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At least 1,585 records · Page 88Linked to original sources
PubMed2006

Matching-pursuit split-operator Fourier-transform simulations of excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)-oxazole.

The excited-state intramolecular proton-transfer dynamics associated with the keto-enolic tautomerization reaction in 2-(2(')-hydroxyphenyl)-oxazole is simulated according to a numerically exact quantum-dynamics propagation method and a full-dimensional excited-state potential energy surface, based on an ab initio reaction surface Hamiltonian. The reported simulations involve the propagation of 35-dimensional wave packets according to the recently developed matching-pursuit/split-operator-Fourier-transform (MP/SOFT) method by Wu and Batista. The underlying propagation scheme recursively applies the time-evolution operator as defined by the Trotter expansion to second order accuracy in dynamically adaptive coherent-state expansions. Computations of time-dependent survival amplitudes, photoabsorption cross sections, and time-dependent reactant(product) populations are compared to the corresponding calculations based on semiclassical approaches, including the Herman-Kluk semiclassical initial value representation method. The reported results demonstrate the capabilities of the MP/SOFT method as a valuble computational tool to study ultrafast reaction dynamics in polyatomic systems as well as to validate semiclassical simulations of complex (nonintegrable) quantum dynamics in multidimensional model systems.

Journal Article↗
PubMed2005

Large amplitude quantum mechanics in polyatomic hydrides. I. A particles-on-a-sphere model for XH(n).

A framework is presented for converged quantum mechanical calculations on large amplitude dynamics in polyatomic hydrides (XH(n)) based on a relatively simple, but computationally tractable, "particles-on-a-sphere" (POS) model for the intramolecular motion of the light atoms. The model assumes independent two-dimensional (2D) angular motion of H atoms imbedded on the surface of a sphere with an arbitrary interatomic angular potential. This assumption permits systematic evolution from "free rotor" to "tunneling" to "quasi-rigid" polyatomic molecule behavior for small, but finite, values of total angular momentum J. This work focuses on simple triatom (n=2) and tetratom (n=3) systems as a function of interatomic potential stiffness, with explicit consideration of H2O, NH3, and H3O+ as limiting test cases. The POS model also establishes the necessary mathematical groundwork for calculations on dynamically much more challenging XH(n) species with n>3 (e.g., models of CH5+) where such a reduced dimensionality approach offers prospects for being quantum mechanically tractable at low J values (i.e., J=0, 1, 2) characteristic of supersonic jet expansion conditions.

Journal Article↗
PubMed2001

[Radionuclide tests of cerebral circulation in the assessment of treatment of sensorineural hearing loss].

Examination and treatment results are available for 59 patients with acute and chronic neurosensory hypoacusis. Cerebral circulation was investigated at radionuclide carotid arteriography and radionuclide single-photon emission computed tomography. Arterial flow to the brain, microcirculation of the gray matter and venous outflow from the cranial cavity were studied before and after the course of quantum hemotherapy.

Blood Transfusion, Autologous↗
PubMed2005

Evaluation and testing of computed radiography systems.

The implementation of film replacement digital radiographic imaging systems throughout Europe is now gathering momentum. Such systems create the foundations for totally digital departments of radiology, since radiographic examinations constitute the most prevalent modality. Although this type of development will lead to improvements in the delivery and management of radiological service, such widespread implementation of new technology must be carefully monitored. The implementation of effective QA tests on installation, at periodic intervals and as part of a routine programme will aid this process. This paper presents the results of commissioning tests undertaken on a number of computed radiography imaging systems provided by different manufacturers. The aim of these tests was not only to provide baseline performance measurements against which subsequent measurements can be compared but also to explore any differences in performance, which might exist between different units. Results of measurements will be presented for (1) monitor and laser printer set-up; (2) imaging plates, including sensitivity, consistency and uniformity; (3) resolution and contrast detectability; and (4) signal and noise performance. Results from the latter are analysed in relationship with both system and quantum noise components.

Computers↗
PubMed2006

Overlayers on silver nanotriangles: Field confinement and spectral position of localized surface plasmon resonances.

We studied the spectral evolution of plasmon modes associated with silver nanotriangles as a function of dielectric overlayer thickness in the range of 5-300 nm. A substantial red-shift of the resonance is observed that oscillates with increasing over-layer thickness. We explain this previously unreported oscillation through the cavity quantum electrodynamical effect of the array of triangles combined with the dielectric overlayer. The red-shift, though substantial, is less than expected. Comparison with numerical models indicates that this discrepancy is due to very tight field confinement around the tips of the triangles.

Computer Simulation↗
PubMed2006

Spin-orbit ab initio investigation of the photolysis of bromoiodomethane.

The photodissociation of bromoiodomethane has been investigated by spin-orbit ab initio calculations. The experimentally observed A- and B-bands and the corresponding photoproducts were assigned by multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction potential energy curves, vertical excitation energies, and oscillator strengths of low-lying excited states. The present conclusions with respect to the dissociation process in the B-band are different compared with those of previous studies. The reaction between the iso-CH(2)Br-I and iso-CH(2)I-Br species has also been studied. Finally, a set of stable excited states was identified for both isomers. These species might be of importance in the recombination process that follows the photodissociation in a solvent.

Computer Simulation↗
PubMed2006

Interatomic potential models for natural apatite crystals: incorporating strontium and the lanthanides.

A comprehensive set of interatomic potential parameters for modeling natural apatite crystals is presented. These potentials build on those previously used in research on apatites with new potentials fitted empirically to crystal structures and their properties using the GULP program. We demonstrate that the new potentials produce good models for the different compounds used for fitting, as well as for several natural apatites. Also presented are predicted enthalpies of mixing of strontium and calcium apatites and predicted cation site preferences in strontium calcium fluorapatite.

Apatites↗
PubMed2006

Accurate relativistic adapted Gaussian basis sets for Cesium through Radon without variational prolapse and to be used with both uniform sphere and Gaussian nucleus models.

Accurate relativistic adapted Gaussian basis sets (RAGBSs) from Cs (Z = 55) through Rn (Z = 86) without variational prolapse were developed by using the polynomial version of the Generator Coordinate Dirac-Fock method. The RAGBSs presented here can be used with any of two popular finite nucleus models, the uniform sphere and the Gaussian models. The largest RAGBS error is 4.5 mHartree for Radon with a size of 30s27p17d11f.

Cesium↗
PubMed1976

Molecular orbital calculations for parabens: a possible mechanism of action.

Molecular orbital calculations were carried out on the series benzoic acid, p-hydroxybenzoic acid, methylparaben, and ethylparaben. The characteristic trends of this entire series of preservatives are demonstrated by these members. The computer program used was Iterated Extended Hückel Theory. These systems were characterized in terms of atomic charge distributions, effect of hydroxyl and ester groups (both sterically and electronically), preferred molecular conformations, and distinguishing features of molecular orbitals. Through this approach, it was hoped that a common mode of action for all members of this series could be detected and that the trend(s) in this feature would correlate with known trends in activity of members of this series.

Chemical Phenomena↗
PubMed1997

Three-dimensional reconstruction from reduced sets of very noisy images acquired following a single-axis tilt schema: application of a new three-dimensional reconstruction algorithm and objective comparison with weighted backprojection.

In this work we propose a reconstruction algorithm (ART with blobs) that has not been previously used in electron Tomography and we compare it with the standard method in the field (weighted back projection, WBP). We assume that only a limited set of very noisy images, collected around a single axis tilt, is available; which is a typical situation in Electron Tomography. In general, the reconstruction problem is underdetermined (due to the limited number of projections) and the data are inconsistent (due to the high level of noise). The evaluation of the results is performed in a rigorous way by a task-oriented approach which makes use of numerical observers. ART with blobs outperforms WBP for a number of key tasks. Results are presented both for simplified line integral data and for realistic simulations of macromolecular structures embedded in amorphous ice.

Algorithms↗
PubMed2004

Quantum magnets with anisotropic infinite range random interactions.

Using exact diagonalization techniques, we study the dynamical response of the anisotropic disordered Heisenberg model for systems of S=1/2 spins with infinite range random exchange interactions at temperature T=0. The model can be considered as a generalization, to the quantum case, of the well-known Sherrington-Kirkpatrick classical spin glass model. We also compute and study the behavior of the Edwards Anderson order parameter and energy per spin as the anisotropy evolves from the Ising to the Heisenberg limits.

Anisotropy↗
PubMed2004

Pulsed optically detected NMR of single GaAs/AlGaAs quantum wells.

While nuclear magnetic resonance (NMR) is one of the most important experimental tools for the analysis of bulk materials, the low sensitivity of conventional NMR makes it unsuitable for the investigation of small structures. We introduce an experimental scheme that makes NMR spectra of single, nanometer-sized quantum wells possible with excellent sensitivity and selectivity while avoiding the spectral broadening associated with some alternative techniques. The scheme combines optical pumping and pulsed radiofrequency excitation of the nuclei with time-resolved detection of the free induction decay through the polarization of the photoluminescence.

Aluminum Compounds↗
PubMed2004

Simulation of emission molecular spectra by a semi-automatic programme package: the case of C2 and CN diatomic molecules emitting during laser ablation of a graphite target in nitrogen environment.

Some emission spectra of diatomic molecules were calculated by a semi-automatic programme package in order to infer the rotational and vibrational temperatures in Boltzmann distribution by comparing them with the corresponding experimental ones. The calculation procedure was applied in the case of CN radical and C2 molecule whose optical emission spectra were recorded during pulsed excimer laser ablation of a graphite target in low-pressure nitrogen environment. Computed similar or dissimilar values of rotational and vibrational temperatures let to verify the existence or not of local thermodynamic equilibrium and to hypothesise the temporal range necessary to establish it in such experiments.

Carbon↗
PubMed2005

Improving implicit solvent simulations: a Poisson-centric view.

Recent developments in implicit solvent models may be compared in terms of accuracy and computational efficiency. Based on improvements in the accuracy of generalized Born methods and the speed of Poisson-Boltzmann solvers, it appears that the two techniques are converging to a point at which both will be suitable for simulating certain types of biomolecular systems over sizable time and length scales.

Algorithms↗