bluuebunny/crossref_metadata_2025 huggingface dataset:
Geomorphic analysis of tidal creek networks
Authors: Novakowski Karyn Inez , Torres Raymond , Gardner L. Robert , Voulgaris George
Published: May 2004

The purpose of this study is to determine if concepts in terrestrial channel network analysis provide insight on intertidal creek network development and to present new metrics for their analysis. We delineated creek network geometry using high‐resolution digital images of intertidal marsh near Georgetown, South Carolina. Analyses reveal that intertidal creek networks may be topologically random. Length‐area relationships suggest that salt marsh and terrestrial networks have similar scaling properties, although the marsh networks are more elongate than terrestrial networks. To account for recurrent water exchange between creek basins at high tide, we propose that the landscape unit of geomorphic analyses should be the salt marsh island as opposed to salt marsh creek drainage basin area. Using this approach, the relationship between maximum channel length per island and island area is well described by a power function. A similar power relationship exists for cumulative channel length versus island area, giving a nearly unit slope; this implies that marsh islands have a spatially uniform drainage density. Since the island boundaries are easily identified in remote sensing, taking the island as the unit of geomorphic analysis will eliminate discrepancies in delineating basin boundaries and preclude the need for defining basin area in intertidal landscapes. Analyses and results presented here may be used to quantify salt marsh reference condition and provide indicator variables to assess salt marsh disturbance.
A sequential algorithm for testing climate regime shifts
Authors: Rodionov Sergei N.
Published: May 2004

Empirical studies of climate regime shifts typically use confirmatory statistical techniques with an a priori hypothesis about the timing of the shifts. Although there are methods for an automatic detection of discontinuities in a time series, their performance drastically diminishes at the ends of the series. Since all the methods currently available require a substantial amount of data to be accumulated, the regime shifts are usually detected long after they actually occurred. The proposed sequential algorithm allows for early detection of a regime shift and subsequent monitoring of changes in its magnitude over time. The algorithm can handle the incoming data regardless whether they are presented in the form of anomalies or absolute values. It can be easily used for an automatic calculation of regime shifts in large sets of variables.
Splitting of the Philippine Sea Plate and a magma chamber beneath Mt. Fuji
Authors: Aizawa Koki , Yoshimura Ryokei , Oshiman Naoto
Published: May 2004

Mt. Fuji is located in a tectonically unique area, but various aspects of the structure have not been fully explained. Here we show the results from a magnetotelluric survey across Mt. Fuji along a 70 km observation line. The profile shows that a conductive body is located between two resistive bodies at depths greater than 15 km. Low frequency earthquakes occur above the conductor. We interpret these results in a model where beneath Mt. Fuji, the subducting Philippine Sea Plate is split into two parts, and a magma chamber is located in the gap. Due to this unique structure, Mt. Fuji may be able to sustain a high basalt magma flux throughout its entire history.
On the effect of air humidity on soil susceptibility to wind erosion: The case of air‐dry soils
Authors: Ravi Sujith , D'Odorico Paolo , Over Thomas M. , Zobeck Ted M.
Published: May 2004

Wind erosion is a widespread process in drylands, and contributes to loss of soil fertility, alteration of atmospheric radiation, and air pollution. Erosion occurs when wind speed exceeds a certain threshold, which depends on a number of factors, including surface soil moisture. It is shown that in air‐dry soils surface moisture and threshold wind speed depend significantly on air humidity. Thus, in arid regions variations in surface soil moisture can be significantly affected by changes in atmospheric humidity, with an important effect on wind erosion potential. Wind tunnel tests were conducted to investigate this dependence of threshold velocity on air humidity in air‐dry soils. It was found that at these moisture levels, the threshold velocity decreases with an increase in air humidity. This result is explained by the effect of hygroscopic forces and by their dependence on soil matric potential in dry soils.
An experimental approach to correct the microwave radiometer wind speed affected by the change of the brightness temperature due to the relative wind direction
Authors: Konda Masanori , Shibata Akira
Published: May 2004

We try to correct the change of the brightness temperature of the spaceborne microwave radiometer caused by the angle between the wind and the sensor azimuth (relative wind direction) and to reduce the effect on the wind speed by an experimental way. We make the preliminary approach by comparing the SSM/I brightness temperature collocated with the QuikSCAT wind vector cell. The change of brightness temperature in 37/19 GHz in the horizontal polarization caused by wind is defined as the shift from the relationship between the horizontal and the vertical polarizations without surface wind (PS37/PS19). We propose two hopeful approaches to correct the brightness temperatures. By comparing the observed PS37/PS19 with the value in the PS37/PS19 tables, the correction is done without knowing the information of the in situ vector wind. The relative wind direction effect on the retrieved wind speed is almost halved by this approach.
Light‐driven cycling of dimethylsulfide (DMS) in the Sargasso Sea: Closing the loop
Authors: Toole Dierdre A. , Siegel David A.
Published: May 2004

The factors driving dimethylsulfide (DMS) cycling in oligotrophic environments are isolated using a time‐series of DMS sampled in the Sargasso Sea. The observed distribution of DMS is inconsistent with bottom‐up processes related to phytoplankton production, biomass, or community structure changes. DMS concentrations and estimates of net biological community production are most highly correlated with physical and optical properties, with the dose of ultraviolet radiation (UVR) accounting for 77% of the variability in mixed layer DMS concentrations. Physiological stresses associated with shallow mixed layers and high UVR are the first order determinant of biological production of DMS, indicating that DMS cycling in open‐ocean regions is fundamentally different than in eutrophic regions where phytoplankton blooms provide the conditions for elevated DMS concentrations. The stress regime presented here effectively closes the DMS‐climate feedback loop for open‐ocean environments. This response may also provide a climatic role for phytoplanktonic processes in response to anthropogenic forcing.
The origin of tectonic fabrics in “undeformed” clays: The early stages of deformation in extensional sedimentary basins
Authors: Cifelli F. , Mattei M. , Hirt A. M. , Günther A.
Published: May 2004

An integrated approach was used to detect tectonic effects in apparently undeformed fine‐grained clay sediments filling extensional basins in Southern Italy. Results from low and high field anisotropy of magnetic susceptibility and mineral fabric, deduced by neutron texture analysis, show penetrative deformation on a grain‐scale. The well‐defined tectonic magnetic lineation is parallel to the stretching direction and located at the intersection of the basal planes of phyllosilicates, forming grain‐scale folds and crenulations. This spatial configuration reflects basin scale deformation processes and suggests the existence of a nearly uniaxial stress field during the incipient extensional phases. The integrated approach of magnetic and mineral fabric investigations is a valid tool to detect grain scale and regional deformation pattern in weakly deformed extensional basins, where macroscopic evidence of deformation is not visible.
Increased exposure of Southern Ocean phytoplankton to ultraviolet radiation
Authors: Lubin Dan , Arrigo Kevin R. , van Dijken Gert L.
Published: May 2004

Satellite remote sensing of both surface solar ultraviolet radiation (UVR) and chlorophyll over two decades shows that biologically significant ultraviolet radiation increases began to occur over the Southern Ocean three years before the ozone “hole” was discovered. Beginning in October 1983, the most frequent occurrences of enhanced UVR over phytoplankton‐rich waters occurred in the Weddell Sea and Indian Ocean sectors of the Southern Ocean, impacting 60% of the surface biomass by the late 1990s. These results suggest two reasons why more serious impacts to the base of the marine food web may not have been detected by field experiments: (1) the onset of UVR increases several years before dedicated field work began may have impacted the most sensitive organisms long before such damage could be detected, and (2) most biological field work has so far not taken place in Antarctic waters most extensively subjected to enhanced UVR.
Metabolic changes in the basal ganglia of patients with Huntington's disease: an in situ hybridization study of cytochrome oxidase subunit I mRNA
Authors: Gourfinkel‐An Isabelle , Vila Miquel , Faucheux Baptiste , Duyckaerts Charles , Viallet François , Hauw Jean‐Jacques , Brice Alexis , Agid Yves , Hirsch Etienne C.
Published: May 2004

On the basis of the functional model of the basal ganglia developed in the 1980s and the neuropathological findings in Huntington's disease (HD), changes in the neuronal activity of the basal ganglia have previously been proposed to explain the abnormal movements observed in this pathology. In particular, it has been stated that the neurodegenerative process affecting the basal ganglia in the disease should provoke a hypoactivity in the internal segment of the pallidum (GPi) that could explain choreic movements observed in the disease. To test this functional hypothesis, we performed an in situ hybridization study on control and HD brains postmortem, taking cytochrome oxidase subunit I (COI) mRNAs expression as index of neuronal activity. As most of the HD patients studied were under chronic neuroleptic (NL) treatment, we also studied the brains of non‐HD patients under chronic NL treatment. Our results show that in HD brain the number of neurons expressing COI mRNA tends to be lower in the striatum, GPe and GPi, suggesting a severe involvement of these structures during the neurodegenerative process. Moreover, COI mRNA level of expression was markedly reduced within neurons of the putamen and GPe. Surprisingly, COI mRNA expression was not modified in the GPi in HD brains compared with controls. This paradoxical result in the GPi may be explained by the antagonistic effect of GPe hypoactivity and the degenerative process involving neurons of GPi. Our results indicate that the functional modifications, and consequently the pathophysiology of␣abnormal movements, observed in HD basal ganglia are more complex than expected from the currently accepted model of the basal ganglia organization.
Adsorption energies of molecular oxygen on Au clusters
Authors: Ding Xunlei , Li Zhenyu , Yang Jinlong , Hou J. G. , Zhu Qingshi
Published: May 2004

The adsorption properties of O2 molecules on anionic, cationic, and neutral Aun clusters (n=1–6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2–0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au3 and Au5 clusters can adsorb one O2 molecule.
Development of an improved four-site water model for biomolecular simulations: TIP4P-Ew
Authors: Horn Hans W. , Swope William C. , Pitera Jed W. , Madura Jeffry D. , Dick Thomas J. , Hura Greg L. , Head-Gordon Teresa
Published: May 2004

A re-parameterization of the standard TIP4P water model for use with Ewald techniques is introduced, providing an overall global improvement in water properties relative to several popular nonpolarizable and polarizable water potentials. Using high precision simulations, and careful application of standard analytical corrections, we show that the new TIP4P-Ew potential has a density maximum at ∼1 °C, and reproduces experimental bulk-densities and the enthalpy of vaporization, ΔHvap, from −37.5 to 127 °C at 1 atm with an absolute average error of less than 1%. Structural properties are in very good agreement with x-ray scattering intensities at temperatures between 0 and 77 °C and dynamical properties such as self-diffusion coefficient are in excellent agreement with experiment. The parameterization approach used can be easily generalized to rehabilitate any water force field using available experimental data over a range of thermodynamic points.
Photoluminescence studies of oligothiophene self-assembled monolayers at low excitation energy
Authors: Mishina E. , Tamura T. , Sakaguchi H. , Kulyuk L. , Nakabayashi S.
Published: May 2004

Photoluminescence spectroscopy studies have been performed on self-assembled monolayers (SAMs) on Au(111) of thiophene oligomers with the number of thiophen rings N=3 and N=4. Photoluminescence spectra of SAMs reveal excitonic behavior with different band resolution and temperature dependence. These differences are attributed to different SAMs structure (degree of ordering).
An efficient method for constructing nonorthogonal localized molecular orbitals
Authors: Feng Huasheng , Bian Jiang , Li Lemin , Yang Weitao
Published: May 2004

A new method for constructing nonorthogonal localized molecular orbitals (NOLMOs) is presented. The set of highly localized NOLMOs is obtained by minimization of the spread functional starting from an initial set of canonical orthogonal molecular orbitals. To enhance the stability and efficiency, the centroids of the NOLMOs are constrained to be those of the corresponding orthogonal localized molecular orbitals (OLMOs), which are obtained with the Boys criterion in advance. In particular, these centroid constraints make the optimization for each NOLMO independent of the others, which is an attractive feature for application to large systems. The minimization with the constraints incorporated through the multiplier-penalty function method is stable and efficient in convergence. While exhibiting the classical bonding pattern in chemistry and sharing a spatial distribution similar to that of the corresponding OLMOs, the obtained NOLMOs are more compact than the corresponding OLMOs with about 10%–28% reduction in the value of the spread functional and devoid of the troublesome “orthogonalization tails.”
Use of the ultraviolet absorption spectrum of CF2 to determine the spatially resolved absolute CF2 density, rotational temperature, and vibrational distribution in a plasma etching reactor
Authors: Bulcourt Nicolas , Booth Jean-Paul , Hudson Eric A. , Luque Jorge , Mok Daniel K. W. , Lee Edmond P. , Chau Foo-Tim , Dyke John M.
Published: May 2004

Broadband ultraviolet absorption spectroscopy has been used to determine CF2 densities in a plasma etch reactor used for industrial wafer processing, using the CF2 Ã 1B1←X̃ 1A1 absorption spectrum. Attempts to fit the experimental spectra using previously published Franck–Condon factors gave poor results, and values for the higher vibrational levels of the Ã state [(0,v2,0), with v2′>6] from the ground state were missing; hence new values were calculated. These were computed for transitions between low-lying vibrational levels of CF2 X̃ 1A1 to vibrational levels of CF2 Ã 1B1 (v1′,v2′,0) up to high values of the vibrational quantum numbers using high level ab initio calculations combined with an anharmonic Franck Condon factor method. The Franck Condon factors were used to determine the absorption cross sections of CF2 at selected wavelengths, which in turn were used to calculate number densities from the experimental spectra. Number densities of CF2 have been determined in different regions of the plasma, including the center of the plasma and outside the plasma volume, and CF2 rotational temperatures and vibrational energy distributions were estimated. For absorption spectra obtained outside the confined plasma volume, the CF2 density was determined as (0.39±0.08)×1013 molecule cm−3 and the vibrational and rotational temperatures were determined as 303 and 350 K, respectively. In the center of the plasma reactor, the CF2 density is estimated as (3.0±0.6)×1013 molecules cm−3 with Trot≈500 K. The fitted vibrational distribution in the CF2 ground state corresponds to two Boltzmann distributions with Tvib≈300 and Tvib≈1000 K, indicating that CF2 molecules are initially produced highly vibrationally excited, but are partially relaxed in the plasma by collision.
Calculation of the fourth-rank molecular hypermagnetizability of some small molecules
Authors: Pagola G. I. , Caputo M. C. , Ferraro M. B. , Lazzeretti P.
Published: May 2004

A computational scheme has been developed within the framework of Rayleigh–Schrödinger perturbation theory to evaluate nonlinear interaction energy contributions for a molecule in the presence of an external spatially uniform, time-independent magnetic field. Terms connected with the fourth power of the perturbing field, representing the fourth-rank hypermagnetizabilities of five small molecules, have been evaluated at the coupled Hartree–Fock level of accuracy within the conventional common-origin approach. Gaugeless basis sets of increasing size and flexibility have been employed in a numerical test, adopting two different coordinate systems to estimate the degree of convergence of theoretical tensor components.
Brownian dynamics simulations of needle chain and nugget chain polymer models—rigid constraint conditions versus infinitely stiff springs
Authors: Magne Ådland Hans , Mikkelsen Arne
Published: May 2004

It is well known that orientational correlations appear in polymer chain models when the subunits are linked by ball-socket joints implemented as rigid constraint conditions. These correlations do not appear when the subunits are connected by springlike potential forces, even in the limit of infinitely stiff springs. In a widely used class of algorithms for Brownian dynamics simulations, inertia effects are ignored. However, in the recently introduced needle chain and nugget chain algorithms, the rigid constraint correlations depend on the mass and moment of inertia. This inconsistency does not appear in the bead–rod (Kramers) polymer chain model, which also has orientational correlations introduced by rigid constraint conditions. Explicit expressions for the correlation functions are given for thermodynamic equilibrium states. Analytical expressions for the associated forces (“metric forces”) and simulation results showing how the rigid constraint correlations influence dynamical properties, are also presented. Further we discuss the physical relevance of these correlations and show via simulations that their influence on stationary and dynamical properties depend significantly on chain length. We further show that if the metric forces are removed, algorithms designed with rigid constraint conditions describe a chain of segments connected by infinitely stiff springs. Finally we show that the results presented here for needle chains are relevant also for the bead–rod (Kramers) chain model, making it possible to simulate a bead–spring chain with infinitely stiff springs.
The hydrophobic effect: Molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces
Authors: Jensen Morten Ø. , Mouritsen Ole G. , Peters Günther H.
Published: May 2004

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C36H74 or n-alcohol C35H71OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.
Phase behavior of aqueous solutions containing dipolar proteins from second-order perturbation theory
Authors: Tavares F. W. , Bratko D. , Striolo A. , Blanch H. W. , Prausnitz J. M.
Published: May 2004

Due to the interplay of Coulombic repulsion and attractive dipolar and van der Waals interactions, solutions of globular proteins display a rich variety of phase behavior featuring fluid–fluid and fluid-solid transitions that strongly depend on solution pH and salt concentration. Using a simple model for charge, dispersion and dipole-related contributions to the interprotein potential, we calculate phase diagrams for protein solutions within the framework of second-order perturbation theory. For each phase, we determine the Helmholtz energy as the sum of a hard-sphere reference term and a perturbation term that reflects both the electrostatic and dispersion interactions. Dipolar effects can induce fluid–fluid phase separation or crystallization even in the absence of any significant dispersion attraction. Because dissolved electrolytes screen the charge-charge repulsion more strongly than the dipolar attraction, the ionic strength dependence of the potential of mean force can feature a minimum at intermediate ionic strengths offering an explanation for the observed nonmonotonic dependence of the phase behavior on salt concentration. Inclusion of correlations between charge–dipole and dipole–dipole interactions is essential for a reliable calculation of phase diagrams for systems containing charged dipolar proteins and colloids.
On the closure conjectures for the Gibbsian approximation model of a binary droplet
Authors: Djikaev Y. S. , Napari Ismo , Laaksonen Ari
Published: May 2004

Within the framework of Gibbsian thermodynamics, a binary droplet is regarded to consist of a uniform interior and dividing surface. The properties of the droplet interior are those of the bulk liquid solution, but the dividing surface is a fictitious phase whose chemical potentials cannot be rigorously determined. The state of the nucleus interior and free energy of nucleus formation can be found without knowing the surface chemical potentials, but the latter are still needed to determine the state of the whole nucleus (including the dividing surface) and develop the kinetics of nucleation. Thus it is necessary to recur to additional conjectures in order to build a complete, thermodynamic, and kinetic theory of nucleation within the framework of the Gibbsian approximation. Here we consider and analyze the problem of closing the Gibbsian approximation droplet model. We identify μ- and Γ-closure conjectures concerning the surface chemical potentials and excess surface coverages, respectively, for the droplet surface of tension. With these two closure conjectures, the Gibbsian approximation model of a binary droplet becomes complete so that one can determine both the surface and internal characteristics of the whole nucleus and develop the kinetic theory, based on this model. Theoretical results are illustrated by numerical evaluations for binary nucleation in a water–methanol vapor mixture at T=298.15 K. Numerical results show a striking increase in the droplet surface tension with decreasing droplet size at constant overall droplet composition. A comparison of the Gibbsian approximation with density functional calculations for a model surfactant system indicate that the excess surface coverages from the Gibbsian approximation are accurate enough for large droplets and droplets that are not too concentrated with respect to the solute.
Product angular distributions in dissociative recombination
Authors: Guberman Steven L.
Published: May 2004

The dependence of the dissociative recombination cross section upon the angle between the incoming electron beam and the ion internuclear axis is determined for diatomic molecules. Product angular distributions are derived for the component partial waves of the Coulomb wave function. In agreement with earlier results for dissociative attachment, it is shown that in the slow rotation approximation, if electron capture is dominated by a single partial wave, the product angular distribution is given by the square of the absolute value of the partial wave spherical harmonic describing the incoming electron.
Mass spectrometry study of the fragmentation of valence and core-shell (Cl 2p) excited CHCl3 and CDCl3 molecules
Authors: Lago A. F. , Santos A. C. F. , de Souza G. G. B.
Published: May 2004

The dissociative photoionization of the chloroform and chloroform-d molecules has been studied in the valence region and around the chlorine 2p edge. Time-of-flight mass spectrometry in the coincidence mode—namely, photoelectron–photoion coincidence (PEPICO)—was employed. He I lamp and tunable synchrotron radiation were used as light sources. Total and partial ion yields have been recorded as a function of the photon energy. Singly, doubly, and triply ionized species have been observed below (195 eV), on (201 eV), and above (230 eV) the Cl 2p resonances. A definite degree of site-selective fragmentation was observed at the Cl 2p resonance as the relative contributions of several ionic species were seen to go through a maximum at 201 eV. At the same time all stable doubly charged ions were also observed at 198 eV (below the 2p resonances), resulting from direct ionization processes. Isotopic substitution is shown to provide a very efficient means of improving the mass resolution and assignment of unresolved peaks in spectra of CHCl3, particularly for those fragments differing by a hydrogen atom. It is suggested that ultrafast fragmentation of the system following 2p excitation to a strongly antibonding state contributes to the large amount of Cl+ observed in the PEPICO spectrum measured at 201 eV. Kinetic energy distributions were determined for the H+, D+, and Cl+ fragments.
Conformational kinetics of disaccharides in aqueous solutions
Authors: Hagen R. , Kaatze U.
Published: May 2004

Between 100 kHz and 2 GHz ultrasonic attenuation spectra of disaccharides in water have been measured at 25 °C. Some additional measurements have been performed at different temperatures between 10 and 35 °C and as a function of saccharide concentration c (0.5 mol/l⩽c⩽1.8 mol/l). The analytical description of the spectra in terms of relaxation spectral functions revealed four relaxation regimes. Three relaxation regions correspond to such revealed by monosaccharides in solution. These relaxations reflect a pseudorotation, an exocyclic hydroxymethyl group rotation, and probably a saccharide–saccharide association. The relaxation term with relaxation time between 3.6 and 19 ns (25 °C) is characteristic for the disaccharide solutions. It has been assigned to a rotation of the disaccharide rings relative to one another.
Low temperature extension of the generalized Zusman phase space equations for electron transfer
Authors: Zhang Ming-Liang , Zhang Shesheng , Pollak Eli
Published: May 2004

In a previous paper [J. Chem. Phys. 119, 11864 (2003)], we derived a set of two coupled equations which describe electron transfer in the presence of dissipation at high temperature. Employing the low temperature extension of the Fokker–Planck operator, suggested by Haake and Reibold [Phys. Rev. A 32, 2462 (1985)] and Ankerhold [Europhys. Lett. 61, 301 (2003)], we show that one may extend the generalized Zusman equations in a similar manner to low temperature. Numerical simulation shows that addition of the temperature-dependent term which couples the coordinate and momentum causes an increase in the electron transfer rate as compared to the rate obtained from the previous high temperature equations. The increase in the rate comes from the increase in the equilibrium variances of the coordinate and momentum. The low temperature quantum theory allows for higher energy portions of phase space to contribute to the electron transfer rate where the rate is higher thus enhancing the overall rate.
Ab initio molecular-dynamics study of supercritical carbon dioxide
Authors: Saharay Moumita , Balasubramanian Sundaram
Published: May 2004

Car–Parrinello molecular-dynamics simulations of supercritical carbon dioxide (scCO2) have been performed at the temperature of 318.15 K and at the density of 0.703 g/cc in order to understand its microscopic structure and dynamics. Atomic pair correlation functions and structure factors have been obtained and good agreement has been found with experiments. In the supercritical state the CO2 molecule is marginally nonlinear, and thus possesses a dipole moment. Analyses of angle distributions between near neighbor molecules reveal the existence of configurations with pairs of molecules in the distorted T-shaped geometry. The reorientational dynamics of carbon dioxide molecules, investigated through first- and second-order time correlation functions, exhibit time constants of 620 and 268 fs, respectively, in good agreement with nuclear magnetic resonance experiments. The intramolecular vibrations of CO2 have been examined through an analysis of the velocity autocorrelation function of the atoms. These reveal a red shift in the frequency spectrum relative to that of an isolated molecule, consistent with experiments on scCO2. The results have also been compared to classical molecular-dynamics calculations employing an empirical potential.
Flexible polyelectrolyte simulations at the Poisson–Boltzmann level: A comparison of the kink-jump and multigrid configurational-bias Monte Carlo methods
Authors: Tsonchev Stefan , Coalson Rob D. , Liu Anping , Beck Thomas L.
Published: May 2004

We present a new approach for simulating the motions of flexible polyelectrolyte chains based on the continuous kink-jump Monte Carlo technique coupled to a lattice field theory based calculation of the Poisson–Boltzmann (PB) electrostatic free energy “on the fly.” This approach is compared to the configurational-bias Monte Carlo technique, in which the chains are grown on a lattice and the PB equation is solved for each configuration with a linear scaling multigrid method to obtain the many-body free energy. The two approaches are used to calculate end-to-end distances of charged polymer chains in solutions with varying ionic strengths and give similar numerical results. The configurational-bias Monte Carlo/multigrid PB method is found to be more efficient, while the kink-jump Monte Carlo method shows potential utility for simulating nonequilibrium polyelectrolyte dynamics.
An ice phase of lowest thermal conductivity
Authors: Andersson Ove , Johari G. P. , Suga H.
Published: May 2004

On pressurizing at temperatures near 130 K, hexagonal and cubic ices transform implosively at 0.8–1 GPa. The phase produced on transformation has the lowest thermal conductivity among the known crystalline ices and its value decreases on increase in temperature. An ice phase of similar thermal conductivity is produced also when high-density amorphous ice kept at 1 GPa transforms on slow heating when the temperature reaches ∼155 K. These unusual formation conditions, the density and its distinguished thermal conductivity, all indicate that a distinct crystal phase of ice has been produced.
Correlation of femtosecond wave packets and fluorescence interference in a conjugated polymer: Towards the measurement of site homogeneous dephasing
Authors: Milota F. , Sperling J. , Szöcs V. , Tortschanoff A. , Kauffmann H. F.
Published: May 2004

Probing electronic femtosecond (fs) coherence among segmental sites that are congested by static and dynamic site disorder and subject to structural relaxation is a big, experimental challenge in the study of photophysics of poly(p-phenylenevinylene). In this work, fs-wave-packet fluorescence interferometry experiments are presented that measure macroscopic coherent kernels and their phase-relaxation in the low-temperature, bottom-state regime of the density-of-states below the migrational threshold energy where downhill site-to-site transfer is marginal. By using freely propagating and tunable 70 fs excitation/probing pulses and employing narrow-band spectral filtering of wave packets, fluorescence interferograms with strongly damped beatings can be observed. The coherences formally follow the in-phase superpositions of two site-optical free-induction-decays and originate from distinct pairs of coherent doorway-states, different in energy and space, each of them being targeted, by two discrete quantum-arrival-states 1α and 1β, via independent, isoenergetic 0→1 fluorescence transitions. The coherent transients are explained as site-to-site polarization beatings, caused by the interference of two fluorescence correlation signals. The numerical analysis of the damping regime, based upon second-order perturbational solutions, reveals the lower limit value of homogeneous dephasing in the range from T2≃100 fs to T2≃200 fs depending on the site-excitation energy of the bottom-states. The experiments enable to look into the formation of the relaxed state as a special molecular process of electron–phonon coupling and hence open-up a quite new perspective in the puzzle of multichromophore optical dynamics and structural relaxation in conjugated polymers.
Comment on “An exact quantum Monte Carlo calculation of the helium–helium intermolecular potential” [J. Chem. Phys. 115, 4546 (2001)]
Authors: Anderson James B.
Published: May 2004

We report a further reduction in the statistical error for “exact” quantum Monte Carlo calculations for the potential energy of interaction of two helium atoms. For the equilibrium internuclear distance of 5.6 bohr, the calculated electronic energy is −5.807 483 583±0.000 000 016 hartrees and the corresponding well depth is (ε/k) 10.995±0.005 K.
The diazocarbene (CNN) molecule: Characterization of the X̃ 3Σ− and Ã 3Π electronic states
Authors: Yamaguchi Yukio , Schaefer Henry F.
Published: May 2004

The ground (X̃ 3Σ−) and first excited triplet (Ã 3Π) electronic states of diazocarbene (CNN) have been investigated systematically starting from the self-consistent-field theory and proceeding to the coupled cluster with single, double, and full triple excitations (CCSDT) method with a wide range of basis sets. While the linear X̃ 3Σ− ground state of CNN has a real degenerate bending vibrational frequency, the Ã 3Π state of CNN is subject to the Renner–Teller effect and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequencies of the Ã 3Π state were evaluated via the equation-of-motion coupled cluster (EOM-CC) techniques. The significant sensitivity to level of theory in predicting the ground-state geometry, harmonic vibrational frequencies, and associated infrared intensities has been attributed to the fact that the reference wave function is strongly perturbed by the excitations of 1π→3π followed by a spin flip. At the highest level of theory with the largest basis set, correlation-consistent polarized valence quadruple zeta (cc-pVQZ) CCSDT, the classical X̃–Ã splitting (Te value) was predicted to be 68.5 kcal/mol (2.97 eV, 24 000 cm−1) and the quantum mechanical splitting (T0 value) to be 69.7 kcal/mol (3.02 eV, 24 400 cm−1), which are in excellent agreement with the experimental T0 values, 67.5–68.2 kcal/mol (2.93–2.96 eV, 23 600–23 900 cm−1). With the EOM-CCSD method the Renner parameter (ε) and averaged bending vibrational frequency (ω̃2) for the Ã 3Π state were evaluated to be ε=−0.118 and ω̃2=615 cm−1, respectively. They are in fair agreement with the experimental values of ε=−0.07 and ν̃2=525 cm−1.
Comparison of density functionals for energy and structural differences between the high- [5T2g: (t2g)4(eg)2] and low- [1A1g: (t2g)6(eg)] spin states of the hexaquoferrous cation [Fe(H2O)6]2+
Authors: Fouqueau Antony , Mer Sébastien , Casida Mark E. , Lawson Daku Latevi Max , Hauser Andreas , Mineva Tsonka , Neese Frank
Published: May 2004

A comparison of density functionals is made for the calculation of energy and geometry differences for the high- [5T2g: (t2g)4(eg)2] and low- [1A1g: (t2g)6(eg)0] spin states of the hexaquoferrous cation [Fe(H2O)6]2+. Since very little experimental results are available (except for crystal structures involving the cation in its high-spin state), the primary comparison is with our own complete active-space self-consistent field (CASSCF), second-order perturbation theory-corrected complete active-space self-consistent field (CASPT2), and spectroscopy-oriented configuration interaction (SORCI) calculations. We find that generalized gradient approximations (GGAs) and the B3LYP hybrid functional provide geometries in good agreement with experiment and with our CASSCF calculations provided sufficiently extended basis sets are used (i.e., polarization functions on the iron and polarization and diffuse functions on the water molecules). In contrast, CASPT2 calculations of the low-spin–high-spin energy difference ΔELH=ELS−EHS appear to be significantly overestimated due to basis set limitations in the sense that the energy difference of the atomic asymptotes (5D→1I excitation of Fe2+) are overestimated by about 3000 cm−1. An empirical shift of the molecular ΔELH based upon atomic calculations provides a best estimate of 12 000–13 000 cm−1. Our unshifted SORCI result is 13 300 cm−1, consistent with previous comparisons between SORCI and experimental excitation energies which suggest that no such empirical shift is needed in conjunction with this method. In contrast, after estimation of incomplete basis set effects, GGAs with one exception underestimate this value by 3000–4000 cm−1 while the B3LYP functional underestimates it by only about 1000 cm−1. The exception is the GGA functional RPBE which appears to perform as well as or better than the B3LYP functional for the properties studied here. In order to obtain a best estimate of the molecular ΔELH within the context of density functional theory (DFT) calculations we have also performed atomic excitation energy calculations using the multiplet sum method. These atomic DFT calculations suggest that no empirical correction is needed for the DFT calculations.
Adsorption of 3-pyrroline on Si(100) from first principles
Authors: Romero Aldo H. , Sbraccia Carlo , Silvestrelli Pier Luigi
Published: May 2004

The chemisorption of 3-pyrroline (C4H7N) on Si(100) is studied from first principles. Three different structures can be realized for which, depending on the temperature, the chemisorption process is facile (for two of them it is essentially barrierless); among these configurations the most favored one, from a thermodynamical point of view, is a dissociated structure obtained through an exothermic reaction characterized by the formation of a N–Si bond and a H–Si bond in which the H atom is detached from the molecule. Several other chemisorption structures are possible which, however, require overcoming a significant energy barrier and often breaking multiple bonds. A number of reaction paths going from one stable structure to another have been investigated. We have also generated, for the two basic adsorption structures, theoretical scanning tunneling microscopy images which could facilitate the interpretation of experimental measurements, and we propose a possible reaction mechanism for nitrogen incorporation.
Kinetics of phase transformation on a Bethe lattice in the presence of spin exchange
Authors: Berim Gersh O. , Ruckenstein Eli
Published: May 2004

Kinetics of phase transformation on a Bethe lattice governed by single-spin-flip Glauber and spin-exchange Kawasaki dynamics is examined. For a general Glauber dynamics for which all processes (splitting and coagulation, growth and decay of clusters, as well as creation and annihilation of single-spin clusters) take place, the addition of the Kawasaki dynamics accelerates the transformation process without changing the qualitative behavior. In the growth-decay regime of the Glauber dynamics, regime in which the splitting and coagulation, and creation and annihilation processes due to single-spin flips are negligible, the Kawasaki dynamics strongly increases the fraction of transformed phase because of the splitting and coagulation of clusters induced by the spin-exchange processes. Acting alone, the Kawasaki dynamics leads to the growth of the clusters of each of the phases after the quenching of the temperature to a lower value. When the final temperature Tf is smaller than a certain temperature Tf0, the average cluster radius grows linearly with time during both the initial and intermediate stages of the kinetic process, and diverges as log2(td−t)−1 when the time t approaches the value td at which infinite clusters arise. It is shown that, among the various spin-exchange processes involved in Kawasaki dynamics, the main contribution is provided by those which decrease or increase the number of clusters by unity.
Application of density functional theory to capillary phenomena in cylindrical mesopores with radial and longitudinal density distributions
Authors: Ustinov E. A. , Do D. D.
Published: May 2004

In this paper, we applied a version of the nonlocal density functional theory (NLDFT) accounting radial and longitudinal density distributions to study the adsorption and desorption of argon in finite as well as infinite cylindrical nanopores at 87.3 K. Features that have not been observed before with one-dimensional NLDFT are observed in the analysis of an inhomogeneous fluid along the axis of a finite cylindrical pore using the two-dimensional version of the NLDFT. The phase transition in pore is not strictly vapor–liquid transition as assumed and observed in the conventional version, but rather it exhibits a much elaborated feature with phase transition being complicated by the formation of solid phase. Depending on the pore size, there are more than one phase transition in the adsorption–desorption isotherm. The solid formation in finite pore has been found to be initiated by the presence of the meniscus. Details of the analysis of the extended version of NLDFT will be discussed in the paper.
A new correlation effect in the Helmholtz and surface potentials of the electrical double layer
Authors: González-Tovar Enrique , Jiménez-Ángeles Felipe , Messina René , Lozada-Cassou Marcelo
Published: May 2004

The restricted primitive model of an electrical double layer around a spherical macroparticle is studied by using integral equation theories and Monte Carlo simulations. The resulting theoretical curves for the Helmholtz and surface potentials versus the macroparticle charge show an unexpected positive curvature when the ionic size of uni- and divalent electrolyte species is increased. This is a novel effect that is confirmed here by computer experiments. An explanation of this phenomenon is advanced in terms of the adsorption and layering of the electrolytic species and of the compactness of the diffuse double layer. It is claimed that the interplay between electrostatic and ionic size correlation effects, absent in the classical Poisson–Boltzmann view, is responsible for this singularity.
Solvation force, structure and thermodynamics of fluids confined in geometrically rough pores
Authors: Ghatak Chandana , Ayappa K. G.
Published: May 2004

The effect of periodic surface roughness on the behavior of confined soft sphere fluids is investigated using grand canonical Monte Carlo simulations. Rough pores are constructed by taking the prototypical slit-shaped pore and introducing unidirectional sinusoidal undulations on one wall. For the above geometry our study reveals that the solvation force response can be phase shifted in a controlled manner by varying the amplitude of roughness. At a fixed amplitude of roughness, a, the solvation force for pores with structured walls was relatively insensitive to the wavelength of the undulation, λ for 2.3<λ/σff<7, where σff is the Lennard-Jones diameter of the confined fluid. This was not the case for smooth walled pores, where the solvation force response was found to be sensitive to the wavelength, for λ/σff<7.0 and amplitudes of roughness, a/σff⩾0.5. The predictions of the superposition approximation, where the solvation force response for the rough pores is deduced from the solvation force response of the slit-shaped pores, was in excellent agreement with simulation results for the structured pores and for λ/σff⩾7 in the case of smooth walled pores. Grand potential computations illustrate that interactions between the walls of the pore can alter the pore width corresponding to the thermodynamically stable state, with wall–wall interactions playing an important role at smaller pore widths and higher amplitudes of roughness.
Control of Turing pattern by weak spatial perturbation
Authors: Li Qian Shu , Ji Lin
Published: May 2004

The control of Turing pattern formation by weak spatial perturbation is investigated. The weak spatial perturbation added before Turing pattern stabilization is found to show prominent spatial orientation effect. The control process of perturbation to Turing patterns is tracked. The effect of perturbation factors, such as amplitude and imposing time are also discussed.
Unified treatment of multicenter integrals of integer and noninteger u Yukawa-type screened Coulomb type potentials and their derivatives over Slater orbitals
Authors: Guseinov I. I.
Published: May 2004

Multicenter integrals with integer and noninteger values of indices u of Yukawa-type screened Coulomb-type potentials (SCTPs) fu(η,r)=ru−1e−ηr and their derivatives over Slater-type orbitals (STOs) are evaluated using series expansion formulas obtained from the expansions in terms of complete orthonormal sets of Ψα-exponential-type orbitals (Ψα-ETOs, α=1, 0, −1, −2,…). The final results obtained are valid for the arbitrary values of parameters of SCTPs and STOs.