COSMIC(90): An improved molecular mechanics treatment of hydrocarbons and conjugated systems

Summary Four modifications to the COSMIC molecular mechanics force field are described, which greatly increase both its versatility and the accuracy of calculated conformational energies. The Hill non-bonded van der Waals potential function has been replaced by a two-parameter Morse curve and a new H-H potential, similar to that in MM3, incorporated. Hydrocarbon energies in particular are much improved.A simple iterative Hückel pi-electron molecular orbital calculation allows modelling of conjugated systems. Calculated bond lengths and rotational barriers for a series of conjugated hydrocarbons and nitrogen heterocycles are shown to be as accurate as those determined by the MM2 SCF method.Explicit hydrogen-bonding potentials for H-bond acceptor-donor atom pairs have been included to give better hydrogen bond energies and lengths. The van der Waals radii of protonic hydrogens are reduced to 0.5 Å and the energy well depth is increased to 1.0 kcal mol^-1.Two new general atom types, N⁺ _sp ² and O^- _sp ³ , have been introduced which allow a wide variety of charged conjugated systems to be studied. A minimum of parameterisation is required, as the new types are easily included in the Hückel scheme which automatically adjusts bond and torsional parameters according to the defined bond-order relationships.     

Additional NMR data on compounds of the type cis-Ti(AA)2X2

Previous work has shown that $\text{cis}$-disubstituted (acac)₂ titanium (IV) complexes exist in solution as nonrigid molecules [1]. These studies were followed by variable temperature ¹H NMR spectroscopy. The spectra shows that for the dialkoxy and dihalo complexes the acetylacetonate (acac) rings exchange the methyl groups between the two nonequivalent sites of the cis isomer. These rearrangements take place $\text{via}$ an intramolecular rearrangement: both twisting and one bond rupture mechanisms have been suggested for these stero-chemical rearrangements [2-4].     

Charge calculations in molecular mechanics. Part 8 Partial atomic charges from classical calculations

Summary The CHARGE2 programme, which involves the classical calculation of both the inductive and resonance contributions to the partial atomic charges in molecules is described, and the charges and electrostatic potentials obtained presented for some illustrative examples.In substituted methanes (CH₃X, CF₃X, CCl₃X) the effects of varying the electronegativity of the substituents and the - and -substituent contributions are clearly illustrated for a variety of substituent groups X.The problems involved in the inclusion of silicon into this scheme are detailed, together with the methods of overcoming them. The partial atomic charges ( and contributions) and electrostatic potentials for some silicon oxygen compounds are presented and discussed.The partial atomic charges from CHARGE2 for all the natural amino acids as their N-acetyl, N-methyl-amides are given and compared with those obtained from the AMBER and ECEPP/2 force fields. Considerable differences in these figures are observed, with the AMBER charges consistently much larger than those from the other two methods.The CHARGE2 partial atomic charges and electrostatic potentials for the four common nucleic acids, adenine, cytosine, guanine and thymine, are given and compared with those derived from other calculations. Again there is general similarity but also there are considerable differences, with those from the AMBER force field somewhat larger than the other methods.For previous parts in this series, see Refs. 1-7.     

A semiempirical molecular orbital calculation of borazine and some B-trifluoroborazines

A charge self-consistent semiempirical calculation has been performed on borazine and four fluoroborazines in which all the valence electrons of the molecules studied have been included. The results of the Mulliken population analyses are discussed in terms of the effect of fluorine bonded to boron and/or nitrogen. The correspondence between experiment and calculation for the orbital energies is also discussed.

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Zusammenfassung An Hand der Resultate ladungskonsistenter semi-empirischer Rechnungen für Borazin und vier Fluoroborazine unter Einbeziehung aller Valenzelektronen wird mittels Populationsanalyse die Frage diskutiert, wie sich die Bindung von Fluor an Bor und/oder Stickstoff auswirkt. Außerdem wird auf die Korrespondenz zwischen experimentellen Daten und Orbitalenergien eingegangen.

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Résumé Un calcul semi-empirique avec autocohérence des charges a été effectué sur le borazole et quatre fluoroborazoles avec introduction de tous les électrons de valence. Discussion des résultats de l'analyse de population de Mulliken en termes d'effet du fluor lié au bore et (ou) à l'azote. Discussion de la correspondance entre expérience et théorie pour les énergies orbitales.

     

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A global approach to error estimation and physical diagnostics in multidimensional computational fluid dynamics

An approach for simultaneously assessing numerical accuracy and extracting physical information from multidimensional calculations of complex (engineering) flows is proposed and demonstrated. The method is based on global balance equations, i.e. volume-integrated partial differential equations for primary or derived physical quantities of interest. Balances can be applied to the full computational domain or to any subdomain down to the single-cell level. Applications to in-cylinder flows in reciprocating engines are used for illustration. It is demonstrated that comparison of the relative magnitude of the terms in the balances provides insight into the physics of the flow being computed. Moreover, for quantities that are not conserved at the cell or control volume level in the construction of the numerical scheme, the imbalance allows a direct assessment of numerical accuracy in a single run using a single mesh. The mean kinetic energy imbalance is shown to be a particularly sensitive indicator of numerical accuracy. This simple and powerful diagnostic approach can be implemented for finite-difference, finite-volume or finite-element methods.     

Increasing the RF energy per pulse of an RKO

The Air Force Research Laboratory RKO source has recently demonstrated the ability to convert electron beam power to RF power until the termination of the electron beam pulse, achieving a power of 1.5 GW at an energy of 170 J. These results represent an increase in power of 25-30% in power and energy extracted from this source. This paper discusses the principal research areas encountered in lengthening the RF pulse (FWHM) from 50 ns to the present 120 ns and the associated increase in the RF energy     

Operation of repetitively pulsed virtual cathode oscillators on theTEMPO pulser

The design and performance characteristics of two virtual cathode oscillators operated at a 1-Hz repetition rate for a 10-shot burst using the TEMPO pulser are presented. The 2.4-GHz hardware generated a 300 MW per pulse (radiated) in the TM₀₂ mode with a 1.3% total energy conversion efficiency and with a 10.5% frequency bandwidth. A 2:1 scale-tip of this hardware was used to achieve an 840-MHz operation, but it only radiated 70 MW per pulse in the TM₀₁ mode with a 7.3% bandwidth, since hardware constraints prevented the TM₀₂ component from being radiated. The relatively low beam current density of the TEMPO VCOs yielded a low diode gap closure rate that should make them suitable for long-pulse operation. In addition, the low beam current density minimized damage to the thin anode screen