Recursive intermediate factorization and complete computational linearization of the coupled-cluster single,double, triple,and quadruple excitation equations

Summary The nonlinear CCSDTQ equations are written in a fully linearized form, via the introduction of computationally convenient intermediates. An efficient formulation of the coupled cluster method is proposed. Due to a recursive method for the calculation of intermediates, all computational steps involve the multiplication of an intermediate with aT vertex. This property makes it possible to express the CC equations exclusively in terms of matrix products which can be directly transformed into a highly vectorized program.This work has been supported by the U.S. Air Force Office of Scientific Research, Grant No. 90-0079     

Comparing ab initio density-functional and wave function theories: the impact of correlation on the electronic density and the role of the correlation potential

The framework of ab initio density-functional theory (DFT) has been introduced as a way to provide a seamless connection between the Kohn-Sham (KS) formulation of DFT and wave-function based ab initio approaches [R. J. Bartlett, I. Grabowski, S. Hirata, and S. Ivanov, J. Chem. Phys. 122, 034104 (2005)]. Recently, an analysis of the impact of dynamical correlation effects on the density of the neon atom was presented [K. Jankowski, K. Nowakowski, I. Grabowski, and J. Wasilewski, J. Chem. Phys. 130, 164102 (2009)], contrasting the behaviour for a variety of standard density functionals with that of ab initio approaches based on second-order M?ller-Plesset (MP2) and coupled cluster theories at the singles-doubles (CCSD) and singles-doubles perturbative triples [CCSD(T)] levels. In the present work, we consider ab initio density functionals based on second-order many-body perturbation theory and coupled cluster perturbation theory in a similar manner, for a range of small atomic and molecular systems. For comparison, we also consider results obtained from MP2, CCSD, and CCSD(T) calculations. In addition to this density based analysis, we determine the KS correlation potentials corresponding to these densities and compare them with those obtained for a range of ab initio density functionals via the optimized effective potential method. The correlation energies, densities, and potentials calculated using ab initio DFT display a similar systematic behaviour to those derived from electronic densities calculated using ab initio wave function theories. In contrast, typical explicit density functionals for the correlation energy, such as VWN5 and LYP, do not show behaviour consistent with this picture of dynamical correlation, although they may provide some degree of correction for already erroneous explicitly density-dependent exchange-only functionals. The results presented here using orbital dependent ab initio density functionals show that they provide a treatment of exchange and correlation contributions within the KS framework that is more consistent with traditional ab initio wave function based methods.     

RDX geometries, excited states, and revised energy ordering of conformers via MP2 and CCSD(T) methodologies: insights into decomposition mechanism

The geometries, harmonic frequencies, elec-tronic excitation levels, and energetic orderings of various conformers of RDX have been computed at the ab initio MP2 and CCSD(T) levels, providing more reliable results than have been previously obtained. We observe that the various local minimum-energy conformers are all competitive for being the absolute minimum and that, at reasonable temperatures, several conformers will appreciably contribute to the population of RDX. As a result, we have concluded that any mechanistic study to investigate thermal decomposition can reasonably begin from any one of the cyclohexane conformers of RDX. As such, it is necessary to consider the transition states for each RDX conformer to gauge what the activation energy is. Homolytic bond dissociation has long been speculated to be critical to detonation; we report here the most accurate estimates of homolytic BDEs yet calculated, likely to be accurate within 3 kcal mol(-1). The differences in energy for homolytic BDEs among all the possible RDR conformers are again small, such that most all of the conformers can reasonably be speculated as the next step in the mechanism starting from the RDR radical.     

BaSnF4 fast ion conductor: Variations versus the method of preparation and anomalous temperature variation of the quadrupole splitting

A new method of preparation of high performance fluoride ion conductor, BaSnF₄, by water leaching of newly discovered barium tin(II) chloride fluorides, has been designed, and the materials have been studied and compared to the solid prepared by the usual dry method. The unit-cell parameters and crystallite dimensions were found to vary with the method of preparation. In addition, the crystallite dimensions were found to be highly anisotropic for the samples obtained by the wet method. The Mössbauer spectrum is made of a large tin(II) quadrupole doublet, and a broad tin(IV) oxide peak due to surface oxidation. The tin(II) spectrum is in agreement with covalently bonded tin(II) having a strongly stereoactive lone pair. An unusually high dependence of the quadrupole splitting at low temperatures was observed (5.8 times larger than for α-SnF₂).     

MBPT and coupled cluster calculation on the neon atom with numerical orbitals

Our diatomic coupled cluster method with numerical orbitals has been applied to the Ne atom. Second, third, and full fourth order MBPT correlation corrections as well as coupled cluster correlation energies are reported.     

An investigation of the effects of pressure and temperature on the vapourization characteristics of crude oil using first approximation thermal analysis

This study investigates the effect of pressure and temperature on crude oil vaporization using first approximation thermal analysis. Developed here is a technique for quantifying the vaporization of fractions of a crude oil using an experimentally obtained thermogravimetric (TG) curve of the crude.The initial data utilized is the weight loss versus temperature values at atmospheric pressure, together with physical property data and equilibrium constant,k values.The results of this study allow the estimation of the volume of oil distilled underground when steam is injected into oil reservoirs at varying pressures and temperatures to recover crude oil.

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Zusammenfassung Diese Studie untersucht in erster Näherung mittels Thermoanalyse den Einfluß von Temperatur und Druck auf das Verdampfen von Rohöl. Es wird eine Methode zur quantitativen Bestimmung des Verdampfens von Rohölfraktionen entwickelt, die auf einer experimentell ermittelten thermogravimetrischen (TG) Kurve des Rohstoffes basiert. Als Anfangswerte werden die von der Temperatur abhängige Massenabnahme (unter Normaldruck) zusmmmen mit physikalischen Eigenschaften und der Gleichgewichtskonstantek verwendet. Die Ergebnisse dieser Untersuchung erlauben das Abschätzen des Umfanges des Öldestillationsrückstandes, insofern zur Gewinnung des Rohols Dampf in die Ölreservoir injeziert wird.

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Chiroptische Eigenschaften und Äquilibrierungen stereoisomerer Yohimbane und ihrer 17-Ketoderivate

The chiroptical properties (CD. and ORD.) of four stereoisomeric yohimbanes ( 1–4 ) and the corresponding 17-keto derivatives 5–8 have been studied. The aromatic and the ketonic Cotton effects have been considered in terms of the appropriate Sector and Octant Rules, respectively. Positively charged nitrogen makes an ‘anti-octant’ contribution to the ketonic Cotton effect. The chiroptical properties, the NMR. spectra and dehydrogenation reactions with mercuric acetate together indicate the presence of a C/D-trans-quinolizidine system in all the compounds studied, except those of the pseudo series ( 3 and 7 ), in which a C/D-cis-quinolizidine system is present. An explanation of the unexpectedly slow dehydrogenation of the 3-epi-allo compounds 4 and 8 is suggested. Conformational analysis of the results of equilibration experiments on pairs of 3-epimeric compounds (in both glacial acetic acid at 145° and in K-t-butylate at 211°) leads independently to conclusions which agree with those drawn from the physical methods. The great differences between the equilibrium positions of the free bases and the protonated forms are due to the presence in the latter cases of oriented ion pairs R₃N⁺H…?^?O₂CCH₃.     

The growth of ice crystals in an electric field

Zusammenfassung Das Wachstum von Eiskristallen in einer Diffusionswolkenkammer wird durch ein elektrisches Feld von über 500 V/cm wesentlich geändert. Die Kristalle wachsen sehr schnell in Form langer, dünner Nadeln. Die Ursache des beschleunigten Wachstums ist noch nicht völlig erkannt; aber die Beobachtungen stimmen mit der Hypothese überein, dass die Kristalle durch Einbau von neutralen Wassermolekülen wachsen.     

RPA and the linear hubbard model

The accuracy of RPA is tested semi-empirically by comparison with full Cl.