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At least 1,351 records · Page 75Linked to original sources
PubMed2004

Efficient simulation of one-dimensional quantum many-body systems.

We present a numerical method to simulate the time evolution, according to a generic Hamiltonian made of local interactions, of quantum spin chains and systems alike. The efficiency of the scheme depends on the amount of entanglement involved in the simulated evolution. Numerical analysis indicates that this method can be used, for instance, to efficiently compute time-dependent properties of low-energy dynamics in sufficiently regular but otherwise arbitrary one-dimensional quantum many-body systems. As by-products, we describe two alternatives to the density matrix renormalization group method.

Journal Article↗
PubMed2006

Accurate computational determination of the binding energy of the SO3 x H2O complex.

Reliable thermochemical data for the reaction SO3 + H2O<-->SO3 x H2O (1a) are of crucial importance for an adequate modeling of the homogeneous H2SO4 formation in the atmosphere. We report on high-level quantum chemical calculations to predict the binding energy of the SO3 x H2O complex. The electronic binding energy is accurately computed to De = 40.9+/-1.0 kJ/mol = 9.8+/-0.2 kcal/mol. By using harmonic frequencies from density functional theory calculations (B3LYP/cc-pVTZ and TPSS/def2-TZVP), zero-point and thermal energies were calculated. From these data, we estimate D0 = -Delta H(1a)0(0 K) = 7.7+/-0.5 kcal/mol and Delta H(1a)0(298 K) = -8.3+/-1.0 kcal/mol.

Journal Article↗
PubMed2006

New force field for calcium binding sites in annexin-membrane complexes.

For accurate classical molecular dynamics (MD) simulations of the calcium mediated bound complexes of annexin and membrane we have developed new force-field parameters correctly describing the interaction of the Ca ion with its environment. We have used quantum chemical calculations to investigate the potential energy surface experienced by the Ca ion within the three different binding sites found in domain 1 of annexin V (ANX V/1). Based on these calculations we were able to quantify the charge polarization of atoms within the binding sites, and to determine the geometry and force constants of harmonic restraints between the Ca ion and its coordinating oxygen atoms. Harmonic restraints were introduced to compensate for the deviations between the quantum mechanical potential energy surface and that of the classical force field. Our analysis has shown that using the refined force field for the Ca binding sites enables long-time MD simulations that conserve the initial structure of ANX V/1 significantly better than MD simulations using the standard force field.

Annexin A5↗
PubMed2004

Reconciling semiclassical and Bohmian mechanics. I. Stationary states.

The semiclassical method is characterized by finite forces and smooth, well-behaved trajectories, but also by multivalued representational functions that are ill behaved at caustics. In contrast, quantum trajectory methods--based on Bohmian mechanics (quantum hydrodynamics)--are characterized by divergent forces and erratic trajectories near nodes, but also well-behaved, single-valued representational functions. In this paper, we unify these two approaches into a single method that captures the best features of both, and in addition, satisfies the correspondence principle. Stationary eigenstates in one degree of freedom are the primary focus, but more general applications are also anticipated.

Algorithms↗
PubMed2004

Consistent description of quantum Brownian motors operating at strong friction.

A quantum Smoluchowski equation is put forward that consistently describes thermal quantum states. In particular, it notably does not induce a violation of the second law of thermodynamics. This so modified kinetic equation is applied to study analytically directed quantum transport at strong friction in arbitrarily shaped ratchet potentials that are driven by nonthermal two-state noise. Depending on the mutual interplay of quantum tunneling and quantum reflection these quantum corrections can induce both, a sizable enhancement or a suppression of transport. Moreover, the threshold for current reversals becomes markedly shifted due to such quantum fluctuations.

Biological Transport↗
PubMed2004

Management of pediatric radiation dose using Agfa computed radiography.

Radiation dose to patients and its management have become important considerations in modern radiographic imaging procedures, but they acquire particular significance in the imaging of children. Because of their longer life expectancy, children exposed to radiation are thought to have a significantly increased risk of radiation-related late sequelae compared to adults first exposed to radiation later in life. Therefore, current clinical thinking dictates that dose in pediatric radiography be minimized, while simultaneously ensuring sufficient diagnostic information in the image, and reducing the need for repeat exposures. Dose management obviously starts with characterization and control of the exposure technique. However, it extends farther through the imaging chain to the acquisition system, and even to the image processing techniques used to optimize acquired images for display. Further, other factors, such as quality control procedures and the ability to handle special pediatric procedures, like scoliosis exams, also come into play. The need for dose management in modern radiography systems has spawned a variety of different solutions, some of which are similar across different manufacturers, and some of which are unique. This paper covers the techniques used in Agfa Computed Radiography (CR) systems to manage dose in a pediatric environment.

Child↗
PubMed1980

[Determination of effective atomic number and electronic density by computed tomography with dual energies. Application to the thalamus (author's transl)].

The authors have studied seven thalamic hyperdense lesions of five patients in managing, at the same level, successively two scans with a computer tomograph at two different energies. After they recall their methodology of testing their C.T. and computations, they have found the effective atomic number and the electron density of these lesions. All (one Fahr disease and six old hemorrhages) have both high effective atomic number and electron density. They discuss the limits of this technic but they emphasize the great interest to see the chemical aspects of the structures observed in C.T.

Brain Diseases↗
PubMed2006

Computational study of the process of hydrogen bond breaking: the case of the formamide-formic acid complex.

MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p) quantum calculations are used to study the formamide-formic acid complex (FFAC), a system bound by two hydrogen bonds, N--H...O and O--H...O, that forms a bond ring at equilibrium. When the intermolecular separation between monomers R increases, this ring opens at a distance for which the weaker N--H...O bond breaks remaining the stronger O--H...O bond. The computational study characterizes that process addressing changes of interaction energy DeltaE, structure and properties of the electron density rho(r) as well as spatial distributions of rho(r), the electrostatic potential U(r), and the electron localization function eta(r). It is shown that the spatial derivatives of DeltaE, the topology of rho(r), and qualitative changes noticed in U(r) = 0 isocontours allow to identify a precise distance R for which one can say the N--H...O hydrogen bond has broken. Both levels of theory predict essentially the same changes of structure and electron properties associated to the process of breaking and virtually identical distances at which it takes place.

Computer Simulation↗
PubMed2003

Quantum and thermal fluctuations in the SU(N) Heisenberg spin-glass model near the quantum critical point.

We solve for the SU(N) Heisenberg spin glass in the limit of large N focusing on small S and T. We study the effect of quantum and thermal fluctuations in the frequency dependent response function and observe interesting transfers of spectral weight. We compute the T dependence of the order parameter and the specific heat and find an unusual T2 behavior for the latter at low temperatures in the spin-glass phase. We find remarkable qualitative agreement with various experiments on the quantum frustrated magnet SrCr(9p)Ga(12-9p)O19.

Journal Article↗
PubMed2005

Accurate CSA measurements from uniformly isotopically labeled biomolecules at high magnetic field.

Obtaining chemical shift anisotropy (CSA) principal values from large biomolecular systems is often a laborious process of preparing many singly isotopically labeled samples and performing multiple independent CSA measurements. We present CSA tensor principal values measured in the biomolecular building blocks tyrosine.HCl, histidine.HCl, and all-E-retinal in both isotopically labeled and unlabeled forms at 17.6 T. The measured tensor values are identical for most carbon sites despite significant dipolar couplings between the spins. Quantum mechanical simulations of an arbitrary three spin system were used to evaluate the accuracy of direct CSA measurement as a function of applied magnetic field strength and molecular parameters. It was found that for a CSA asymmetry of 0.2 or more, an accurate measure of the CSA parameters is obtained when the CSA anisotropy is more than six times the largest dipolar coupling in frequency units. If the CSA asymmetry is more than 0.5, this requirement is relaxed, and accurate results are obtained if the anisotropy is more than three times the dipolar coupling. While these limits are insufficient for measurement of CSA's for alpha-carbons and aliphatic sidechain sites in proteins at current field strengths, they open the way for routine systematic CSA measurements of sites with relatively large CSA tensor values in extensively isotopically labeled biomolecules in widely available magnetic fields.

Anisotropy↗
PubMed2006

Reactions of OH and NO radicals with 1,1-dichloroethylene in argon matrices. FTIR and theoretical studies.

HONO/1,1-dichloroethylene/Ar matrices were subjected to UV radiation (lambda > 340 nm) from a medium pressure mercury lamp. The products of the photolysis were studied experimentally by means of FTIR spectroscopy and theoretically using the ab initio MP2 method. Two conformers of 2-nitroso-2,2-dichloroethanol molecule have been identified as the final products of the double addition reaction of the OH, NO radicals to 1,1-dichloroethylene. The additional reactive species observed in the matrix is tentatively identified as an 1,1-dichloro-2-hydroxyethyl radical, an intermediate formed by single addition of OH to 1,1-dichloroethylene. The three photoproducts have been identified and observed for the first time. The identities of the products have been justified by comparison with the experiments with deuterated DONO and by performing concentration and annealing studies as well as by reference to the spectral data of related molecules. The results of the quantum mechanical calculations confirmed both the assignment of the new molecules and mechanism of the reaction observed in our experiment.

Argon↗
PubMed2005

Derivation of quantum langevin equation from an explicit molecule-medium treatment in interaction picture.

A quantum mechanical form of the Langevin equation is derived from an explicit consideration of the molecule-medium interaction, as advocated by Simons in 1978, and by using two identities in the interaction picture. This can be easily reduced to the classical regime, and further simplified to the macroscopic Langevin equation by considering the stochastic Langevin force autocorrelation function. One of the so-called Einstein relations appears as a byproduct. By following the methodology proposed by Simons, an exact expression for the momentum autocorrelation function is obtained. The latter can be used to calculate the zero-frequency macroscopic diffusion coefficient that is observed to satisfy the second Einstein relation. The formalism described above gives rise to the possibility of explicitly computing the transport characteristics such as friction constant and diffusion coefficient from the corresponding quantum statistical mechanical expressions. A discussion on the Langevin equation becomes complete only when the corresponding Fokker-Planck equation is obtained. Therefore, the probability of the evolution of states with a particular absolute magnitude of linear momentum from those of another momentum eigenvalue is quantum mechanically defined. This probability appears as a special average value of a projection operator and as a special projection operator correlation function. A classical identity is introduced that is shown to be valid also for the quantum mechanically defined probability function. By using this identity, the so-called Fokker-Planck equation for the evolution probability is easily established.

Journal Article↗
PubMed2006

Neutron phase imaging and tomography.

We report how a setup consisting of three gratings yields quantitative two- and three-dimensional images depicting the quantum-mechanical phase shifts of neutron de Broglie wave packets induced by the influence of macroscopic objects. Since our approach requires only a little spatial and chromatic coherence it provides a more than 2 orders of magnitude higher efficiency than existing techniques. This dramatically reduces the required measurement time for computed phase tomography and opens up the way for three-dimensional investigations of previously inaccessible quantum-mechanical phase interactions of neutrons with matter.

Journal Article↗
PubMed2001

Photosynthetic performance of the Atlantic brown macroalgae, Cystoseira abies-marina, Dictyota dichotoma and Sargassum vulgare, measured in Gran Canaria on site.

Photosynthetic performance was determined in three common Atlantic brown macroalgae, Cystoseira abies-marina, Dictyota dichotoma and Sargassum vulgare, in Gran Canaria, Canary Islands, on site. The photosynthetic quantum yield was measured with both a portable PAM instrument on site and a diving PAM under water in the habitat. In parallel, solar radiation was measured continuously above and under water by means of two three-channel dosimeters, ELDONET (Real Time Computer, Möhrendorf, Germany), in three wavelength ranges, UV-A, UV-B and PAR. The effective photosynthetic quantum yield decreased in all species in response to exposure to 15 min of solar radiation but recovered in the subsequent shade conditions within several hours. A 30-min exposure caused an even more profound photoinhibition from which the algae recovered only partially. Most of the effect was due to visible radiation, however, the UV wavelength range, and especially UV-B, considerably enhanced the decrease in photosynthetic quantum yield. In all species except Sargassum a significant photoinhibition was detected at their growth sites at high solar angles in the water column, measured with the diving PAM.

Journal Article↗
PubMed2006

Analysis of the vibrational spectra of new OH-containing E-4-arylmethylene-3-isochromanones and 3-arylcoumarins.

A combined experimental and theoretical approach is presented to structural characterization of fairly large, newly synthesized organic molecules in order to enhance the effectiveness of their instrumental analysis by vibrational spectroscopy. The method consists of measurement of FT-IR and Raman spectra of the reaction products and subsequent ab initio or DFT quantum mechanical calculations (prediction) of the vibrational spectra for any anticipated structural varieties of the synthesized molecules. Comparison of the measured and computed frequencies as well as the observed and simulated spectra is performed to resolve any uncertainties in identifying the reaction products. Vibrational frequency and normal mode calculations based on scaled quantum mechanical (SQM) force fields performed at the DFT/B3LYP/6-31G* level of theory are demonstrated to provide a wealth of information that have been used in this work to ascertain the molecular structure, probable conformation and H-bond properties of three new isochromanone or coumarin derivatives, namely: 3-([2'-hydroxymethyl]-phenyl)-coumarin (1), E-4-(3'-hydroxyphenylmethylene)-3-isochromanone (2), and 2-[(2'-hydroxymethyl)phenyl]-3H-naphto[2,1-b]pyran-3-one (3).

Chromones↗
PubMed1996

Biology and the measurement problem.

Present-day molecular biology, despite its name, is almost entirely committed to a macroscopic, classical picture of the organism; one in which quantum aspects play no role, except as a source of noise. Particularly is this true when dealing with informational aspects; especially "genetic information". The pervading metaphor here is an identification of "genetic information" with DNA sequence, and thence with program or software. We take a quite different view herein. If we presume, to the contrary, that microphysical processes play a role in primary genetic processes, then the "information" they can convey consists of observables evaluated on states. It is then natural to analogize a complex, consisting of (observed system + observer) with the biological partition between genome (observed system) and phenotype (observer). Such a picture immediately raises the deep issues surrounding "the measurement problem" in quantum mechanics. In our brief consideration of such matters, we suggest that standard quantum mechanics is too narrow to deal with the biological pictures, because it is inexorably tied to quantifications of classical, conservative systems; there is no such for an organism. Rather, we are led to consider subsystems we call "sites", for which there is in principle no Hamiltonian. We then query the extent to which such "genetic information" is already subsumed in traditional observables a physicist would measure in vitro in a laboratory. We suggest there is no reason to believe that "genetic information", manifested in bioactivities, is reducible to these. Finally, we contrast this view of "genetic information" with more traditional ideas of program and computability. We argue that computability (algorithms) are entirely classical concepts, in a physical sense, and quite inadequate for a biology (or even a physics) in which quantum measurement processes are important.

Computational Biology↗
PubMed2001

Signal velocity, causality, and quantum noise in superluminal light pulse propagation.

We consider pulse propagation in a linear anomalously dispersive medium where the group velocity exceeds the speed of light in vacuum ( c) or even becomes negative. A signal velocity is defined operationally based on the optical signal-to-noise ratio, and is computed for cases appropriate to the recent experiment where such a negative group velocity was observed. It is found that quantum fluctuations limit the signal velocity to values less than c.

Journal Article↗