bluuebunny/crossref_metadata_2025 huggingface dataset:
Postdepositional losses of methane sulfonate, nitrate, and chloride at the European Project for Ice Coring in Antarctica deep‐drilling site in Dronning Maud Land, Antarctica
Authors: Weller R. , Traufetter F. , Fischer H. , Oerter H. , Piel C. , Miller H.
Published: March 2004

We quantified postdepositional losses of methane sulfonate (MSA−), nitrate, and chloride at the European Project for Ice Coring in Antarctica (EPICA) drilling site in Dronning Maud Land (DML) (75°S, 0°E). Analyses of four intermediate deep firn cores and 13 snow pits were considered. We found that about 26 ± 13% of the once deposited nitrate and typically 51 ± 20% of MSA− were lost, while for chloride, no significant depletion could be observed in firn older than one year. Assuming a first order exponential decay rate, the characteristic e‐folding time for MSA− is 6.4 ± 3 years and 19 ± 6 years for nitrate. It turns out that for nitrate and MSA− the typical mean concentrations representative for the last 100 years were reached after 5.4 and 6.5 years, respectively, indicating that beneath a depth of around 1.2–1.4 m postdepositional losses can be neglected. In the area of investigation, only MSA− concentrations and postdepositional losses showed a distinct dependence on snow accumulation rate. Consequently, MSA− concentrations archived at this site should be significantly dependent on the variability of annual snow accumulation, and we recommend a corresponding correction. With a simple approach, we estimated the partial pressure of the free acids MSA, HNO3, and HCl on the basis of Henry's law assuming that ionic impurities of the bulk ice matrix are localized in a quasi‐brine layer (QBL). In contrast to measurements, this approach predicts a nearly complete loss of MSA−, NO3−, and Cl−.
Impacts of data length on optimal parameter and uncertainty estimation of a land surface model
Authors: Xia Youlong , Yang Zong‐Liang , Jackson Charles , Stoffa Paul L. , Sen Mrinal K.
Published: April 2004

The optimal parameters and uncertainty estimation of land surface models require that appropriate length of forcing and calibration data be selected for computing error functions. Most of the previous studies used less than two years of data to optimize land surface models. In this study, 18‐year hydrometeorological data at Valdai, Russia, were used to run the Chameleon Surface Model (CHASM). The optimal parameters were obtained by employing a global optimization technique called very fast simulated annealing. The uncertainties of model parameters were estimated by the Bayesian stochastic inversion technique. Forty‐four experiments were conducted by using different lengths of data from the 18‐year record, and a total of about 3 million parameter sets were produced. This study found that different calibration variables require different lengths of data to obtain optimal parameters and uncertainty estimates which are insensitive to the period selected. In the case of optimal parameters, monthly root‐zone soil moisture, runoff, and evapotranspiration require 8, 3, and 1 years of data, respectively. In the case of uncertainty estimates, monthly root‐zone soil moisture, runoff, and evapotranspiration require 8, 8, and 3 years of data, respectively. Spin‐up has little impact on the selection of optimal parameters and uncertainty estimates when evapotranspiration and runoff were calibrated. However, spin‐up affects the selection of optimal parameters when soil moisture was calibrated.
Hydration state of the Martian coarse‐grained hematite exposures: Implications for their origin and evolution
Authors: Baldridge Alice M. , Calvin Wendy M.
Published: April 2004

The discovery of areas of bulk gray hematite on the surface of Mars is among the most interesting of the results from Mars Global Surveyor (MGS). The individual outcrops are concentrated in equatorial, low‐albedo regions from the Valles Marineris to the Meridiani Planum region to the east. While the Thermal Emission Spectrometer (TES) team favored an aqueous process for the formation of this deposit, recent geologic studies have interpreted deposits in the region to have a volcanic origin. Competing ideas for the method of formation of bulk hematite range from aqueous alteration, burial diagenesis, and metamorphism to igneous processes. Here we show that some of the spectra collected over Meridiani Planum and Aram Chaos from the Mariner 6 and 7 spacecraft have a high 3 μm band depth. This feature is associated with hydrated minerals in the surface layer, and the hydrated spectra coincide with the mapped hematite regions. This supports an aqueous mode of hematite formation and, as hematite itself is not hydrated, suggests the presence of other hydrous mineral phases in these areas. We discuss the implications of this association for origin and evolution of these deposits.
Reply [to “Comment on ‘Righting the balance: Gender diversity in the geosciences’”] by Floyd Herbert
Authors: Bell Robin , Kane Mark , Miller Roberta B. , Mutter John C. , Pfirman Stephanie , Kastens Kim
Published: March 2004

In our article “On Gender Equity in the Geosciences,” we are not advocating a quota of 50% women in the geosciences, but rather, suggesting that the field would be strengthened if all the individuals with doctorates in the geosciences (including the 28% of all Ph.D.s who are women) were to remain active in these fields [National Science Board, 2002; Kastens, 2003].

Currently, only about 13% of employed Ph.D.s in these fields are women, which means that there is a considerable loss of trained scientists in the Earth, atmospheric, and oceanographic sciences. Even the entry‐level hires at Ph.D.‐granting institutions are only 20% female [Holmes et al, 2003].
In Memoriam
Authors: Anonymous
Published: March 2004

Leo C. Benz,80, 14 March 2003, Retired Life Member, Hydrology, 1962.

David M. Chittenden II, 67, 2 January 2004, Atmospheric Sciences, 1996.
Synchrotron 3D microtomography of halite aggregates during experimental pressure solution creep and evolution of the permeability
Authors: Renard François , Bernard Dominique , Thibault Xavier , Boller Elodie
Published: April 2004

Pressure solution creep is one of the possible processes of mechano‐chemical deformation that controls porosity and permeability variations in the upper crust. The three‐dimensional geometry of the porous network of halite aggregates was imaged during compaction driven by pressure solution creep using X‐ray synchrotron computed microtomography. This technique can be used to monitor individual grain contacts and whole aggregate textural changes during deformation. By reconstructing subvolumes, the 3D porosity of the aggregates was extracted. The time‐resolved decrease in permeability during porosity reduction was calculated by solving the Stokes equations. The permeability remained isotropic and decreased from 2.1 to 0.15 Darcy after 18.2% compaction. Two microscopic mechanisms can explain the permeability reduction: grain indentation and pore connectivity reduction by precipitation on the free surface of pore throats.
Postseismic viscoelastic‐gravitational half space computations: Problems and solutions
Authors: Fernández J. , Rundle J. B.
Published: April 2004

We consider the problem of surface deformation arising from a fault in a semi‐infinite, elastic‐gravitational, and/or viscoelastic‐gravitational, plane‐layered medium, subject to an externally imposed gravitational acceleration g. Rundle [1981, 1982] presented a calculation in which self‐gravitation, represented by terms proportional to G are neglected, and the externally imposed acceleration due to gravity, g, is considered constant in the medium. Because of the recent strong interest in computations of this type, we examine the assumptions involved in these computations. We show that these assumptions are not likely to have serious consequences in the relatively near‐field viscoelastic displacements, where the earth's curvature is neglected. We also show that the approximation described by Rundle [1981, 1982], which was technically not regular as z → ∞, can easily be regularized using a new approach without appreciable change in the resulting displacement field.
Potential energy surfaces for HenNe+ ions: ab initio and diatomics-in-molecule results
Authors: Seong Jeonghee , Rohrbacher Andreas , Li Zhi Ru , Janda Kenneth C. , Tao Fu-Ming , Spiegelman Fernand , Halberstadt Nadine
Published: April 2004

The potential energy surface of He2Ne+ has been reinvestigated using a combination of ab initio and diatomics-in-molecule (DIM) calculations. In contrast to the reports of two recent studies the ion is found to have an asymmetric linear He–Ne–He structure, with no barrier to formation from the separated atoms on the ground-state surface. The He–Ne+ bond lengths at the potential minimum are 1.51 and 1.81 Å, and the total bonding energy is 0.717 eV. Comparing the He2Ne+ energy to that of HeNe+, the bonding energy for the second helium atom is 0.06 eV, about 10% of that of the first He atom. The saddle point between the two equivalent minima is a symmetric structure, 0.0074 eV above the potential minimum. A symmetric geometry becomes the overall potential minimum if the 2s hole on the Ne is excluded from the reference states of a multireference configuration interaction calculation. A DIM potential was created for the HenNe+ family of ions. The DIM potential is consistent with the asymmetric He2Ne+ ion serving as a core; it predicts a slightly more asymmetric geometry than the ab initio results. Additional helium atoms form five-membered rings around the bonds of the core ion to fill the first shell and then add to the ends of the cluster. The asymmetric core ion and the highly compact structure help to account for the lack of apparent shell structure in the mass spectrometry of HenNe+ clusters. Finally, we recommend that the value De=0.63±0.04 eV be adopted for the ground state of HeNe+.
Structural properties of reciprocal form factor in neutral atoms and singly charged ions
Authors: Romera E. , Angulo J. C.
Published: April 2004

Structural characteristics of the spherically averaged internally folded density or reciprocal form factor B(r) are studied within the Hartree–Fock framework for 103 neutral atoms, 54 singly charged cations, and 43 anions in their ground state. The function B(r) is classified throughout the Periodic Table into three types: (i) monotonic decrease from the origin, (ii) maximum at r=0 and a negative minimum at r>0, and (iii) a local maximum at r=0 and a pair maximum–minimum out of the origin. A detailed study of the corresponding properties for individual subshells as well as their relative weight for the total B(r) is also carried out. For completeness, the analytical B(r) for hydrogenlike atoms in both ground and excited states is also analyzed.
Dynamically weighted multiconfiguration self-consistent field: Multistate calculations for F+H2O→HF+OH reaction paths
Authors: Deskevich Michael P. , Nesbitt David J. , Werner Hans-Joachim
Published: April 2004

A novel method of dynamically adjusted weighting factors in state-averaged multiconfiguration self-consistent-field calculations (SA–MCSCF) is described that is applicable to systems of arbitrary dimensionality. The proposed dynamically weighted approach automatically weights the relevant electronic states in each region of the potential energy surface, smoothly adjusting between these regions with an energy dependent functional. This method is tested on the F(2P)+H2O→HF+OH(2Π) reaction, which otherwise proves challenging to describe with traditional SA–MCSCF methods due to (i) different asymptotic degeneracies of reactant (threefold) and product (twofold) channels, and (ii) presence of low-lying charge transfer configurations near the transition state region. The smoothly varying wave functions obtained by dynamically weighted multiconfigurational self-consistent field represent excellent reference states for high-level multireference configuration interaction calculations and offer an ideal starting point for construction of multiple state potential energy surfaces.
Computer simulations of localized small polarons in amorphous polyethylene
Authors: Cubero David , Quirke Nicholas
Published: April 2004

We use a simple mean field scheme to compute the polarization energy of an excess electron in amorphous polyethylene that allows us to study dynamical properties. Nonadiabatic simulations of an excess electron in amorphous polyethylene at room temperature show the spontaneous formation of localized small polaron states in which the electron is confined in a spherically shaped region with a typical dimension of 5 Å. We compute the self-trapping energy to be −0.06±0.03 eV, with a lifetime on the time scale of a few tens of picoseconds.
Analytic evaluation of nonadiabatic coupling terms at the MR-CI level. II. Minima on the crossing seam: Formaldehyde and the photodimerization of ethylene
Authors: Dallos Michal , Lischka Hans , Shepard Ron , Yarkony David R. , Szalay Péter G.
Published: April 2004

The method for the analytic calculation of the nonadiabatic coupling vector at the multireference configuration-interaction (MR-CI) level and its program implementation into the COLUMBUS program system described in the preceding paper [Lischka et al., J. Chem. Phys. 120, 7322 (2004)] has been combined with automatic searches for minima on the crossing seam (MXS). Based on a perturbative description of the vicinity of a conical intersection, a Lagrange formalism for the determination of MXS has been derived. Geometry optimization by direct inversion in the iterative subspace extrapolation is used to improve the convergence properties of the corresponding Newton-Raphson procedure. Three examples have been investigated: the crossing between the 1 1B1/2 1A1 valence states in formaldehyde, the crossing between the 2 1A1/3 1A1 π-π* valence and ny-3py Rydberg states in formaldehyde, and three crossings in the case of the photodimerization of ethylene. The methods developed allow MXS searches of significantly larger systems at the MR-CI level than have been possible before and significantly more accurate calculations as compared to previous complete-active space self-consistent field approaches.
Molecular-dynamics simulation of the effect of ions on a liquid–liquid interface for a partially miscible mixture
Authors: Wardle Kent E. , Carlson Eric , Henderson Douglas , Rowley Richard L.
Published: April 2004

Molecular-dynamics simulations were performed to model the effect of added salt ions on the liquid–liquid interface in a partially miscible system. Simulations of the interface between saturated phases of a model 1-hexanol+water system show a bilayer structure of 1-hexanol molecules at the interface with −OH heads of the first layer directed into the water phase and the opposite orientation for the second layer. The alignment of the polar −OH groups at the interface stabilizes a charge separation of sodium and chloride ions when salt is introduced into the aqueous phase, producing an electrical double layer. Chloride ions aggregate nearer the interface and sodium ions move toward the bulk water phase, consistent with the explanation that the −OH alignment presents a region of partial positive charges to which the hydrated chloride atoms are attracted. Ions near the interface were found to be less solvated than those in the bulk phase. An electric field was also applied to drive ions through the interface. Ions crossing the interface tended to shed water molecules as they entered the hexanol bilayer, leaving a trail of water molecules. Stabilization and facilitated transport of the ion by interactions with the second layer of hexanol molecules appeared to be an important step in the mechanism of sodium ion transport.
An empirical charge transfer potential with correct dissociation limits
Authors: Valone Steven M. , Atlas Susan R.
Published: April 2004

The empirical valence bond (EVB) method [J. Chem. Phys. 52, 1262 (1970)] has always embodied charge transfer processes. The mechanism of that behavior is examined here and recast for use as a new empirical potential energy surface for large-scale simulations. A two-state model is explored. The main features of the model are: (1) explicit decomposition of the total system electron density is invoked; (2) the charge is defined through the density decomposition into constituent contributions; (3) the charge transfer behavior is controlled through the resonance energy matrix elements which cannot be ignored; and (4) a reference-state approach, similar in spirit to the EVB method, is used to define the resonance state energy contributions in terms of “knowable” quantities. With equal validity, the new potential energy can be expressed as a nonthermal ensemble average with a nonlinear but analytical charge dependence in the occupation number. Dissociation to neutral species for a gas-phase process is preserved. A variant of constrained search density functional theory is advocated as the preferred way to define an energy for a given charge.
Infrared spectroscopy of physisorbed and chemisorbed O2 on Pt(111)
Authors: Gustafsson K. , Andersson S.
Published: April 2004

The adsorption of O2 on the Pt(111) surface, with particular emphasis on the influence of substrate temperature, has been studied by infrared reflection absorption spectroscopy (IRAS). In the temperature range 30–90 K the IRAS spectra reveal three different molecular adsorption states. A physisorbed state appears below 40 K while chemisorbed peroxo- and superoxo-like states are observed in the whole temperature range, the characteristic vibrational frequencies are at full coverages of O162, 1543 cm−1 and around 700 cm−1 and 870 cm−1, respectively. Flash heating from 30 K to 45 K reveal that the physisorbed state acts as a precursor to the superoxo chemisorption. Theoretical calculations suggest that peroxo molecules may occupy both fcc and hcp threefold sites on the Pt(111) surface. However, within the high resolution of the IRAS measurements we only observe one peroxo state in the temperature range 45–90 K, assigned to occupy the fcc site. The peroxo adsorption probability is significantly lower at 45 K than at 90 K, presumably due to reduced thermal activation from the physisorbed precursor state. A longer lifetime in this precursor state at the low temperature results in formation of larger superoxo islands already at low oxygen coverage.
Inverse isotope effects and electron-phonon coupling in the positively charged deutero- and fluoroacenes
Authors: Kato Takashi , Yamabe Tokio
Published: April 2004

Electron-phonon interactions in the monocations of deutero- and fluoroacenes are studied and compared with those in the monocations of acenes and those in the monoanions of fluoroacenes. Because of the significant phase pattern difference between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO), the frequency modes lower than 500 cm−1 and the high-frequency modes around 1400 cm−1 couple more strongly to the LUMO than to the HOMO, while the frequency modes around 500 cm−1 and the frequency modes around 1600 cm−1 couple more strongly to the HOMO than to the LUMO in fluoroacenes with D2h geometry. The total electron-phonon coupling constants for the monocations (lHOMO) are estimated and compared with those for the monoanions (lLUMO) in deutero- and fluoroacenes. The lHOMO values are estimated to be 0.418, 0.399, 0.301, 0.255, and 0.222 eV for C6F6 (1f), C10F8 (2f), C14F10 (3f), C18F12 (4f), and C22F14 (5f), respectively. The lHOMO values are smaller than the lLUMO values in small fluoroacenes. But the lHOMO value decreases with an increase in molecular size less rapidly than the lLUMO value in fluoroacenes, and the lHOMO value of 0.074 eV is much larger than the lLUMO value of 0.009 eV in polyfluoroacene. The logarithmically averaged phonon frequencies for the monocations (ωln,HOMO) are estimated to be larger than those for the monoanions (ωln,LUMO) in fluoroacenes. This is because the C-C stretching modes around 1600 cm−1 couple most strongly to the HOMO, and those around 1400 cm−1 couple the most strongly to the LUMO in fluoroacenes. The significant phase pattern difference between the HOMO and the LUMO is the main reason for the calculational results. The lHOMO values increase much more significantly by H-F substitution than by H-D substitution in acenes. The possible inverse isotope effects in the electron-phonon interactions as a consequence of deuteration in the monocations of nanosized molecules are suggested.
Electronic and infrared absorption spectra of linear and cyclic C6+ in a neon matrix
Authors: Fulara Jan , Riaplov Evgueni , Batalov Anton , Shnitko Ivan , Maier John P.
Published: April 2004

Electronic and infrared absorption spectra of mass-selected C6+, generated by dissociative electron impact ionization of C6Cl6 and C6Br6, have been recorded in 6 K neon matrices. Linear and cyclic forms of C6+ have been observed. The Π2g←XΠ2u electronic transition of linear C6+ has its origin band at 646 nm whereas for the (2) B22←XA21 system of the cyclic isomer it lies at 570 nm. An infrared active fundamental mode in the ground electronic state of C6+ is observed at 2092 and 1972 cm−1 for the linear and cyclic isomer, respectively.
Globally uniform semiclassical surface-hopping wave function for nonadiabatic scattering
Authors: Herman Michael F. , El Akramine Ouafae , Moody Michael P.
Published: April 2004

A globally uniform time-independent semiclassical wave function for nonadiabatic scattering is presented. This wave function, which takes the form of a surface-hopping expansion, is motivated by the globally uniform semiclassical wave function of Kay and co-workers for the single-surface case. The surface-hopping expansion is similar to a previously presented primitive semiclassical wave function for nonadiabatic problems. This earlier wave function has the important feature that it correctly incorporates all phase terms, allowing for an accurate treatment of quantum interference effects. The globally uniform expression has important numerical advantages over the primitive formulation. The globally uniform wave function does not have caustic singularities, and the globally uniform calculation avoids a root search for trajectories obeying double-ended boundary conditions that is required by the primitive semiclassical calculation.
State-to-state rotational relaxation rate constants for CO+Ne from IR–IR double-resonance experiments: Comparing theory to experiment
Authors: Hostutler David A. , Smith Tony C. , Hager Gordon D. , McBane George C. , Heaven Michael C.
Published: April 2004

IR–IR double-resonance experiments were used to study the state-to-state rotational relaxation of CO with Ne as a collision partner. Rotational levels in the range Ji=2–9 were excited and collisional energy transfer of population to the levels Jf=2–8 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions. Fitting laws based on the modified exponential gap (MEG) and statistical power exponential gap (SPEG) models were used; the MEG model performed better than the SPEG model. A rate constant matrix was also generated from scattering calculations that employed the ab initio potential energy surface of McBane and Cybulski [J. Chem. Phys. 110, 11 734 (1999)]. This theoretical rate constant matrix yielded kinetic simulations that agreed with the data nearly as well as the fitted MEG model and was unique in its ability to reproduce both the rotational energy transfer and pressure broadening data for Ne–CO. The theoretical rate coefficients varied more slowly with the energy gap than coefficients from either of the fitting laws.
Real versus artifactual symmetry-breaking effects in Hartree–Fock, density-functional, and coupled-cluster methods
Authors: Russ Nicholas J. , Crawford T. Daniel , Tschumper Gregory S.
Published: April 2004

We have examined the relative abilities of Hartree–Fock, density-functional theory (DFT), and coupled-cluster theory in describing second-order (pseudo) Jahn–Teller (SOJT) effects, perhaps the most commonly encountered form of symmetry breaking in polyatomic molecules. As test cases, we have considered two prototypical systems: the Σ2u+ states of D∞h BNB and C3+ for which interaction with a low-lying Σ2g+ excited state leads to symmetry breaking of the nuclear framework. We find that the Hartree–Fock and B3LYP methods correctly reproduce the pole structure of quadratic force constants expected from exact SOJT theory, but that both methods appear to underestimate the strength of the coupling between the electronic states. Although the Tamm–Dancoff (CIS) approximation gives excitation energies with no relationship to the SOJT interaction, the random-phase-approximation (RPA) approach to Hartree–Fock and time-dependent DFT excitation energies predicts state crossings coinciding nearly perfectly with the positions of the force constant poles. On the other hand, the RPA excited-state energies exhibit unphysical curvature near their crossings with the ground (reference) state, a problem arising directly from the mathematical structure of the RPA equations. Coupled-cluster methods appear to accurately predict the strength of the SOJT interactions between the Σ2u+ and Σ2g+ states, assuming that the inclusion of full triple excitations provides a suitable approximation to the exact wave function, and are the only methods examined here which predict symmetry breaking in BNB. However, coupled-cluster methods are plagued by artifactual force constant poles arising from the response of the underlying reference molecular orbitals to the geometric perturbation. Furthermore, the structure of the “true” SOJT force constant poles predicted by coupled-cluster methods, although correctly positioned, has the wrong structure.
Adsorption of O2 and oxidation of CO at Au nanoparticles supported by TiO2(110)
Authors: Molina L. M. , Rasmussen M. D. , Hammer B.
Published: April 2004

Density functional theory calculations are performed for the adsorption of O2, coadsorption of CO, and the CO+O2 reaction at the interfacial perimeter of nanoparticles supported by rutile TiO2(110). Both stoichiometric and reduced TiO2 surfaces are considered, with various relative arrangements of the supported Au particles with respect to the substrate vacancies. Rather stable binding configurations are found for the O2 adsorbed either at the trough Ti atoms or leaning against the Au particles. The presence of a supported Au particle strongly stabilizes the adsorption of O2. A sizable electronic charge transfer from the Au to the O2 is found together with a concomitant electronic polarization of the support meaning that the substrate is mediating the charge transfer. The O2 attains two different charge states, with either one or two surplus electrons depending on the precise O2 adsorption site at or in front of the Au particle. From the least charged state, the O2 can react with CO adsorbed at the edge sites of the Au particles leading to the formation of CO2 with very low (≈0.15 eV) energy barriers.
Relation between different variants of the generalized Douglas–Kroll transformation through sixth order
Authors: van Wüllen Christoph
Published: April 2004

Wolf et al. have recently investigated a generalized Douglas–Kroll transformation. From a general class of unitary transformations that can be used in the Douglas–Kroll transformation, they pick one which is supposed to give, at a given order, an optimal transformed Dirac Hamiltonian. Results were presented through the fifth order. However, no data were given to demonstrate to which extent the so-called “optimal” Douglas–Kroll transformation is superior to other choices. In this work, the Douglas–Kroll transformation is extended to the sixth order for the first time, using computer algebra algorithms to obtain the working equations. It is shown how, at a given order, different variants of the Douglas–Kroll Hamiltonians are related. Various choices of the generalized transformation are examined numerically for the ground states of the one-electron atomic ions with nuclear charges Z=20, 40, 60, 80, 100, and 120. It is shown that compared to the improvement obtained by including the next order, the differences between various choices for the generalized Douglas–Kroll transformation are almost negligible. Results closest to the Dirac eigenvalues are not obtained with the optimal Douglas–Kroll transformation given by Wolf et al., but with the parametrization originally suggested by Douglas and Kroll.
A cell model of a liquid droplet
Authors: Barrett Jonathan C.
Published: April 2004

An expression for the free energy of a droplet composed of attracting hard spheres is found using a simple cell model. The hard-sphere repulsion is assumed to act only between molecules in the same cell, whereas attraction extends over many cells. A maximum term analysis gives rise to a mean-field free energy which includes terms proportional to the first and second power of the droplet radius R with coefficients which can be related to the planar surface tension and Tolman length. Certain Gaussian fluctuations about the maximum term are also considered, corresponding to droplet translation and capillary wave fluctuations. Inclusion of these fluctuations is necessary to ensure that the nucleation rate is proportional to the system volume. They also reduce the planar surface tension and introduce a logarithmic term, −43 ln R, into the free energy. The inclusion of other fluctuations and the relationship between these equations and those arising in density-functional theories of nucleation is discussed.
Atomic and electronic structure of unreduced and reduced CeO2 surfaces: A first-principles study
Authors: Yang Zongxian , Woo Tom K. , Baudin Micael , Hermansson Kersti
Published: April 2004

The atomic and electronic structure of (111), (110), and (100) surfaces of ceria (CeO2) were studied using density-functional theory within the generalized gradient approximation. Both stoichiometric surfaces and surfaces with oxygen vacancies (unreduced and reduced surfaces, respectively) have been examined. It is found that the (111) surface is the most stable among the considered surfaces, followed by (110) and (100) surfaces, in agreement with experimental observations and previous theoretical results. Different features of relaxation are found for the three surfaces. While the (111) surface undergoes very small relaxation, considerably larger relaxations are found for the (110) and (100) surfaces. The formation of an oxygen vacancy is closely related to the surface structure and occurs more easily for the (110) surface than for (111). The preferred vacancy location is in the surface layer for CeO2(110) and in the subsurface layer (the second O-atomic layer) for CeO2(111). For both surfaces, the O vacancy forms more readily than in the bulk. An interesting oscillatory behavior is found for the vacancy formation energy in the upper three layers of CeO2(111). Analysis of the reduced surfaces suggests that the additional charge resulting from the formation of the oxygen vacancies is localized in the first three layers of the surface. Furthermore, they are not only trapped in the 4f states of cerium.
Alignment of molecules in pulsed resonant laser fields
Authors: Granucci Giovanni , Persico Maurizio , Van Leuven Piet
Published: April 2004

We investigate by numerical simulations the dynamics of alignment of linear molecules in resonant pulsed laser fields and its dependence on pulse length, field strength, and molecular parameters. We propose an analytical short-time approximation for the time-dependent wave packets. We provide a theoretical basis for the occurrence of saturation in the rotational pumping. We present a formula to predict the time at which the maximum alignment occurs. We discuss the magnitude of the laser-induced alignment and we relate it to a theoretical upper limit.
A computational study of microsolvation effect on ethylene glycol by density functional method
Authors: Chaudhari Ajay , Lee Shyi-Long
Published: April 2004

This study focuses on the conformational analysis of ethylene glycol–(water)n (n=1–3) complex by using density functional theory method and the basis set 6-311++G*. Different conformers are reported and the basis set superposition error corrected total energy is −306.767 5171, −383.221 3135, and −459.694 1528 for lowest energy conformer with 1, 2, and 3 water molecules, respectively, with corresponding binding energy −7.75, −15.43, and −36.28 kcal/mol. On applying many-body analysis it has been found that relaxation energy, two-body, three-body energy have significant contribution to the binding energy for ethylene glycol–(water)3 complex whereas four-body energies are negligible. The most stable conformers of ethylene glycol–(water)n complex are the cyclic structures in which water molecules bridge between the two hydroxyl group of ethylene glycol.
Spectroscopic properties and potential energy curves of low-lying electronic states of RuC
Authors: Guo Rui , Balasubramanian K.
Published: April 2004

The RuC molecule has been a challenging species due to the open-shell nature of Ru resulting in a large number of low-lying electronic states. We have carried out state-of-the-art calculations using the complete active space multiconfiguration self-consistent field followed by multireference configuration interaction methods that included up to 18 million configurations, in conjunction with relativistic effects. We have computed 29 low-lying electronic states of RuC with different spin multiplicities and spatial symmetries with energy separations less than 38 000 cm−1. We find two very closely low-lying electronic states for RuC, viz., Σ1 + and Δ3 with the Σ1 + being stabilized at higher levels of theory. Our computed spectroscopic constants and dipole moments are in good agreement with experiment although we have reported more electronic states than those that have been observed experimentally. Our computations reveal a strongly bound Σ1 + state with a large dipole moment which is most likely the experimentally observed ground state and an energetically close Δ3 state with a smaller dipole moment. Overall our computed spectroscopic constants of the excited states with energy separations less than 18 000 cm−1 agree quite well with those of the corresponding observed states.
Transport of matter and energy in a mesoscopic thermo-hydrodynamic approach
Authors: Madureira Justino R.
Published: April 2004

We derive a thermo-hydrodynamic theory for particles and energy flow, based on a nonequilibrium grand-canonical ensemble formalism. The time-dependent kinetic coefficients are explicitly given in terms of microscopic mechanical quantities. The time evolution equations describing the coupled flow of energy and particles are derived. The second-rank tensorial fluxes of current of energy and particles present in the nonequilibrium ensemble are nondiagonal. We obtain a generalized Fick’s Law, which presents the effect of the energy flow on the particle diffusion equation.
Electronic transport in Z-junction carbon nanotubes
Authors: Zhang Jintao , Shi Q. W. , Yang Jinlong
Published: April 2004

In this paper, the electronic transport in different Z-shape carbon nanotubes containing double knee junction structures on the same tube is studied. One consists of (5,5)-(9,0)-(5,5) double knee nano-metal-metal-metal junctions and another consists (6,6)-(10,0)-(6,6) double knee nano-metal-semiconductor-metal junctions. With the nearest-neighbor π-orbital tight-binding model, quantum conductances of these double knee junctions are calculated using the Landauer formula. The interesting conductance curves are provided to exhibit a potential application in the arena of molecular electronics.
Clustering and percolation in lithium borate glasses
Authors: Vegiri Alice , Varsamis Cristos-Platon E.
Published: April 2004

Molecular dynamics simulations are carried out in xLi2O–(1−x)B2O3 glasses (x=0.2–0.6) at T=1250 K, where cluster size distributions for Li cations and nonbridging oxygen (NBO) atoms are calculated. The existence of percolating clusters above x=0.3 places the percolation threshold between x=0.3 and 0.4 for the system under investigation, which is consistent with the abrupt increase of the diffusion coefficient of Li cations observed at x=0.4. It is also shown that the clusters of Li cations consist mainly of Li atoms found in the vicinity of NBO atoms. This result explains the higher mobility exhibited by this type of cations compared to the mobility of Li cations in the vicinity of bridging oxygen atoms.
Atomic and molecular intracules for excited states
Authors: Besley Nicholas A. , Gill Peter M. W.
Published: April 2004

Intracules in position space, momentum space and phase space have been calculated for low-lying excited states of the He atom, Be atom, formaldehyde and butadiene. The phase-space intracules (Wigner intracules) provide significantly more information than the position- and momentum-space intracules, particularly for the Be atom. Exchange effects are investigated through the differences between corresponding singlet and triplet states.
Collision complexes in dissociative single electron transfer between Ne2+ and N2
Authors: Harper Sarah M. , Hu Sunny W.-P. , Price Stephen D.
Published: April 2004

Experiments involving the coincident detection of the two monocationic products (Ne+ and N+) from the dissociative electron transfer reaction between Ne2+ and N2 at 7.8 eV collision energy allow the nascent velocity vectors of the ionic and neutral (N) products to be determined. Examination of the correlations between these vectors shows that one pathway to the products involves the dissociation of a transitory collision complex (N2Ne2+).
Cumulative inactivation of N-type CaV2.2 calcium channels modified by alternative splicing
Authors: Thaler Christopher , Gray Annette C. , Lipscombe Diane
Published: April 2004

The CaV2 family of voltage-gated calcium channels, present in presynaptic nerve terminals, regulates exocytosis and synaptic transmission. Cumulative inactivation of these channels occurs during trains of action potentials, and this may control short-term dynamics at the synapse. Inactivation during brief, repetitive stimulation is primarily attributed to closed-state inactivation, and several factors modulate the susceptibility of voltage-gated calcium channels to this form of inactivation. We show that alternative splicing of an exon in a cytoplasmic region of the CaV2.2 channel modulates its sensitivity to inactivation during trains of action potential waveforms. The presence of this exon, exon 18a, protects the CaV2.2 channel from entry into closed-state inactivation specifically during short (10 ms to 3 s) and small depolarizations of the membrane potential (–60 mV to –50 mV). The reduced sensitivity to closed-state inactivation within this dynamic range likely underlies the differential responsiveness of CaV2.2 splice isoforms to trains of action potential waveforms. Regulated alternative splicing of CaV2.2 represents a possible mechanism for modulating short-term dynamics of synaptic efficacy in different regions of the nervous system.
Metalloregulation of yeast membrane steroid receptor homologs
Authors: Lyons Thomas J. , Villa Nancy Y. , Regalla Lisa M. , Kupchak Brian R. , Vagstad Anna , Eide David J.
Published: April 2004

Zinc is an essential micronutrient that can also be toxic. An intricate mechanism exists in yeast that maintains cellular zinc within an optimal range. The centerpiece of this mechanism is the Zap1p protein, a transcription factor that senses zinc deficiency and responds by up-regulating genes involved in zinc metabolism. A microarray screen for novel Zap1p target genes suggested a role in zinc homeostasis for four homologous yeast genes. The expression of two of these genes,
YDR492w
and
YOL002c
, suggested direct regulation by Zap1p, whereas the expression of
YOL002c
and a third homologous gene,
YOL101c
, was induced by high zinc.
YDR492w
and
YOL002c
are confirmed to be direct Zap1p target genes. The induction of
YOL002c
and
YOL101c
by toxic metal ion exposure is shown to be mediated by the Mga2p hypoxia sensor. Furthermore,
YOL101c
is induced by deletion of the Aft1p iron-responsive transcription factor. These three genes, along with a fourth yeast homolog,
YLR023c
, have phenotypic effects on zinc tolerance and Zap1p activity. Because of their metalloregulation, zinc-related phenotypes, and highly conserved motifs containing potential metal-binding residues, this family has been renamed the
IZH
gene family (Implicated in Zinc Homeostasis). Furthermore, these genes are regulated by exogenous fatty acids, suggesting a dual role in lipid metabolism. The
IZH
genes encode membrane proteins that belong to a ubiquitous protein family that includes hemolysin III and vertebrate membrane steroid receptors. We propose that the
IZH
genes affect zinc homeostasis either directly or indirectly by altering sterol metabolism.
Helically chiral polymers: A class of ligands for asymmetric catalysis
Authors: Reggelin Michael , Doerr Sebastian , Klussmann Martin , Schultz Melanie , Holbach Michael
Published: March 2004

Helically chiral polymers from achiral monomers containing N and P atoms have been shown to be ligands for transition metals such as Pd and Rh. The Rh complex of the phosphane-containing polyisocyanate p(18-
co
-17) was an active albeit hardly enantioselective catalyst in the asymmetric hydrogenation of the dehydro amino acid
N
-acetamidocinnamic acid (15% enantiomeric excess). The most active catalyst obtained until now was the Pd-complexed polymethacrylate Pd-p12, which catalyzes the allylic substitution reaction of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate even at –20°C in quantitative yield, although again the enantioselectivity was unsatisfactory. The most successful application of a helically chiral polymer in asymmetric catalysis with respect to both reactivity and enantioselectivity is the polymethacrylate p(5-
co
-8). Its palladium complex catalyzes the above-mentioned reaction at 0°C with quantitative yield and 60% enantiomeric excess.
COMP-Ang1: A designed angiopoietin-1 variant with nonleaky angiogenic activity
Authors: Cho Chung-Hyun , Kammerer Richard A. , Lee Hyuek Jong , Steinmetz Michel O. , Ryu Young Shin , Lee Sung Ho , Yasunaga Kunio , Kim Kyung-Tae , Kim Injune , Choi Han-Ho , Kim Won , Kim Sung Hyun , Park Sung Kwang , Lee Gyun Min , Koh Gou Young
Published: April 2004

Angiopoietin-1 (Ang1) has potential therapeutic applications in inducing angiogenesis, enhancing endothelial cell survival, and preventing vascular leakage. However, production of Ang1 is hindered by aggregation and insolubility resulting from disulfide-linked higher-order structures. Here, by replacing the N-terminal portion of Ang1 with the short coiled-coil domain of cartilage oligomeric matrix protein (COMP), we have generated a soluble, stable, and potent Ang1 variant, COMP-Ang1. This variant is more potent than native Ang1 in phosphorylating the tyrosine kinase with Ig and epidermal growth factor homology domain 2 (Tie2) receptor and Akt in primary cultured endothelial cells, enhancing angiogenesis
in vitro
and increasing adult angiogenesis
in vivo
. Thus, COMP-Ang1 is an effective alternative to native Ang1 for therapeutic angiogenesis
in vivo
.
A single amino acid substitution in human APOBEC3G antiretroviral enzyme confers resistance to HIV-1 virion infectivity factor-induced depletion
Authors: Xu Hongzhan , Svarovskaia Evguenia S. , Barr Rebekah , Zhang Yijun , Khan Mohammad A. , Strebel Klaus , Pathak Vinay K.
Published: March 2004

HIV-1 and other retroviruses occasionally undergo hypermutation, characterized by a high rate of G-to-A substitution. Recently, the human apolipoprotein B mRNA-editing, enzyme-catalytic, polypeptide-like 3G (APOBEC3G), first identified as CEM15, was shown to be packaged into retroviral virions and to deaminate deoxycytidine to deoxyuridine in newly synthesized viral minus-strand DNA, thereby inducing G-to-A hypermutation. This innate mechanism of resistance to retroviral infection is counteracted by the HIV-1 viral infectivity factor (Vif), which protects the virus by preventing the incorporation of APOBEC3G into virions by rapidly inducing its ubiquitination and proteasomal degradation. To gain insights into the mechanism by which Vif protects HIV-1 from APOBEC3G, we substituted several amino acids in human APOBEC3G with equivalent residues in simian APOBEC3Gs that are resistant to HIV-1 Vif and determined the effects of the mutations on HIV-1 replication in the presence and absence of Vif. We found that a single amino acid substitution mutant of human APOBEC3G (D128K) can interact with HIV-1 Vif but is not depleted from cells; thus, it inhibits HIV-1 replication in an HIV-1 Vif-resistant manner. Interestingly, rhesus macaque simian immunodeficiency virus 239 or HIV-2 Vif coexpression depleted the intracellular steady state levels of the D128K mutant and abrogated its antiviral activity, indicating that it can be a substrate for the proteasomal pathway. The HIV-1 Vif-resistant mutant APOBEC3G could provide a gene therapy approach to combat HIV-1 infection.
Acculturation of Greek Americans: Change and continuity in cognitive schemas guiding intimate relationships
Authors: Koutrelakos James
Published: April 2004

The study compares Greek Americans to Greeks and to third‐generation white Americans in their endorsement of two cognitive schemas guiding intimate relationships. Greek Americans were more rejecting of low self‐disclosure in intimate relationships than were Greeks but did not differ from them on how strongly they advocated sacrificing the self for one's partner. By contrast, Greek Americans did not differ from Americans in their rejection of low self‐disclosure and more strongly endorsed self‐sacrifice in intimate relationships than did Americans. These findings were interpreted as indicating that Greek Americans have acculturated to a more individualistic orientation in terms of self‐disclosure while maintaining a collectivistic orientation regarding self‐sacrifice in intimate relationships. Respondents' age, cultural group, and whether they were college students or professionals interacted with how strongly individuals rejected low self‐disclosure and showed that age and status differences were more pronounced between rather than within the three cultural groups. It revealed that the initial finding, showing that Greeks and Americans differed, was based on the scores of students; professionals, with one exception, did not differ in their disagreement with low self‐disclosure, regardless of their age and cultural group. The exception was the older Greek American professional subgroup, whose stronger disagreement with low self‐disclosure may be an overreaction to the acculturation process. Age and status differences were not significant in the American group, while there was a pattern in Greece for professionals to reject low self‐disclosure more strongly than did students. Women were more rejecting of both low self‐disclosure and self‐sacrifice in intimate relationships than were men. Older women most strongly disagreed with the self‐sacrifice principle and older men adhered to it more strongly with increasing age.