Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “quantum computing”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,171 records · Page 65Linked to original sources
PubMed2005

Ab initio molecular dynamics simulations of an excited state of X(-)(H(2)O)(3) (X = Cl, I) complex.

Upon excitation of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters, the electron transfers from the anionic precursor to the solvent, and then the excess electron is stabilized by polar solvent molecules. This process has been investigated using ab initio molecular dynamics (AIMD) simulations of excited states of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters. The AIMD simulation results of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) are compared, and they are found to be similar. Because the role of the halogen atom in the photoexcitation mechanism is controversial, we also carried out AIMD simulations for the ground-state bare excess electron -- water trimer [e(-)(H(2)O)(3)] at 300 K, the results of which are similar to those for the excited state of X(-)(H(2)O)(3) with zero kinetic energy at the initial excitation. This indicates that the rearrangement of the complex is closely related to that of e(-)(H(2)O)(3), whereas the role of the halide anion is not as important.

Bromides↗
PubMed2005

Theoretical study of vibrational and optical spectra of methylene-bridged oligofluorenes.

We report a systematic characterization of methylene-bridged fluorene oligomers constructed of two, four, six, and eight aromatic rings using time-dependent density functional theory (TDDFT), the ab initio approximate coupled-cluster singles and doubles (CC2) method, and semiempirical spectroscopic Zerner's intermediate neglect of differential overlap method (ZINDO/S). Geometry optimizations have been performed for the ground state and for the first electronically excited state. Vertical excitations and the fluorescence transition from the lowest excited state have been calculated. Computed ground-state geometries and infrared spectra for fluorene are in good agreement with experimental results. The RI-CC2 and ZINDO/S absorption and fluorescence spectra agree very well with the available experimental data for studied fluorene oligomers and for para oligophenylenes films. On the other hand, TDDFT calculations underestimate excitation and fluorescence energies systematically for larger systems (N > 4) in comparison with the above-mentioned results. The effective conjugation length was estimated to 13-14 repeat units. The computed radiative lifetimes for the fluorene molecule show good agreement with experiment within realistic expectations. The decrease of the radiatiave fluorescence lifetime with the increase in the conjugation length has been discussed also.

Computer Simulation↗
PubMed2006

Oxygen atom transfer energetics: assessment of the effect of method and solvent.

Several density functional methods, the semiempirical methods AM1 and PM3, Hartree-Fock, and Gaussian3 theories were applied to compute the oxygen atom transfer enthalpies for 14 X/XO couples (inorganic and organic systems, charged and neutral species, light and heavy main group element containing molecules). The calculated reaction enthalpies were compared to available experimental data. The G3 method alone was found to perform within the experimental error, while the popular B3LYP and BLYP functionals provided inadequate results. Solvent effects were estimated for 19 neutral and anionic X/XO couples by using the conductor-like polarizable continuum model and several cavity models coupled with the B3LYP/6-31++G(2d,2p) level of theory. Surprisingly, the magnitude of the aqueous solvent correction was found to vary significantly for different solute cavity models, occasionally giving larger errors than the gas-phase calculation.

Computer Simulation↗
PubMed2006

Theoretical investigation of the temperature dependence of the fifth-order Raman response function of fluid and liquid xenon.

The temperature dependence of the fifth-order Raman response function, R(5)(t1,t2), is calculated for fluid xenon by employing a recently developed time-correlation function (TCF) theory. The TCF theory expresses the two-dimensional (2D) Raman quantum response function in terms of a two-time, computationally tractable, classical TCF. The theory was shown to be in excellent agreement with existing exact classical MD calculations for liquid xenon as well as reproducing line shape characteristics predicted by earlier theoretical work. It is applied here to investigate the temperature dependence of the fifth-order Raman response function in fluid xenon. In general, the characteristic line shapes are preserved over the temperature range investigated (for the reduced temperature points T* = 0.5, 1.0, and 2.0); differences in the signal decay times and a large decline in intensity with decreasing temperature (and associated anharmonicity) are observed. In addition, there are some signature features that were not observed in earlier results for T* = 1. The most dramatic difference in line shape is observed for the polarization condition, xxzzxx, that shows a vibrational echo peak. In contrast, the fully polarized signal changes mainly in magnitude.

Journal Article↗
PubMed2006

Nonadiabatic molecular dynamics simulations of correlated electrons in solution. 1. Full configuration interaction (CI) excited-state relaxation dynamics of hydrated dielectrons.

The hydrated dielectron is composed of two excess electrons dissolved in liquid water that occupy a single cavity; in both its singlet and triplet spin states there is a significant exchange interaction so the two electrons cannot be considered to be independent. In this paper and the following paper,we present the results of mixed quantum/classical molecular dynamics simulations of the nonadiabatic relaxation dynamics of photoexcited hydrated dielectrons, where we use full configuration interaction (CI) to solve for the two-electron wave function at every simulation time step. To the best of our knowledge, this represents the first systematic treatment of excited-state solvation dynamics where the multiple-electron problem is solved exactly. The simulations show that the effects of exchange and correlation contribute significantly to the relaxation dynamics. For example, spin-singlet dielectrons relax to the ground state on a time scale similar to that of single electrons excited at the same energy, but spin-triplet dielectrons relax much faster. The difference in relaxation dynamics is caused by exchange and correlation: The Pauli exclusion principle imposes very different electronic structure when the electrons' spins are singlet paired than when they are triplet paired, altering the available nonadiabatic relaxation pathways. In addition, we monitor how electronic correlation changes dynamically during nonadiabatic relaxation and show that solvent dynamics cause electron correlation to evolve quite differently for singlet and triplet dielectrons. Despite such differences, our calculations show that both spin states are stable to excited-state dissociation, but that the excited-state stability has different origins for the two spin states. For singlet dielectrons, the stability depends on whether the solvent structure can rearrange to create a second cavity before the ground state is reached. For triplet dielectrons, in contrast, electronic correlation ensures that the two electrons do not dissociate, even if the dielectron is artificially kept from reaching the ground state. In addition, both singlet and triplet dielectrons change shape dramatically during relaxation, so that linear response fails to describe the solvation dynamics for either spin state. In the following paper (Larsen, R. E.; Schwartz, B. J. J. Phys. Chem. B 2006, 110, 9692), we use these simulations to calculate the pump-probe spectroscopic signal expected for photoexcited hydrated dielectrons and to predict an experiment to observe hydrated dielectrons directly.

Computer Simulation↗
PubMed2006

A theoretical study of properties and reactions involving arsenic and selenium compounds present in coal combustion flue gases.

Species of arsenic and selenium thought to be present in coal combustion flue gases were studied using density functional theory and a broad range of ab initio methods. At each level of theory, the calculated geometries and vibrational frequencies of each species as well as the reaction enthalpies of anticipated reactions were compared with experimental data where available. Comparisons between each calculation are given along with a discussion of the better performance of some theoretical calculations for a given species/reaction.

Air Pollutants↗
PubMed2006

Double proton transfer reactions with plateau-like transition state regions: pyrazole-trifluoroacetic acid clusters.

The double proton transfer reactions between carboxylic acids and pyrazole were studied by computational methods up to the coupled-cluster level. Introduction of substituents allowed for a systematic modulation of the reaction profile, resulting in imaginary frequencies of the associated transition states between -1180 and -45 cm(-1). In the latter case, a local transition state is replaced by an extremely flat (plateaulike) transition region, which constitutes the transition from a concerted toward a stepwise mechanism. Vertical excitation energies along the reaction path reveal that the feature of a plateau in the ground state is mirrored in the excited states.

Hydrogen Bonding↗
PubMed2005

Six-dimensional potential energy surface for H2 at Ru(0001).

The six-dimensional (6D) potential energy surface (PES) for the H(2) molecule interacting with a clean Ru(0001) surface has been computed accurately for the first time. Density functional theory (DFT) and a pseudopotential based periodic plane-wave approach have been used to calculate the electronic interactions between the molecule and the surface. Two different generalized gradient approximation (GGA) exchange-correlation functionals, PW91 and RPBE, have been adopted. Based on the DFT/GGA calculated potential energies, an analytical 6D PES has been constructed using the corrugation reducing procedure. A very accurate representation of the DFT/GGA data has been achieved, with an average error in the interpolation of about 3 meV and a maximum error not larger than about 30 meV. The top site is found to be the most reactive site for both functionals used, but PW91 predicts a higher reactivity than RPBE, with lower-energy and earlier-located dissociation barriers. The energetic corrugation displayed by the RPBE PES is larger than the PW91 PES while the geometric corrugation is smaller. The differences between the two PESs increase as the distance of the molecular center of mass to the surface decreases. A direct comparison with experimental investigations on H(2)/Ru(0001) could shed light on the suitability of these XC potentials often used in DFT calculations.

Adsorption↗
PubMed2006

Molecular simulation of adsorption of alkanes in sodium MOR-type zeolites using a new force field.

The applicability of a recently proposed force field of Calero et al. (J. Am. Chem. Soc., 2004, 126, 11377) to Na-MOR zeolites is evaluated. The Henry law coefficients of ethane and C(5)-C(9) as well as the adsorption isotherms of ethane, propane, butane, and hexane in various Na-MOR zeolites are computed and compared with experimental values. These comparisons show that the new force field is suitable for Na-MOR zeolites. Furthermore, this force field is used to study the effects of sodium cations on the adsorption behavior of larger alkanes, such as C(4)-C(7), in MOR-type zeolites. These simulations give a better understanding of the underlying mechanisms of the cations' position and density influence on adsorption. In addition, a characteristic pressure named "reversal pressure" is introduced which characterizes the efficiency of the presence of cations in zeolites.

Adsorption↗
PubMed2005

Intermolecular multiple quantum coherences at high magnetic field: the nonlinear regime.

Experiments have been carried at magnetic-field strengths of 9.4, 14.1, and 17.6 T to explore the evolution of intermolecular multiple quantum coherences in the nonlinear regime where the system evolves for times that are much greater than the characteristic time of action of the long-range dipolar field, tau(d). The results show the expected Bessel function form of the recorded signal as a function of time of evolution, with evident zeros and sign changes. As expected, the rate of signal evolution increases at higher-field strengths as a result of the increased equilibrium magnetization. A numerical method for calculating the evolution of magnetization under the action of the distant dipolar field, relaxation, and diffusion that is based on Fourier analysis of the magnetization distribution has been applied to the correlated two-dimensional spectroscopy revamped by asymmetric z-gradient echo detection sequence in the nonlinear regime and shown to produce results that are in good agreement with experimental data acquired at different magnetic fields and rates of spatial modulation. Experiments and simulations have also been used to explore the evolution of magnetization in a mixture of two interacting spin species in the nonlinear regime.

Algorithms↗
PubMed2006

Energetics of formation of TiGa3As4 and TiGa3P4 intermediate band materials.

Using density functional theory quantum methods, total energy values and vibrational properties have been computed, and thermodynamic properties evaluated, for Ti-substituted GaAs and GaP, proposed as candidates for intermediate band photovoltaic cells. The calculations predict that the formation of these materials from the binary compounds implies an increase in total energy (that is ascribed largely to the change in coordination undergone by Ti, from six-fold to four-fold), and thus phase separation rather than mixed compound formation would be favored. However, the mentioned increase is not larger (for the arsenide case it is actually smaller) than that predicted for Mn-substituted GaAs, a material which has been experimentally made, and therefore the obtention of these Ti-substituted materials is expected to be feasible as well. Vibrational and disorder entropy contributions to the formation free energy of the ternary compounds have been also computed; they compensate partially for the total energy increase, and indicate that the thermodynamic feasibility of the materials synthesis improves for low Ti concentrations and high temperature conditions.

Journal Article↗
PubMed2006

Efficiency analysis of reaction rate calculation methods using analytical models I: The two-dimensional sharp barrier.

We analyze the efficiency of different methods for the calculation of reaction rates in the case of a simple two-dimensional analytical benchmark system. Two classes of methods are considered: the first is based on the free energy calculation along a reaction coordinate and the calculation of the transmission coefficient, the second on the sampling of dynamical pathways. We give scaling rules for how this efficiency depends on barrier height and width, and we hand out simple optimization rules for the method-specific parameters. We show that the path sampling methods, using the transition interface sampling technique, become exceedingly more efficient than the others when the reaction coordinate is not the optimal one.

Algorithms↗
PubMed2005

Linearized path integral approach for calculating nonadiabatic time correlation functions.

We show that quantum time correlation functions including electronically nonadiabatic effects can be computed by using an approach in which their path integral expression is linearized in the difference between forward and backward nuclear paths while the electronic component of the amplitude, represented in the mapping formulation, can be computed exactly, leading to classical-like equations of motion for all degrees of freedom. The efficiency of this approach is demonstrated in some simple model applications.

Biophysics↗
PubMed2005

High-resolution imager for digital mammography: physical characterization of a prototype sensor.

The physical performance characteristics of a high-resolution sensor module for digital mammography were investigated. The signal response of the imager was measured at various detector entrance air kerma and was found to be linear. The spatial resolution was determined by measuring the presampling modulation transfer function, MTF(f), of the system. The noise power spectra, NPS(f), of the system were estimated using 26 kVp: Mo/Mo, 28 kVp: Mo/Rh and 30 kVp: Rh/Rh, with polymethyl methacrylate (PMMA) 'tissue equivalent material' of thickness 20, 45 and 57 mm for each of three x-ray spectra at detector entrance air kerma in the range between approximately 80.2 and 92.3 microGy. The noise equivalent quanta, NEQ(f), and detective quantum efficiencies, DQE(f), for the various spectral conditions were computed. In addition, dose dependence of NPS(f) and DQE(f) was studied at various detector entrance air kerma ranging from 9.4 to 169.7 microGy. A spatial resolution of about 10 cycles mm(-1) was obtained at the 10% MTF(f) level. A small increase in NEQ(f)was observed under higher energy spectral conditions while the DQE(f) decreased marginally. For a given spectrum, increasing PMMA filtration produced negligible change in DQE(f). The estimated DQE values at zero frequency were in the range between 0.45 and 0.55 under the conditions investigated in this study.

Equipment Design↗
PubMed2006

Magnon decay in gapped quantum spin systems.

In the continuum O(3) sigma-model description of gapped spin systems, S = 1 magnons can only decay into three lower energy magnons. We argue that the symmetry of the quantum spin Hamiltonian often allows decay into two magnons, and compute this decay rate in model systems. Two-magnon decay is present in Haldane gap S = 1 spin chains, even though it cannot be induced by any allowed term written in powers and gradients of the sigma-model field. We compare our results with recent measurements of Stone et al. on a two-dimensional spin system.

Journal Article↗
PubMed2000

Clinical Consequences of New Insights in the Pathophysiology of Disorders of Iron and Heme Metabolism.

This review examines the clinical consequences for the practicing hematologist of remarkable new insights into the pathophysiology of disorders of iron and heme metabolism. The familiar proteins of iron transport and storage-transferrin, transferrin receptor, and ferritin-have recently been joined by a host of newly identified proteins that play critical roles in the molecular management of iron homeostasis. These include the iron-regulatory proteins (IRP-1 and -2), HFE (the product of the HFE gene that is mutated in most patients with hereditary hemochromatosis), the divalent metal transporter (DMT1), transferrin receptor 2, ceruloplasmin, hephaestin, the "Stimulator of Fe Transport" (SFT), frataxin, ferroportin 1 and others. The growing appreciation of the roles of these newly identified proteins has fundamental implications for the clinical understanding and laboratory evaluation of iron metabolism and its alterations with iron deficiency, iron overload, infection, and inflammation. In Section I, Dr. Brittenham summarizes current concepts of body and cellular iron supply and storage and reviews new means of evaluating the full range of body iron stores including genetic testing for mutations in the HFE gene, measurement of serum ferritin iron, transferrin receptor, reticulocyte hemoglobin content and measurement of tissue iron by computed tomography, magnetic resonance imaging and magnetic susceptometry using superconducting quantum interference device (SQUID) instrumentation. In Section II, Dr. Weiss discusses the improved understanding of the molecular mechanisms underlying alterations in iron metabolism due to chronic inflammatory disorders. The anemia of chronic disorders remains the most common form of anemia found in hospitalized patients. The network of interactions that link iron metabolism with cellular immune effector functions involving pro- and anti-inflammatory cytokines, acute phase proteins and oxidative stress is described, with an emphasis on the implications for clinical practice. In Section III, Dr. Brissot and colleagues discuss how the diagnosis and management of hereditary hemochromatosis has changed following the identification of the gene, HFE, that is mutated in most patients with hereditary hemochromatosis, and the subsequent development of a genotypic test. The current understanding of the molecular effects of HFE mutations, the usefulness of genotypic and phenotypic approaches to screening and diagnosis and recommendations for management are summarized.

Journal Article↗
PubMed2005

External beam radiotherapy as curative treatment of prostate cancer.

External beam radiotherapy (RT) has been used as a curative treatment of prostate cancer for more than 5 decades, with the "modern" era emerging more than 3 decades ago. Its history is marked by gradual improvements punctuated by several quantum leaps that are increasingly driven by advancements in the computer and imaging sciences and by its integration with complementary forms of treatment. Consequently, the contemporary use of external beam RT barely resembles its earliest form, and this must be appreciated in the context of current patient care. The influence of predictive factors on the use and outcomes of external beam RT is presented, as is a selected review of the methods and outcomes of external beam RT as a single therapeutic intervention, in association with androgen suppression, or as a postoperative adjunct. Thus, the "state of the (radiotherapeutic) art" is presented to enhance the understanding of this treatment approach with the hope that this information will serve as a useful resource to physicians as they care for patients with prostate cancer.

Androgen Antagonists↗