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  • 1
    Electronic Resource
    Electronic Resource
    On the problem of a suitable definition of the cluster in embedded-cluster treatments of defects in crystals (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 4195-4204 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The perturbed-cluster embedding scheme [J. Chem. Phys. 92, 7448 (1990)] adopts a self-consistent procedure where, at each cycle, a molecular cluster solution is calculated, and subsequently corrected for realizing its coupling to the crystalline environment. In the present work, it is shown that a proper design of the cluster is of critical importance for the scheme to work satisfactorily. Indications are given for defining clusters that are at the same time "self-satisfied,'' and optimally fitted to the environment, so that correction terms are minimized. The procedure is tested in the case of defects in crystalline silicon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463922
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  • 2
    Electronic Resource
    Electronic Resource
    Ab initio approach to molecular crystals: A periodic Hartree–Fock study of crystalline urea (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 7402-7411 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic structure of crystalline urea (two molecules, 16 atoms per unit cell) is investigated at an ab initio level with CRYSTAL, a Hartree–Fock linear combination of atomic orbitals (LCAO) program for periodic systems. The influence of the basis set and of the computational parameters which control the treatment of the Coulomb and exchange series and the reciprocal space integration is documented; results include total and interaction energy, Mulliken analysis data and interaction (solid minus molecules) density maps, band structure, and density of states. The crystal field modifies the electronic structure of the isolated molecule, the main effect being an increase in the ionicity of bonds. The interaction energy obtained with a 6-21** basis set is 28 kcal/mol, (16 kcal/mol after a correction of the basis set superposition error by using the counterpoise method) to be compared with 21±0.5 kcal/mol from experiment. This preliminary application shows that accurate ab initio calculations of hydrogen bonded molecular crystals with a relatively small unit cell (less than 30 atoms/cell) are now within reach.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458592
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  • 3
    Electronic Resource
    Electronic Resource
    Adsorption energies of NH3 and NH+4 in zeolites corrected for the long-range electrostatic potential of the crystal (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5865-5874 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The calculation of adsorption energies in zeolites in the cluster approximation has the advantage of a reduced computational effort compared to that of the periodic approach. However, in the cluster approximation, the long-range electrostatic effects of the infinite crystal are ignored and there are boundary effects. In order to remove a part of the disadvantages of the cluster approach, we developed a method to embed a cluster in a zeolite crystal by imposing an electrostatic potential on it. This potential adds the long-range electrostatic effects of the crystal and subtracts the electrostatic potential of the boundary of the cluster. It is calculated from the charge distribution of the crystal obtained with an ab initio calculation using the crystal program. We calculated the adsorption energies of NH3 and NH+4 on three different clusters embedded in the potential of a chabazite crystal, and we have compared them with the adsorption energies of NH3 and NH+4 in the crystal. If a cluster is used that has dangling bonds four, or more, bonds away from the adsorbate, i.e., the adsorption site is described well covalently, embedding the cluster reduces the error in adsorption energies (relative to the crystal calculations) from ≈20 to ≈2–3 kJ/mol.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467303
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  • 4
    Electronic Resource
    Electronic Resource
    Treatment of the exchange interactions in Hartree-Fock LCAO calculations of periodic systems (1988)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 92 (1988), S. 909-913 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100315a010
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  • 5
    Electronic Resource
    Electronic Resource
    Hydrogen abstraction from methane by Li doped MgO: A periodic quantum mechanical study (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 8937-8943 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The catalytic abstraction of hydrogen from methane on the (001) surface of Li-doped MgO has been studied at the ab initio level within the periodic unrestricted Hartree–Fock approximation. The catalyst is modeled with an infinite slab, periodic in two dimensions and of finite thickness. Active sites for the reaction are surface O− radical ions resulting from coupling with the Li+ dopants; in the periodic model they are regularly distributed upon the surface. At 0 K the energy difference between reactants and products is close to zero, but a potential barrier of 18 kcal/mol is found to reach the transition state. Relaxation of ions in the zone of the Li+/O− couple stabilizes the reaction products, and plays a fundamental role in the energy balance. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472623
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  • 6
    Electronic Resource
    Electronic Resource
    An Analytical Solution of Cyclic Mass Transfer Operations (1977)
    s.l. : American Chemical Society
    Industrial and engineering chemistry 16 (1977), S. 400-405 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/i260063a026
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  • 7
    Electronic Resource
    Electronic Resource
    Fish yields in Cuban reservoirs and relationships with some morphometric and edaphic parameters (1999)
    Oxford, UK : Blackwell Science Ltd
    Lakes & reservoirs 4 (1999), S. 0 
    Details
    ISSN: 1440-1770
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geography
    Notes: Cuba is located between 19°49′ and 23°18′ north latitude and 74°08′ and 84°57′ west longitude. It is the largest island in the Caribbean Sea and the paucity of natural lakes and the prolonged dry season makes it necessary to construct reservoirs. During the past 36 years, Cuba has built 220 large reservoirs (〉 100 ha) and 800 smaller ones (〈 100 ha). Reservoirs are the most important standing waters in Cuba and their fish, although of low diversity, support productive fisheries, with tilapias contributing 92% of the total production by weight. The fisheries in reservoirs located in the eastern and central parts of the island were analysed during the period 1983–92. The mean depth (z) of reservoirs here is 5.9 m, ranging from 1.8 to 16.0 m. The best correlation coefficients and fit regression in 54 Cuban reservoirs were between FY (fish yield) and DE (diversity of exploitation); when data were analysed with a multiplicative statistical model, r = 0.70, FY = 4.795 DE0.9292; and between TC (total catch) and A (area), but in this case data were analysed with a linear statistical model, r = 0.91, TC = 17209A–6635.56. Regression between FY versus z was relatively low and inverse, r = −0.20, FY = 249.145–11.04z, although all regressions were statistically significant (P 〈 0.01).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1440-1770.1999.00082.x
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  • 8
    Electronic Resource
    Electronic Resource
    Review article: the pathophysiology of gastro-oesophageal reflux disease − oesophageal manifestations (2004)
    Oxford, UK : Blackwell Publishing Ltd
    Alimentary pharmacology & therapeutics 20 (2004), S. 0 
    Details
    ISSN: 1365-2036
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The pathogenesis of gastro-oesophageal reflux disease (GERD) is multifactorial, involving transient lower oesophageal sphincter relaxations (TLESRs) as well as other lower oesophageal sphincter (LES) pressure abnormalities. GERD is associated with a decrease in LES pressure, which can be provoked by factors such as foods (fat, chocolate, etc.), alcohol, smoking and medications. These factors have also been shown to increase TLESRs. As a result, reflux of acid, bile, pepsin and pancreatic enzymes occurs, leading to oesophageal mucosal injury, which can potentially progress to oesophageal adenocarcinoma in a minority of patients with Barrett's metaplasia. In addition, duodenogastric contents can also contribute to oesophageal injury. Other factors contributing to the pathophysiology of GERD include hiatal hernia, poor oesophageal clearance, delayed gastric emptying and impaired mucosal defensive factors. Hiatal hernia has a permissive role in the pathogenesis of reflux oesophagitis by promoting LES dysfunction. Delayed gastric emptying, resulting in gastric distension, can significantly increase the rate of TLESRs, contributing to postprandial GER. The mucosal defensive factors have an important role in GERD. When excessive acid causes a breakdown in oesophageal epithelial defenses, epithelial resistance may be reduced. Nocturnal GERD is associated with prolonged acid exposure and proximal extent of acid contact, which elevates the risk for oesophageal damage and GERD-related complications. In sum, GERD is a complex problem caused by many factors that are exacerbated when the patient is in the supine position.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2036.2004.02238.x
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  • 9
    Electronic Resource
    Electronic Resource
    Efficacy of omeprazole versus ranitidine for symptomatic treatment of poorly responsive acid reflux disease—a prospective, controlled trial (1999)
    Oxford UK : Blackwell Science Ltd
    Alimentary pharmacology & therapeutics 13 (1999), S. 0 
    Details
    ISSN: 1365-2036
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: : H2-receptor antagonists are widely used in patients with gastro-oesophageal reflux disease (GERD) and are frequently continued when symptoms persist. Aim: To compare the efficacy of omeprazole 20 mg once daily with that of ranitidine 150 mg twice daily in relieving GERD symptoms, in patients who remained symptomatic following a 6-week course of ranitidine therapy.〈section xml:id="abs1-2"〉〈title type="main"〉Methods: Patients with heartburn on at least 4 days/week but who did not have endoscopy to assess oesophageal mucosa could participate. This two-phase, prospective trial included a 6-week open-label phase (phase I), followed by an 8-week double-blind phase (phase II). Patients still symptomatic following treatment with ranitidine 150 mg twice daily (phase I) were randomized to double-blind treatment (phase II) with either omeprazole 20 mg once daily or ranitidine 150 mg twice daily. The primary efficacy variable was the proportion of patients with heartburn resolution during weeks 4 and 8 of phase II.〈section xml:id="abs1-3"〉〈title type="main"〉Results: Of the 533 patients with GERD who received ranitidine in phase I, 348 patients (65%) were still symptomatic. A total of 317 patients (59%) were randomized to double-blind treatment (phase II). At week 8, a significantly (P 〈 0.0004) greater proportion of omeprazole-treated patients (70%) experienced no more than mild heartburn compared with ranitidine-treated patients (49%). Complete resolution of heartburn also occurred in a significantly (P 〈 0.00001) greater proportion of omeprazole-treated patients (46% vs. 16% of the ranitidine group at week 8).〈section xml:id="abs1-4"〉〈title type="main"〉Conclusions: After 6 weeks of ranitidine treatment, the majority of patients with GERD were still experiencing moderate to severe heartburn. Omeprazole was significantly more effective than ranitidine in resolving heartburn in this group of patients.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2036.1999.00527.x
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  • 10
    Electronic Resource
    Electronic Resource
    Three-dimensional coupled thermo-magnetic analysis of toroidal field coils of resistive high field tokamaks by FEM (1998)
    Bradford : Emerald
    Compel 17 (1998), S. 576-584 
    Details
    ISSN: 0332-1649
    Source: Emerald Fulltext Archive Database 1994-2005
    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics
    Notes: In this paper a code for the solution of nonlinear thermo-magnetic problems, arising in the transient analysis of thermal and electromagnetic fields in resistive toroidal field coils of tokamak devices, is presented. A formulation based on the electric vector potential T and the magnetic total scalar potential ? for the analysis of the thermo-magnetic skin effect phenomenon in the toroidal field coils of proposed tokamaks has been used, and the results obtained are reported, reviewed and discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1108/03321649810220874
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