eine sog. Nicht-SCF-Theorie aufgestellt. An Beispielen werden die Ergebnisse von SCF- und Nicht-SCF-Rechnungen verglichen; dabei erweisen sich die Energien des Grundzustandes als nahezu unverändert, während die Energien der Orbitale näher bei den experimentellen Werten liegen. Die zero-differential-overlap-Näherung ist immer dann in der neuen Theorie anwendbar, wenn sie in der SCF-Theorie anwendbar ist.

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Résumé Une matrice de répulsion électronique indépendante de R ^o est construite, remplaçant la matrice hamiltonienne dépendant de R ^o (R ^o matrice de densité). Une théorie non SCF est développée afin de résoudre l'équation aux valeurs propres sans itérations. Trois méthodes de résolution du problème aux valeurs propres sont proposées. Des calculs illustrent la comparaison entre les théories SCF et non SCF. Les résultats des calculs sont comme prévus: l'énergie de l'état fondamental varie peu alors que les énergies orbitales sont plus proches des résultats expérimentaux. D'autres propriétés physiques ainsi que des grandeurs spectrales sont comparées. On trouve que l'approximation du recouvrement différentiel nul est applicable dans la théorie non SCF si elle est applicable dans la théorie SCF.

     

Shaping the density to fit one-electron properties: Constrained RHF calculations on N2, FH,CO, and LiH

The molecular density required to give the correct values for one-electron properties is rarely given by wave functions obtained from variation methods based on the total energy or the eigenvalues. Perhaps if we knew how the density should be shaped in any particular volume to fit a particular property, the whole molecular density might then be properly described to fit the whole volume. The secant-parametrization procedure is used to constrain minimum basis set RHF wave functions for N₂, FH, CO, and LiH to determine the effects of different constraints on RHF wave functions, and to see how constraints improve the quality of small basis set RHF wave functions. One-electron property expectation values, energies, and unweighted and property weighted populations and electron density difference profiles are used to analyze the constrained wavefunctions. With the information from the constrained wave functions it should be possible to select a LCAO -CI basis and states to give the correct density for all properties. This should map onto the constrained wave function in the region of the constraint and at the same time minimize the energy of the total molecular wave function. Such a density would be suitable for the density analyses favored by Bader and Nguyen-Dang [Adv. Quantum Chem. 14 , 113 (1981)], Mezey [Theor. Chim. Acta 54 , 95 (1980); 58 , 309 (1981); 59 , 321 (1981)], and March [Theoretical Chemistry (Royal Society of Chemistry, London, 1981), Vol. 4, p. 158], and show the real atom needed to generate the molecule.     

Non self-consistent field theory

Roothaan's SCF method [2] is reformulated so that two non-SCP methods are developed to solve the eigenequation. The results from these methods can be used as starting eigenfunctions for Roothaan's SCF method.

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Zusammenfassung Die self-consistent field Methode von Roothaan [2] wird neu formuliert: dabei werden zwei Nicht-SCF Methoden zur Lösung der Eigenwertgleichung entwickelt. Deren Resultate können als Anfangsfunktionen bei Rechnungen mit der SCF-Methode von Roothaan benutzt werden.

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Résumé La méthode SCF de Roothaan [2] est reformulée en développant deux méthodes non-SCF pour la solution de l'équation aux valeurs propres. Les résultats de ces deux méthodes peuvent être utilisées comme fonctions de départ pour la méthode SCF de Roothaan.

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The title Theoretical Chemistry has been transferred to the Division of Physical Chemistry.     

Transferable atom equivalent multicentered multipole expansion method

The transferability of atomic and functional group properties is an implicit concept in chemistry. The work presented here describes the use of Transferable Atom Equivalents (TAE) to represent molecular electrostatic potential fields through the use of integrated atomic multipole moments that are associated with each TAE atom type used in the reconstruction. TAE molecular surface distributions of electrostatic potentials are compared with analytical ab initio and empirical (Gasteiger) partial charge reference models for several conformations of test peptides. Surface electrostatic potential distributions computed using TAE multipole representations were found to converge at the octopole level, with incremental improvement observed when hexadecapoles were included. Molecular electrostatic potential fields that were produced using the TAE method were observed to be responsive to conformational changes and to compare well with ab initio reference distributions. Generation of TAE atom types and their associated multipoles does not involve fitting to sample electrostatic potential fields, but rather utilizes integrated AIM atomic electron density distributions within representative chemical environments. The RECON program was used for TAE reconstruction. RECON is capable of processing 5,000 drug-sized molecules or 25 proteins per minute per 1.7 GHz P4 Linux processor.     

A simple atmospheric microwave-excited emissive detector for gas chromatography

An atmospheric pressure microwave-excited emissive argon plasma is evaluated for use as a detector in conventional gas-chromatographic analysis. A range of carbon-, oxygen-, nitrogen- and halogen-containing compounds was investigated in order to optimize the sensitivity, linear working range and plasma operating conditions. For the non-selective determination of carbon-containing compounds, the atomic carbon emission at 247.9 nm was found to be most useful. At this wavelength the sensitivities for all the compounds investigated were directly proportional to the amount of carbon in the compounds. The limit of detection for these compounds at 247.9 nm was 1.94·10^-l0 g of carbon per sec. The atomic lines at 206.2 nm (I) and 470.5 nm (Br) were the best for the selective determination of iodine- and bromine-containing compounds respectively, and a new band emission at 256 nm, characteristic of chlorinecontaining compounds, gave good sensitivity.     

Determination of riboflavin in pharmaceutical products by automatic discrete-sample analysis

A procedure for the determination of iron, copper, nickel, cobalt, manganese and chromium down to 0.01 μg g^-1 in sodium calcium silicate glass, sodium borosilicate glass, sodium carbonate and calcium carbonate is described. The analytical procedure depends on the separation at pH 6 of the metal diethyldithiocarbamates into isobutyl methyl ketone, and their determination by flameless atomic absorption spectrometry, with a Massmann-type graphite furnace. The limiting factors on the detection limits attainable are discussed and related to the purity of the acids used for sample solution, sample contamination during chemical separation and the sensitivity of the analytical technique.     

Photoresponsive supramolecular systems: self-assembly of azodibenzoic acid linear tapes and cyclic tetramers

A new strategy to effect photoinduced control over molecular self-assembly is reported. This strategy uses the reversible trans-cis photoisomerization of a novel azobenzene system, where the trans- and cis-forms self-assemble into dramatically different higher-order structures. The trans-azobenzene form of this molecule associates into infinite hydrogen-bonded linear tapes, while the cis-azobenzene form undergoes hydrogen-bonded self-assembly into cyclic tetramers. This results in a second level of association, where the cis-hydrogen-bonded supramolecular cycles ultimately form long, rod-like aggregates through stacking interactions.     

Studies on azaspiracid biotoxins. I. Ultrafast high-resolution liquid chromatography/mass spectrometry separations using monolithic columns

In this study, the performance of monolithic columns was evaluated for ultrafast liquid chromatography/mass spectrometry (LC/MS) analyses and for high-resolution separations of several azaspiracid biotoxin analogs. Because of their high permeability, monolithic columns offer a number of advantages over conventional packed columns; viz., very low backpressures and relatively flat van Deemter curves at high flow rates. That is, very high flow rates can be used for ultrafast analyses or, by using longer than normal columns, high-resolution separations are possible. In a series of experiments, we varied the mobile phase flow rates between 1 and 8 mL/min, and studied their impact on chromatographic parameters such as retention time, resolution, number of plates and pressure. The chromatographic run times could be reduced to ca. 30 s without a significant change in the separation efficiency. A signal intensity comparison revealed interesting differences between atmospheric-pressure chemical ionization (APCI) and electrospray ionization (ESI) in their flow-rate dependency. An explanation with respect to the behavior as of a mass-flow or a concentration-dependent device is given in the paper. Additionally, the column length was varied between 10 and 70 cm. As a result, the number of theoretical plates increased substantially. In the example shown in the report, an increase from 13 000 plates for a 10-cm column to 80 000 for a 70-cm column is demonstrated. In addition, the potential of the monolithic columns for high-resolution LC/MS separations is shown for a complex biotoxin mixture, which was separated on a 40-cm-long column.     

Recent advances in analytical construct resins

The area of solid-phase synthesis has witnessed exponential growth in the last fifteen years, but difficulties associated with the monitoring and analysis of resin-bound reactions and products have been apparent due to a limited number of analytical methods available. With the substrate tethered to an insoluble support traditional chromatographic monitoring is only possible after cleavage. In order to address this 'analytical bottleneck' Geysen, in 1996 elaborated Merrifield's initial dual linker strategy by incorporating an encoding system between two in-line linkers. These analytical construct resins represented a new approach for both the quality control of solid-phase combinatorial libraries and for the development of new synthetic sequences on solid-support. This review will summarize the development and application of analytical construct resins focusing on recent applications of the technology.