bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics study of water clusters, liquid, and liquid–vapor interface of water with many-body potentials
Authors: Dang Liem X. , Chang Tsun-Mei
Published: July 2002

The molecular dynamics computer simulation technique is used to develop a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the thermodynamic and structural properties of the liquid and the liquid/vapor interface of water. The minimum energy configurations and the binding energies for these clusters are in reasonable agreement with accurate electronic structure calculations. The model predicts that the water trimer, tetramer, and pentamer have cyclic planar minimum energy structures. A prismlike structure is predicted to be lowest in energy for the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The computed thermodynamic properties for the model, at room temperature, including the liquid density, the enthalpy of vaporization, as well as the diffusion coefficient, are in excellent agreement with experimental values. Structural properties of liquid water, such as the radial distribution functions, neutron, and x-ray scattering intensities, were calculated and critically evaluated against the experimental measurements. In all cases, we found the agreement between the observed data and the computed properties to be quite reasonable. The computed density profile of the water’s liquid/vapor interface shows that the interface is not sharp at a microscopic level and has a thickness of 3.2 Å at 298 K. These results are consistent with those reported in earlier work on the same systems. The calculated surface tension at room temperature is in reasonable agreement with the corresponding experimental data. As expected, the computed average dipole moments of water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to their bulk values.
A simple model for Coulombic systems. Thermodynamics, correlation functions and criticality
Authors: Stafiej J. , Badiali J. P.
Published: July 2002

A simple Landau–Ginzburg Hamiltonian is proposed to study an example of critical behavior in ionic systems. We focus on the role of asymmetry between ions which is common in real systems. The free energy calculated via a functional integration contains a divergence due to the Coulombic self energy. The finite part is just the Debye–Huckel limiting law with the correction due to the asymmetry. The criticality in our model is associated with an attractive non-Coulombic interaction between ions whatever their sign. At the level of our model there is no criticality for the restricted primitive model. The correlation functions are calculated. Out of the criticality the charge–charge correlation function conforms to the Stillinger–Lovett conditions and the charge–density coupling vanishes in the long wavelength limit. At the criticality due to the asymmetry the effective density of free charge carriers has to be reduced to verify the second Stillinger–Lovett condition and the charge-density correlation function exhibits a long wavelength coupling.
Electron–electron–nuclear three-spin mixing in spin-correlated radical pairs
Authors: Jeschke Gunnar
Published: July 2002

Electron–electron–nuclear three-spin mixing occurs in radical pairs in solid-state matrices if the radicals feature a significant electron–electron spin coupling and an anisotropic hyperfine coupling. The perturbation of nuclear frequencies by the electron–electron spin coupling and the three-spin mixing have to be generally considered in the calculation of transition frequencies and probabilities in such radical pairs. Analytical descriptions of three-spin mixing for different ratios of the spin Hamiltonian parameters are introduced. It is found that nuclear frequencies are strongly perturbed if the difference of the Zeeman energies of the two electron spins is matched to half the hyperfine coupling and that three-spin mixing is maximum, if also the nuclear Zeeman frequency matches the former two interactions. Such double matching situations may be encountered for pairs of organic radicals under the conditions where transient electron spin resonance (ESR) experiments are usually performed. If three-spin mixing is significant, spin-correlated radical pairs are born in a state that features nuclear coherence in addition to the electron spin zero-quantum coherence that is created irrespective of this mixing. The possibility is discussed to detect such chemically induced nuclear coherence (CINC) in transient electron spin resonance experiments by selective microwave irradiation. It is shown that subsequent electron transfer reactions can yield chemically induced dynamic nuclear coherence (CIDNC) in isolated radicals and chemically induced dynamic nuclear polarization (CIDNP) in diamagnetic products if three-spin mixing is significant. The novel CINC, CIDNC, and CIDNP effects in the solid state might be used in the structure determination of spin-correlated radical pairs with applications to photosynthesis research.
A state-selected study of the ion–molecule reactions O+(4S,2D,2P)+N2
Authors: Li X. , Huang Y.-L. , Flesch G. D. , Ng C. Y.
Published: July 2002

Absolute state-selected cross sections for the reactions O+(4S,2D,2P)+N2→N2++O, NO++N, and N++NO (and/or N++N+O) have been measured in the center-of-mass collision energy (Ec.m.) range of 0.06–40 eV employing the differential retarding potential method and the O+(2D) and O+(2P) ion state-selection schemes we developed recently. Charge transfer is the overwhelming product channel for the O+(2D)+N2 and O+(2P)+N2 reactions. Contrary to the results of previous experiments, the charge transfer cross sections for O+(2P)+N2 are found to be 30%–100% greater than those for O+(2D)+N2. This observation suggests that N2 is an excellent quenching gas for O+(2D,2P). While the Ec.m. dependencies for the cross sections of NO+ from O+(4S)+N2 and O+(2D)+N2 are similar, exhibiting a broad maximum in the Ec.m. range of 1.5–8 eV, the cross section for NO+ from O+(2P)+N2 is found to decrease as Ec.m. is decreased. The N+ signal observed in the O+(4S)+N2 reaction is attributed to the formation of N++N+O. The pathway of O++N2→N++NO to generate N+ is strongly suggested as the major channel in the reactions of O+(2D,2P)+N2, as evidenced by the observation of N+ well below the thermochemical thresholds of O+(2D,2P)+N2→N++N+O.
Distributed approximating functional fit of the H3 ab initio potential-energy data of Liu and Siegbahn
Authors: Frishman Anatoli , Hoffman David K. , Kouri Donald J.
Published: July 2002

We report a distributed approximating functional (DAF) fit of the ab initio potential-energy data of Liu [J. Chem. Phys. 58, 1925 (1973)] and Siegbahn and Liu [ibid. 68, 2457 (1978)]. The DAF-fit procedure is based on a variational principle, and is systematic and general. Only two adjustable parameters occur in the DAF leading to a fit which is both accurate (to the level inherent in the input data; RMS error of 0.2765 kcal/mol) and smooth (“well-tempered,” in DAF terminology). In addition, the LSTH surface of Truhlar and Horowitz based on this same data [J. Chem. Phys. 68, 2466 (1978)] is itself approximated using only the values of the LSTH surface on the same grid coordinate points as the ab initio data, and the same DAF parameters. The purpose of this exercise is to demonstrate that the DAF delivers a well-tempered approximation to a known function that closely mimics the true potential-energy surface. As is to be expected, since there is only roundoff error present in the LSTH input data, even more significant figures of fitting accuracy are obtained. The RMS error of the DAF fit, of the LSTH surface at the input points, is 0.0274 kcal/mol, and a smooth fit, accurate to better than 1 cm−1, can be obtained using more than 287 input data points.
Study of the depolarized light scattering spectra of supercooled liquids by a simple mode-coupling model
Authors: Krakoviack V. , Alba-Simionesco C. , Krauzman M.
Published: July 2002

By using simple mode coupling equations, we investigate the depolarized light scattering spectra of two so-called “fragile” glassforming liquids, salol (phenylsalicylate) and CKN (Ca0.4K0.6(NO3)1.4), measured by Cummins and co-workers. Nonlinear integrodifferential equations for the time evolution of the density-fluctuations autocorrelation functions are the basic input of the mode coupling theory. Restricting ourselves to a small set of such equations, we fit the numerical solution to the experimental spectra. It leads to a good agreement between model and experiment, which allows us to determine how a real system explores the parameter space of the model, but it also leads to unrealistic effective vertices in a temperature range where the theory makes critical asymptotic predictions. We finally discuss the relevance and the range of validity of these universal asymptotic predictions when applied to experimental data on supercooled liquids.
Molecular dynamics simulations of flexible liquid crystal molecules using a Gay-Berne/Lennard-Jones model
Authors: Wilson Mark R.
Published: July 2002

Molecular dynamics simulations are described for liquid crystal molecules composed of two Gay-Berne particles connected by an eight-site Lennard-Jones alkyl chain. Calculations have been carried out for 512 molecules in the NVE and NPT ensembles for simulation times of up to 6.4 ns. The system exhibits the sequence of phases: isotropic liquid, smectic-A, smectic-B, and the simulations demonstrate the spontaneous growth of a smectic-A liquid crystal over a period of approximately 6 ns on cooling from the isotropic liquid. Model molecules are seen to remain flexible and able to change conformation in the smectic-A phase. As temperature is reduced molecules become elongated as the number of gauche conformations drops, leading to a small increase in the spacing of smectic layers. The latter is seen through the temperature dependence of the Gay-Berne radial distribution function resolved parallel to the direction of orientational order. Results are presented which show an odd–even variation of orientational order parameters for bonds in the alkyl chain, and a change in effective torsional potentials as the system is cooled from isotropic liquid to a smectic-A phase.
Ab initio theoretical study of dipole-bound anions of molecular complexes: (HF)3− and (HF)4− anions
Authors: Ramaekers Riet , Smith Dayle M. A. , Smets Johan , Adamowicz Ludwik
Published: July 2002

Ab initio calculations have been performed to determine structures and vertical electron detachment energy (VDE) of the hydrogen fluoride trimer and tetramer anions, (HF)3− and (HF)4−. In these systems the excess electron is bound by the dipole field of the complex. It was determined that, unlike the neutral complexes which prefer the cyclic structures, the equilibrium geometries of the anions have “zig–zag” shapes. For both complexes the predicted VDEs are positive [210 meV and 363 meV for (HF)3− and (HF)4−, respectively], indicating that the anions are stable systems with respect to the vertical electron detachment. These results were obtained at the coupled-cluster level of theory with single, double and triple excitations [CCSD(T) method; the triple-excitation contribution in this method is calculated approximately using the perturbation approach] with the anion geometries obtained using the second-order Mo/ller–Plesset perturbation theory (MP2) method. The same approach was also used to determine the adiabatic electron affinities (AEA) of (HF)3 and (HF)4. In addition to the electronic contribution, we also calculated the contributions (using the harmonic approximation) resulting from different zero-point vibration energies of the neutral and anionic clusters. The calculations predicted that while the AEA of (HF)3 is positive (44 meV), the AEA for (HF)4 is marginally negative (−16 meV). This suggests that the (HF)3− anion should be a stable system, while the (HF)4− is probably metastable.
Interaction of oxygen with Al(111) at elevated temperatures
Authors: Trost J. , Brune H. , Wintterlin J. , Behm R. J. , Ertl G.
Published: July 2002

The interaction of oxygen with Al(111) was investigated by STM at temperatures between 350 and 530 K, by annealing an oxygen precovered surface and by adsorption of oxygen on the hot surface. For exposures up to 10 L and temperatures up to 470 K a considerable part of the oxygen exists still in the chemisorbed state, another part transforms into Al oxide. In contrast to 300 K chemisorbed Oad atoms are mobile at elevated temperatures, and compact, hexagonal (1×1)Oad islands develop by an ordinary nucleation and growth scheme. This evidences attractive interactions between the oxygen atoms on (1×1) sites. From the lateral distribution of Oad islands a diffusion barrier of 1.0–1.1 eV is derived. The imaging of the islands of the (1×1) phase by STM depends on their size, which is understood by a different imaging of the Oad/Al adsorbate complexes at the island borders. Defects in the islands and bright features at the edges are interpreted as nuclei of aluminum oxide. Additional features which appear as topographic holes may be attributed to nonconducting Al oxide grains.
Ab initio model potential embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6
Authors: Al-Abdalla Abdalla , Barandiarán Zoila , Seijo Luis , Lindh Roland
Published: July 2002

In this paper we present the results of an ab initio model potential (AIMP) embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6; complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on CrCl63− and CrBr63− clusters embedded in ab initio model potential representations of the surrounding lattices Cs2NaYCl6  and Cs2NaYBr6. The experimental structural data are revisited and some new results are found which differ significantly from those available in the literature. The calculated local structure parameters and electronic transition energies which can be compared to experiments are found to be very good; new structural and spectroscopic results are produced which have been neither measured nor calculated, which are complementary to the available ones, and whose quality is expected to be high as well. In particular, the question of the competition of the excited-state absorptions with the potential vibronic laser emission has been adressed: A considerable overlap between the broad Eg2→A1g2 excited-state absorption and A2g4←4 T2g emission bands is predicted in both materials, which must result in a reduction in the emission efficiency. Finally, it is shown that the quantum mechanical embedding effects due to the fact that the external Cs+, Na+, Y3+, Cl−, and Br− ions are not point charges, are non-negligible; lacking of these effects must be one of the reasons which make previous Density Functional Theory calculations show significantly larger discrepancies with the available experiments.
Pump–probe diffraction imaging of vibrational wave functions
Authors: Geiser Joseph D. , Weber Peter M.
Published: July 2002

The theory of pump–probe diffraction experiments shows that probability density distributions of vibrational wave functions are experimentally observable. In the experiment a laser prepares a molecule in a selected vibrational state, in either the same or a different electronic manifold. The diffraction pattern of the molecule in the excited state is the Fourier transform image of the nuclear probability density distribution, as determined by the vibrational eigenfunction of the molecule. This suggests the possibility to directly observe components of molecular vibrational wave functions. Model calculations illustrate the concept on iodine molecules, and sodium dimers. The relevance to time-resolved pump–probe experiments that prepare vibrational wave packets is discussed.
A self-consistent version of quasidegenerate perturbation theory
Authors: Khait Yuriy G. , Hoffmann Mark R.
Published: July 2002

A new quasidegenerate perturbation theory is developed that describes the interactions of electronic states of interest with energetically low-lying excited states variationally and with more high-lying excited states perturbatively. The states of interest, the low-lying excited states and the more high-lying excited states, define primary, secondary, and external subspaces, respectively. The task of determination of the lowest solutions of the full configuration interaction (CI) problem is shown to be equivalent to the task of searching iteratively for an optimal primary subspace within the model space spanned by the initial unperturbed primary and secondary states. It is also shown that the present approach, which we refer to as the self-consistent quasidegenerate perturbation theory (SC-QDPT), theoretically satisfies the following criteria: (1) it avoids instabilities due to intruder states; (2) it ensures the additivity of the energy for noninteracting subsystems; (3) the projection of the correlated wave functions on the model space coincides with the optimal primary subspace; and (4) the energies of the primary states will be restricted below by the full CI limit. Furthermore, by use of an exponential ansatz the model space effective Hamiltonian takes into account finite-order primary-external perturbations exactly. Some of these conclusions are corroborated by the results of application of the lowest-order approximation, SC-QDPT(SD), of the method on the beryllium atom and on the reaction Be+H2, using a simple computer realization.
Thermodynamic and structural properties of the path-integral quantum hard-sphere fluid
Authors: Sesé Luis M.
Published: July 2002

An extensive study of the path-integral quantum hard-sphere fluid far from exchange is presented. The results cover the calculation of mechanical, thermal, r-space and k-space pair properties. Path-integral Monte Carlo simulations involving the Cao–Berne’s propagator provide the internal energies and pair radial distribution functions (instantaneous, linear response, and necklace center of mass). For the sake of comparison, Barker’s and Jaccuci–Omerti’s image propagators are also applied at several state points. To obtain k-space properties use of the Gaussian Feynman–Hibbs picture for representing quantum systems is made. This picture is known to yield two Ornstein–Zernike equations; one for true quantum particles (linear response) and the other for the centers of mass of quantum particles. Direct correlation functions and static structure factors are obtained via Baxter’s partition complemented with Dixon–Hutchinson’s variational procedure. Wherever possible, the present results are compared with semiclassical (Yoon–Scheraga’s) and path-integral (Runge–Chester’s) data available in the literature. The limits of validity of the Gaussian Feynman–Hibbs picture in this context are also established.
On the simulation of vapor–liquid equilibria for alkanes
Authors: Nath Shyamal K. , Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

A Monte Carlo simulation study has been conducted to assess the ability of recently proposed force fields to predict orthobaric densities, second virial coefficients, and P-V-T data for short and long alkanes. A new, modified force field is proposed that provides good agreement with experimental phase equilibrium and second virial coefficient data over wide ranges of temperature and chain length.
Gas diffusion in random binary media
Authors: Burganos Vasilis N.
Published: July 2002

The binary representation of porous media can provide a direct means for the characterization of their internal structure and is becoming progressively fashionable as a result of the rapid development of novel, powerful image analysis techniques. In the present work we provide accurate predictions of the Knudsen and intermediate diffusivities based on molecular trajectory computations in three-dimensional pixelized porous media. The main advantage of this approach is that it avoids resorting to the commonly used concepts of pore, grain, or fiber models, which introduce inevitably a significant degree of approximation to the actual structure. A second advantage is that this method is valid even far from the continuum limit where rarefied transport prevails. An analytical expression for the Knudsen diffusivity in random binary media is suggested that combines simplicity and accuracy over the entire porosity range. In the intermediate diffusion regime, it is found that the serial combination of the overall bulk and Knudsen diffusional resistances provides excellent estimates of the effective diffusional resistance and, hence, a Bosanquet type of approximation is quite valid. Finally, numerical results for the accessible porosity and specific surface area in random binary media are presented using large random realizations that, practically, eliminate statistical errors.
Alternating electrical conductivity of polyaniline
Authors: Bianchi R. F. , Leal Ferreira G. F. , Lepienski C. M. , Faria R. M.
Published: July 2002

The ac complex conductivity σ*(f) of polyaniline (PAN) films at different doping levels and different temperatures, in the 1–100 KHz frequency range, are reported. The results are typical of a disordered medium where the real component of ac conductivity is frequency independent at low frequencies, rising for higher values of frequencies. In order to interpret both the real and the imaginary components of σ*(f), we developed a model which considers the doped PAN as a disordered insulating matrix, sprinkled with conductive islands generated by doping, as indicated by energy dispersed x-ray microanalysis. The conduction through the insulating matrix obeys the random free energy barrier model, while in the conductive islands a metallic frequency-independent conductivity is considered. From the fittings we obtained the activation energy value of the maximum energy barrier of the doping mechanism and estimated the concentration of hopping sites.
Photodissociating trimethylamine at 193 nm to probe dynamics at a conical intersection and to calibrate detection efficiency of radical products
Authors: Forde Nancy R. , Morton Melita L. , Curry Stephen L. , Wrenn S. Jarrett , Butler Laurie J.
Published: July 2002

This paper reports crossed laser-molecular beam scattering experiments measuring the photofragment velocities and product branching in the photodissociation of trimethylamine (N(CH3)3) at 193 nm. We have observed two primary N–CH3 bond fission channels that ultimately produce different nitrogen-containing species, CH3+N(CH3)2 (X̃ 2B1); CH3+N(CH3)2*→CH3+CH2NCH3+H. The data also indicate that a third minor channel may contribute to the dissociation dynamics, CH3+N(CH3)2†→CH3+NC2H4+H2. The experiments show that ground state N(CH3)2 radicals are formed in the photodissociation, in sharp contrast to the exclusive production of NH2 (Ã 2A1) in a similar molecule, methylamine. We discuss how this results from the differing dynamics through the S1–S0 conical intersection in the exit channel in these two dissociating amines. We also use the photodissociation results to calibrate the mass spectrometric sensitivity at the m/e=15 daughter ion for methyl radicals vs N(CH3)2 and CH2NCH3 products. This provides the only necessary calibration to determine an absolute branching ratio in any system producing a methyl radical in one reaction channel and N(CH3)2 (X̃ 2B1) or CH2NCH3 in other reaction channels.
Influence of particle shape on the nematic—isotropic transition of colloidal platelet systems
Authors: Bates Martin A.
Published: July 2002

We study the nematic–isotropic transition in model colloidal systems composed of platelets of various shapes using grand canonical simulations. This is of relevance for recently synthesized hard platelet systems, since the platelets in such systems are found to be not circular but irregular hexagons, and so cannot be described by the simulation data currently available. We show that the coexistence densities scale with an effective volume related to the isotropic orientation-averaged excluded volume of a pair of platelets. This excluded volume can be obtained from the perimeter of the face of the particles and so can be easily calculated for both regular and irregular particle shapes.
Transport and spectroscopy of the hydrated proton: A molecular dynamics study
Authors: Vuilleumier Rodolphe , Borgis Daniel
Published: July 2002

In order to study the microscopic nature of the hydrated proton and its transport mechanism, we have introduced a multi-state empirical valence bond model, fitted to ab-initio results [J. Phys. Chem. B 102, 4261 (1998) and references therein]. The model makes it possible to take into account an arbitrary number N of valence states for the system proton+water and the electronic ground-state is obtained by diagonalization of a N×N interaction matrix. The resulting force field was applied to the study, at low computational cost, of the structure and dynamics of an excess proton in liquid water. The quantum character of the proton is included by means of an effective parametrization of the model using a preliminary path-integral calculation. In the light of the simulations, the mechanism of proton transfer is interpreted as the translocation of a privileged H5O2+ structure along the hydrogen bond network, with at any time a special O–H+–O bond, rather than a series of H3O++H2O→H2O+H3O+ reactions. The translocation of the special bond can be described as a diffusion process with a jump time of 1 ps on average and distributed according to a Poisson law. A time dependent correlation function analysis of the special pair relaxation yields two times scales, 0.3 and 3.5 ps. The first time is attributed to the interconversion between a delocalized (H5O2+-like) and a localized (H9O4+-like) form of the hydrated proton within a given special pair. The second one is the relaxation time of the special pair, including return trajectories. The computed diffusion constant (8×10−5 cm2/s) as well as the isotopic substitution effect (1.15), are in good agreement with experiment. The broad infrared absorption spectrum which characterizes the excess proton in liquid water is also computed and interpreted. The main contribution to the broad bands between 1000 and 1800 cm−1 is a combination of the bends and asymmetric O–H+ stretch of the H5O2+ complex. The continuum of absorption between 2000 and 3000 cm−1 is attributed to the interconversion between symmetric and asymmetric structures within a given special bond.
Vibrational and structural properties of OH adsorbed on Pt(111)
Authors: Bedürftig K. , Völkening S. , Wang Y. , Wintterlin J. , Jacobi K. , Ertl G.
Published: July 2002

OH species adsorbed on Pt(111) were studied in a combined investigation using scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). OH was formed by two different reactions, by reaction of H2O with O, and as an intermediate in the reaction of O with hydrogen to H2O. In both cases, two ordered OH phases were observed, a (√3×√3)R30° and a (3×3) structure, for which models are proposed. Both structures have OH coverages of 2/3, and their formation is driven by hydrogen bond formation between the adparticles; the OH adsorption site is most likely on top. OH molecules at defects in the adlayer, in particular at island edges, are spectroscopically distinguishable and contribute significantly to the vibrational spectra in disordered OH layers. This is important for the water formation reaction, where the OH islands are small. The discrepancies between previous HREELS studies on OH can be explained by the different degree of order under the various formation conditions.
Development of a linear response theory based on a state-specific multireference coupled cluster formalism
Authors: Chattopadhyay Sudip , Mahapatra Uttam Sinha , Mukherjee Debashis
Published: July 2002

We present in this paper a linear response theory based on our recently developed state-specific multireference coupled cluster (SS-MRCC) method to compute excited state energies for systems whose ground state has a pronounced multireference character. The SS-MRCC method is built on complete active space reference functions, and is designed to treat quasidegeneracy of varying degrees while bypassing the intruder problem. The linear response theory based on such a function [multireference coupled cluster based linear response theory (MR-CCLRT)] offers a very convenient access to computation of excited states and, in particular, to generation of potential energy surfaces (PES) for excited states where a traditional response formulation based on a single reference theory will fail due to the quasidegeneracy at some regions of the PES and the effective Hamiltonian-based multireference response methods would be plagued by intruders. An attractive feature of the MR-CCLRT is that the computed excitation energies are size intensive in the sense that they become asymptotically equal to the sum of fragment excitation energies in the limit of noninteracting fragments. Illustrative numerical results are presented for the excited state PES of the rectangular H4 (P4) model, the trapezoidal H4 (H4) model, for Li2, and for some sample points on the excited states PES of the BeH2 complex. The ground states of all the three examples possess quasidegeneracy at some point on the PES, and there are potential intruders at some other points in the PES, and hence are appropriate to test the efficacy of the MR-CCLRT. A comparison with the (CI) full configuration interaction and MR-CCLRT results in the same basis for all the molecules shows very good performance of the theory in general, and indicates the efficacy of the method.
Structure sensitivity and cluster size convergence for formate adsorption on copper surfaces: A DFT cluster model study
Authors: Hu Zhenming , Boyd Russell J.
Published: July 2002

The structure sensitivity and cluster size convergence for formate adsorption on the Cu(100), Cu(110) and Cu(111) surfaces have been investigated systematically using density functional theory and the cluster model containing up to 40 Cu atoms. The copper core–valence correlation effect on the adsorbate–surface interaction is examined by using three different basis sets and effective core potentials. The calculated geometries and vibrational frequencies are in good agreement with experimental data even on the small clusters and are not surface sensitive. However, the adsorption energies show strong dependence on the surface structure and the cluster size. The adsorption energies are shown to converge very well for the large clusters, and the activity of the Cu planes for formate adsorption is in the order of Cu(110)>Cu(100)>Cu(111), the same as that observed experimentally for methanol synthesis. Regardless of the basis set, cluster size and surface structure, all results show an anionic formate adsorption species. The chemisorption mechanism and the local structure of formate on the three copper surfaces are essentially very similar. Some discussion about cluster modeling is presented.
The dynamics in polyethyleneoxide–alkali iodide complexes investigated by neutron spin-echo spectroscopy and molecular dynamics simulations
Authors: Mos B. , Verkerk P. , Pouget S. , van Zon A. , Bel G.-J. , de Leeuw S. W. , Eisenbach C. D.
Published: July 2002

We determined the self part of the intermediate scattering function in liquid polyethyleneoxide (PEO) and PEO–alkali iodide complexes by means of neutron spin-echo spectroscopy and molecular dynamics (MD) computer simulations. We present the first accurate quantitative results on the segmental dynamics in the time range up to 1 ns and the wave-vector range from a few nm−1 to approximately 20 nm−1. We investigate the influence of polymer chain length, salt concentration, and cation type. We find that the neutron data and MD data for pure PEO agree very well. A relatively small concentration of dissolved salt (1 metal ion per 15 monomers) leads to a slowing down of the segmental motions by an order of magnitude. Here, the MD simulations agree qualitatively. Increasing the chain length from 23 to 182 monomers has no significant effect except at the highest salt concentration. Similarly, changing the cation from Li to Na hardly makes any difference. The Rouse model does not adequately describe our data.
Electrohydrodynamic stability of two cylindrical interfaces under the influence of a tangential periodic electric field
Authors: El Dabe Nabil T. , El Shehawey El Sayed F. , Moatimid Galal M. , Mohamed Abou El Magd A.
Published: July 2002

The electrohydrodynamic stability of two cylindrical interfaces influenced by a periodic tangential field is studied. The model allows for general forms of deformations of the interfaces. Two simultaneous ordinary differential equations of the Mathieu type are obtained. The coupled equations are solved by the method of multiple scales and stability conditions are discussed. It is found that the constant tangential field has a stabilizing effect while the tangential periodic field has a stabilizing influence except at resonance points. Graphs are drawn to illustrate the resonance regions in a parameter space. It is also found that the thickness of the jet plays a role in the stability criterion. The frequency of the modulated field can be used to control the position of the resonance regions. The special cases of large modulation and small modulation are also examined. It is found that for large modulation the electric field exhibits an enhanced destabilizing influence.
A class of linearizable models and generation of material response functions to nonlinear hyperbolic heat conduction
Authors: Fusco D. , Manganaro N.
Published: July 2002

An approach is proposed for reducing to linear form 2×2 quasilinear nonhomogeneous systems of partial differential equations of first order. In the process, classes of material response functions to governing models of physical interest can be determined. An application to nonlinear hyperbolic heat conduction is given.
A trace identity and its application to integrable systems of 1 + 2 dimensions
Authors: Tu G. Z. , Andrushkiw R. I. , Huang X. C.
Published: July 2002

A trace identity is proposed and it is shown that this identity can be effectively applied to establish the Hamiltonian structure of the KP, DS hierarchies and a new hierarchy of (1+2)-dimensional systems.
The quantum Heisenberg group H(1)q
Authors: Celeghini Enrico , Giachetti Riccardo , Sorace Emanuele , Tarlini Marco
Published: July 2002

The structure of the quantum Heisenberg group is studied in the two different frameworks of the Lie algebra deformations and of the quantum matrix pseudogroups. The R-matrix connecting the two approaches, together with its classical limit r, are explicitly calculated by using the contraction technique and the problems connected with the limiting procedure discussed. Some unusual properties of the quantum enveloping Heisenberg algebra are shown.
The squeezed state approach of the semiclassical limit of the time-dependent Schrödinger equation
Authors: Combescure M.
Published: July 2002

It is by now well recognized how the quantum time evolution for time-dependent Hamiltonians that are quadratic both in position and momentum variables generates ‘‘squeezing.’’ On the other hand, ‘‘squeezed states’’ and their ‘‘high-n generalizations’’ are known to provide a simple classical mechanical description of general quantum time evolution. In this paper, these ideas are combined to show the relevance of ‘‘generalized squeezed states’’ in the semiclassical limit of the evolution of time-dependent wave packets for general time-dependent Hamiltonians, including the multidimensional case.
Quantum logic and the histories approach to quantum theory
Authors: Isham C. J.
Published: July 2002

An extensive analysis is made of the Gell-Mann and Hartle axioms for a generalized ‘histories’ approach to quantum theory. Emphasis is placed on finding analogs of the lattice structure employed in standard quantum logic. Particular attention is given to ‘quasitemporal’ theories in which the notion of time-evolution in conventional Hamiltonian physics is replaced by something that is much broader; theories of this type are expected to arise naturally in the context of quantum gravity and quantum field theory in a curved space–time. The quasitemporal structure is coded in a partial semigroup of ‘temporal supports’ that underpins the lattice of history propositions. Nontrivial examples include quantum field theory on a non-globally-hyperbolic space–time, and a possible cobordism approach to a theory of quantum topology. A key result is the demonstration that the set of history propositions in standard quantum theory can be realized in such a way that each such proposition is represented by a genuine projection operator. This gives valuable insight into the possible lattice structure in general history theories.
Super-Toda lattices
Authors: van der Lende Epco
Published: July 2002

The Lax formalism, as described by Oevel et al. and in an earlier and more fundamental form by Semenov, Kostant, Symes, and Adler, can easily be generalized to the case where anticommuting variables are involved, the so-called supercase. In this article this super-Lax formalism is applied to the well-known associative superalgebra G=Mat(m,n,Λ). Subspaces of G to which the super-Poisson structures can be restricted arise in a natural way. Taking L in one of these subspaces formally leads to superextensions of the hierarchy of nonrelativistic Toda lattices. In the simplest case, where only nearest-neighbor interaction is involved, the equations are explicitly solved. Furthermore, the relevant two super-Hamiltonian structures are explicitly calculated. Finally a superextension of the relativistic Toda lattice with a super-Hamiltonian structure is described herein.
The noncommutative geometry of the quantum Hall effect
Authors: Bellissard J. , van Elst A. , Schulz- Baldes H.
Published: July 2002

An overview of the integer quantum Hall effect is given. A mathematical framework using nonommutative geometry as defined by Connes is prepared. Within this framework, it is proved that the Hall conductivity is quantized and that plateaux occur when the Fermi energy varies in a region of localized states.
The complete Kepler group can be derived by Lie group analysis
Authors: Nucci M. C.
Published: July 2002

It is shown that the complete symmetry group for the Kepler problem, as introduced by Krause, can be derived by Lie group analysis. The same result is true for any autonomous system.
Symmetrizable quantum affine superalgebras and their representations
Authors: Zhang R. B.
Published: July 2002

Aspects of the algebraic structure and representation theory of the quantum affine superalgebras with symmetrizable Cartan matrices are studied. The irreducible integrable highest weight representations are classified, and shown to be deformations of their classical counterparts. It is also shown that Jimbo-type quantum affine superalgebras can be obtained by deforming universal enveloping algebras of ordinary (i.e., nongraded) affine algebras supplemented by certain parity operators.
The algebra of two symmetric matrices: Proving completeness and deriving syzygies for a set of invariants of the Riemann tensor
Authors: Bonanos S.
Published: July 2002

A large number of Riemann tensor invariants can be written as traces of products of two 3×3 matrices, representing the Weyl tensor and the Weyl-like square of the Ricci tensor. It is pointed out that finding a complete set, ℐ, for all of these invariants is a simple consequence of the more general problem of finding a complete set of symmetric matrices, ℳ, in terms of which all symmetric matrix polynomials in these two matrices can be expressed. Such a set is constructed and a formal proof of its completeness is given. Several matrix identities and a scalar syzygy, obtained recently by Sneddon, are rederived and their interrelationships clarified. They are shown to be, ultimately, consequences of the Cayley–Hamilton theorem. A “minimal set” of invariants, that must be contained in the complete set of invariants of the general problem, is identified and it is concluded that no set proposed so far is complete.
q
-Trinomial identities
Authors: Warnaar S. Ole
Published: July 2002

We obtain connection coefficients between q-binomial and q-trinomial coefficients. Using these, one can transform q-binomial identities into q-trinomial identities and back again. To demonstrate the usefulness of this procedure we rederive some known trinomial identities related to partition theory and prove many of the conjectures of Berkovich, McCoy and Pearce, which have recently arisen in their study of the φ2,1 and φ1,5 perturbations of minimal conformal field theory.
On the integrability of linear and nonlinear partial differential equations
Authors: Fokas A. S.
Published: July 2002

A new method for studying boundary value problems for linear and for integrable nonlinear partial differential equations (PDE’s) in two dimensions is reviewed. This method provides a unification as well as a significant extension of the following three seemingly different topics: (a) The classical integral transform method for solving linear PDE’s and several of its variations such as the Wiener–Hopf technique. (b) The integral representation of the solution of linear PDE’s in terms of the Ehrenpreis fundamental principle. (c) The inverse spectral (scattering) method for solving the initial value problem for nonlinear integrable evolution equations. The detailed implementation of the method is presented for: (a) An arbitrary linear dispersive evolution equation on the half line. (b) The nonlinear Schrödinger equation on the half line. (c) The Laplace, Helmholtz and modified Helmholtz equations in an arbitrary convex polygon. In addition, several other applications are briefly considered. The possible extension of this method to multidimensions is also discussed.
The investigation into the Schwarz–Korteweg–de Vries equation and the Schwarz derivative in (2+1) dimensions
Authors: Toda Kouichi , Yu Song-Ju
Published: July 2002

In this note, we shall introduce a new integrable equation and the Schwarz derivative in (2+1) dimensions. First we show the existence of the Lax pair for an equation which has the relation to the Schwarz–Korteweg–de Vries (SKdV) equation. Next we derive a new equation in (2+1) dimensions by using a well-known higher-dimensional manner to the Lax pair for the SKdV equation. The (2+1) dimensional Schwarz derivative is defined here. Finally we briefly discuss various results which we have obtained about the new equation.
Perturbation by and recovery from bend curvature of a fully developed turbulent pipe flow
Authors: Anwer M. , So R. M. C. , Lai Y. G.
Published: July 2002

An experimental investigation was carried out to study the perturbation by and recovery from bend curvature of a fully developed turbulent pipe flow. The pipe Reynolds number and the pipe-to-bend radius ratio were 50 000 and 0.077, respectively. Hot-wire techniques were used to measure the three components of mean velocity and the six components of the Reynolds stress along a horizontal as well as along a normal plane in streamwise locations extending from 18 diameters upstream to 18 diameters downstream of the U-bend. Results show that a Dean-type secondary motion is established in the bend and is confirmed by a numerical modeling of the flow based on a two-equation closure. Evidence of a second cell is obtained within the bend and again verified by numerical results and previous total pressure measurements. Therefore it is postulated that the pressure field is also responsible for the second cell. The second cell disappears at about one diameter downstream of the curved bend. Turbulent kinetic energy is greatly enhanced by additional production due to extra strain rates created by bend curvature. Since viscous dissipation is the only way in which this extra energy can be destroyed, recovery from bend curvature takes a long distance.
Sedimentation of a dilute suspension
Authors: Rubinstein Jacob , Keller Joseph B.
Published: July 2002

Two methods, based upon the idea of multiple scattering, are used to calculate the sedimentation velocity of a dilute suspension of identical rigid spheres. One method is valid when the correlation length l of the distribution of spheres is large compared to the sphere radius a. For dilute simple cubic arrays it yields Hasimoto’s [J. Fluid Mech. 5, 317 (1959)] result. The second method applies for all values of l/a. For a dilute uniform homogeneous distribution it yields Batchelor’s [J. Fluid Mech. 52, 245 (1972)] result. Both methods apply to other distributions and to other flows of dilute suspensions.
A periodic grain consolidation model of porous media
Authors: Larson R. E. , Higdon J. J. L.
Published: July 2002

Calculations are presented for a periodic grain consolidation model of porous media. The model is an extension of previous work on lattices of spheres, in which the radius of the spheres is allowed to increase past the point of close touching to form a consolidated medium. A collocation method is used for the solution of Stokes flow in terms of Lamb’s general solution in spherical coordinates. Excellent accuracy is achieved with only moderate computational effort. At low concentrations up to the close touching limit excellent agreement is found with the earlier calculations of Zick and Homsy [J. Fluid Mech. 115, 13 (1982)]. For high concentrations above the close touching limit, an asymptotic theory is presented that agrees within a few percent with the numerical computations over the entire consolidated range.
Droplet distributions from the breakup of a cylindrical liquid jet
Authors: Chin L. P. , LaRose P. G. , Tankin R. S. , Jackson T. , Stutrud J. , Switzer G.
Published: July 2002

A phase/Doppler particle analyzer is used to measure the size and velocity distributions of the droplets generated by the disintegration of a cylindrical liquid jet. This type of liquid jet breakup is commonly called Rayleigh breakup. Metered liquid flow rates agree with the rates computed from the droplet measurements made with the phase/Doppler particle analyzer. The maximum entropy principle is used to predict the droplet size and velocity distributions. The constraints imposed in this model involve conservation of mass, momentum, surface energy, and kinetic energy. Agreement between measurements and predictions is very good.
Wall shear stress modulation in unsteady turbulent channel flow with high imposed frequencies
Authors: Tardu F. Sedat , Binder Gilbert
Published: July 2002

Results concerning the modulation of the wall shear stress in an unsteady channel flow are presented. The imposed frequency is increased by up to seven times the mean bursting frequency. The imposed amplitude varies from 10% to 60% of the mean centerline velocity. The characteristics of the modulation of the wall shear stress τ̃ and of the turbulent wall shear stress intensity τ′τ′̃ are discussed. The time mean flow is insensitive to the forcing. There is a coexistence of viscous Stokes flow with a fully developed turbulent mean flow for imposed frequencies f such that 0.02uτ2/ν<ω=2πf<0.045uτ2/ν. The flow leaves the Stokes regime at ω≳0.045uτ2/ν. Although the amplitude of the modulation of the turbulent wall shear stress intensity at first decreases from the quasisteady limit, it surprisingly increases when ω≳0.045uτ2/ν. The time lag of τ′τ′̃ with respect to τ̃ remains constant at 130 wall units for ω<0.014uτ2/ν, but it decreases for higher imposed frequencies.