bluuebunny/crossref_metadata_2025 huggingface dataset:
Density matrix functional theory of four-electron systems
Authors: Cioslowski Jerzy , Buchowiecki Marcin , Ziesche Paul
Published: November 2003

An approximate expression for the electron–electron repulsion energy of a closed-shell four-electron system in terms of the Coulomb and exchange integrals among natural orbitals and the respective occupation numbers is derived. It constitutes a strict upper bound to the exact density matrix functional, yields energy that is lower than that obtained within the antisymmetrized product of strongly orthogonal geminals theory, and thus is exact for two noninteracting two-electron systems. Its relevance to the general case of closed-shell N-electron systems is discussed.
Restricted open-shell Kohn–Sham theory for π–π* transitions. I. Polyenes, cyanines, and protonated imines
Authors: Grimm Stephan , Nonnenberg Christel , Frank Irmgard
Published: November 2003

We present a self-consistent field algorithm for the restricted open-shell Kohn–Sham method which can be used to calculate excited states that have the same spatial symmetry as the corresponding ground states. The method is applied to π–π* transitions in polyenes, cyanines, and protonated imines. Excitation energies obtained with gradient corrected functionals are found to be significantly redshifted; the shift is constant within a homologous series. Planar excited state geometries have been optimized for all systems.
Restricted open-shell Kohn–Sham theory for π–π* transitions. II. Simulation of photochemical reactions
Authors: Nonnenberg Christel , Grimm Stephan , Frank Irmgard
Published: November 2003

We present first principles molecular dynamics simulations of the photochemistry of butadiene and cyclohexadiene. The excited state is described with restricted open-shell Kohn–Sham theory. We observe cis–trans isomerizations for 1,3-butadiene and conrotatory ring opening of cyclohexadiene. The excitation of a sample of several butadiene molecules leads to the formation of an excimer.
Collective and thermal diffusion in dilute, semidilute, and concentrated solutions of polystyrene in toluene
Authors: Rauch J. , Köhler W.
Published: November 2003

Collective mass diffusion (D), thermal diffusion (DT), and Soret coefficients (ST) have been determined for solutions of polystyrene in toluene ranging from dilute (10−4 g/cm3) to concentrated (0.9 g/cm3) by a transient holographic grating technique and photon correlation spectroscopy (collective diffusion only). The molar mass range of the polymer was between 4.75 and 4060 kg/mol. With a slight exception at intermediate concentrations, DT is molar mass independent over the entire concentration range. Above a polymer concentration of C≈0.2 g/cm3 the rising glass transition temperature of the solution leads to a rapid slowing down of both mass and thermal diffusion (Ludwig Soret effect) and the structure relaxation enters the experimental time window. Both D and DT are governed by the same microscopic local viscosity ηeff, which increases dramatically at Tg. It cancels out in ST=DT/D. As a consequence, ST is completely insensitive to the glass transition and shows concentration-dependent scaling as expected for a fictive polymer solution with concentration-independent local friction. Where available, results are compared with literature data.
A comparison of the low-frequency vibrational spectra of liquids obtained through infrared and Raman spectroscopies
Authors: Giraud Gerard , Wynne Klaas
Published: November 2003

Dynamic solvation of charge-distribution rearrangements is often described using a (harmonic) solvent coordinate. It is not a priori clear whether such a solvent coordinate has a real physical meaning. We have studied five polar organic liquids (benzonitrile, benzyl alcohol, N,N-dimethylformamide, ethylene glycol, and glycerol triacetate) with high-resolution high signal-to-noise ultrafast optical heterodyne-detected Raman-induced optical Kerr effect spectroscopy (OHD-RIKES). The data, converted to the frequency domain, were analyzed entirely with a multimode Brownian-oscillator model. The infrared spectra of the same five liquids were obtained with a combination of terahertz spectroscopy and Fourier-transform infrared spectroscopy. The Brownian-oscillator fits to the OHD-RIKES spectra could be converted successfully to IR spectra by using a simple theoretical model and by keeping all Brownian-oscillator parameters the same except for the amplitudes. This suggests that there is a small set of harmonic oscillators describing ultrafast solvent nuclear dynamics that can be used to understand solvation, IR absorption, and Raman scattering spectra.
Dust acoustic solitary waves and double layers in a dusty plasma with trapped electrons
Authors: El-Labany S. K. , El-Taibany W. F.
Published: November 2003

The effect of variable dust charge, dust temperature, and trapped electrons on small amplitude dust acoustic waves is investigated. It is found that both compressive and rarefactive solitons as well as double layers exist depending on the nonisothermality parameter. A modified Korteweg–de Vries is derived. At critical density, the Korteweg–de Vries equation is obtained. Employing quasipotential analysis, the Sagdeev potential equation with the inclusion of different new effects has been derived. Because of the presence of free and trapped electrons, the plasma acoustic wave has gained features of various solitary waves. The Sagdeev potential equation, at a small amplitude, shows that the ordering of nonisothermality plays a unique role. In the case of a plasma with first-order nonisothermality, the Sagdeev potential equation shows the compressive solitary wave propagation, while for plasma with higher-order nonisothermality, the solution of this equation reveals the coexistence of both compressive and rarefactive solitary waves. In addition, for certain plasma parameters, the solitary wave disappears and a double layer is expected. Again, with the better approximation in the Sagdeev potential equation, more features of solitary waves, e.g., spiky and explosive, along with the double layers, are also highlighted. The findings of this investigation may be useful in understanding laboratory plasma phenomena and astrophysical situations.
Charge density and electric charge in quantum electrodynamics
Authors: Morchio G. , Strocchi F.
Published: November 2003

The convergence of integrals over charge densities is discussed in relation with the problem of electric charge and (nonlocal) charged states in quantum electrodynamics. Delicate points like the domain dependence of local charges as quadratic forms and the class of time smearing ensuring strong convergence of integrals of charge densities are analyzed and shown to be crucial in QED, also for the control of vacuum polarization effects leading to time dependence of the charge (Swieca phenomenon). The possibility of constructing physical charged states in the Feynman–Gupta–Bleuler gauge as limits of local state vectors is discussed, compatibly with the vanishing of the Gauss charge on local states. A modification of the Dirac exponential factor which yields the physical Coulomb fields from the Feynman–Gupta–Bleuler fields is shown to remove the infrared divergence of scalar products of local and physical charged states, allowing for a construction of physical charged fields with well-defined correlation functions with local fields.
Integrability characteristics of two-dimensional generalizations of NLS type equations
Authors: Choudhury S. Roy
Published: November 2003

A recent procedure based on truncated Painlevé expansions is used to derive Lax Pairs, Darboux transformations, and various soliton solutions for integrable (2+1) generalizations of NLS type equations. In particular, diverse classes of solutions are found analogous to the dromion, instanton, lump, and ring soliton solutions derived recently for (2+1) Korteweg–de Vries type equations, the Nizhnik–Novikov–Veselov equation, and the (2+1) Broer–Kaup system.
The helically-reduced wave equation as a symmetric-positive system
Authors: Torre C. G.
Published: November 2003

Motivated by the partial differential equations of mixed type that arise in the reduction of the Einstein equations by a helical Killing vector field, we consider a boundary value problem for the helically-reduced wave equation with an arbitrary source in 2+1 dimensional Minkowski space–time. The reduced equation is a second-order partial differential equation which is elliptic in a disk and hyperbolic outside the disk. We show that the reduced equation can be cast into symmetric-positive form. Using results from the theory of symmetric-positive differential equations, we show that this form of the helically-reduced wave equation admits unique, strong solutions for a class of boundary conditions which include Sommerfeld conditions at the outer boundary.
The second variation of a null geodesic
Authors: Tian Guihua , Zheng Zhao
Published: November 2003

Confined to the second derivative of the variation of a null geodesic, the proper acceleration of the timelike curves obtained from the variation goes infinity as they approach the null geodesic except that the variation vector is a generalized Jacobi field on the null geodesic and the second variation β2 is constant on the null geodesic.
Effective interaction between large spheres immersed into a multicomponent hard-sphere fluid
Authors: Henderson Douglas , Wasan Darsh T. , Trokhymchuk Andrij
Published: November 2003

An approach to study the effective interaction between two extremely large spheres, infinitely dilute, in a multicomponent hard-sphere fluid is presented. The proposed approach is an approximation that guarantees at least qualitative consistency with the experimental results for the interaction between two large solutes that is mediated by a background medium. The approach is developed to study the complex colloidal systems, i.e., extremely large (micronsized) spherical particles immersed in suspensions of the smaller (nanosized) colloid particles that are dissolved in a molecular solvents. Numerical results for the effective energy and force between two extremely large spheres in a one-component hard-sphere fluid (when only colloid particles are considered explicitly) and a two-component hard-sphere fluid (when both “solvent” particles and colloid particles are considered explicitly), are presented. The role of the discrete nature of the solvent on the interaction between two extremely large spheres immersed in a colloidal suspension is discussed. To account for the effect of colloid particle size polydispersity, calculations with four-component and ten-component hard-sphere fluids are performed. Using the Derjaguin approximation, the disjoining energy and pressure between two flat hard surfaces separated by a multicomponent hard-sphere fluid could be obtained.
Librational heat capacity of fullerenes in the Einstein model
Authors: Johari G. P.
Published: November 2003

In search of low-energy excitations in rigid molecular crystals, the available heat capacity, Cp, data of 85%C60–15%C70 (wt %) mixture at temperatures in the 2–30 K range have been analyzed. It is found that its Cp/T3 against T plot has an asymmetric peak centered at 4.5 K, which has the features of both the Boson peak and of the peak attributed to librations of guest molecules in clathrates. Appearance of a peak in the Cp/T3 against T plot is a characteristic of the Einstein equation for random phase atomic motions. Most of the Cp of fullerenes at T<10 K is attributed to random phase librations of C60 and C70 molecules at their crystal sites.
Internal noise stochastic resonance in a circadian clock system
Authors: Hou Zhonghuai , Xin Houwen
Published: November 2003

We have studied the influence of internal noise on a circadian clock system using stochastic simulation methods and chemical Langevin equations. It is found that internal noise can induce circadian oscillations, when the corresponding deterministic system does not oscillate. The performance of the noise induced circadian oscillation undergoes a maximum with the variation of the internal noise level, showing the occurrence of internal noise stochastic resonance. Since the magnitude of the internal noise is changed via the variation of the system size, these phenomena also demonstrate a kind of system size resonance.
Polyamorphism in low temperature water: A simulation study
Authors: Guillot Bertrand , Guissani Yves
Published: November 2003

By performing molecular dynamics simulations we have generated several forms of high density amorphous (HDA) water (ρ⩾1.1 g/cm3), either by pressurizing cubic ice or by quenching liquid water maintained under pressure. It is shown that the temperature of amorphization is a key variable in understanding the thermodynamic and structural properties of the resulting amorphous phase. Thus, the higher the temperature of amorphization the denser the amorphous ice recovered at low temperature. Two limiting forms of high density are identified, a less dense one obtained by pressure-induced amorphization of cubic ice at low temperature which can be assigned to the HDA phase commonly investigated experimentally, and a denser form obtained by vitrifying the liquid under pressure whose structure is analogous to the VHDA phase recently discovered. Upon heating, both phases transform into low density phases (LDAI and LDAII), respectively, with slightly different properties. In particular, it is found that the LDAI phase exhibits thermodynamic, structural, and dynamical properties very close to that of hyperquenched glassy water. These findings can be understood by analyzing the relative displacements of the molecules with respect to their original positions in the sample at the beginning of the amorphization process. This study supports the idea of a continuity of metastable amorphous phases in low temperature water.
Effects of chain stiffness on the quenching of an excited polymer by small quenchers
Authors: Kim Ji-Hyun , Lee Sangyoub
Published: November 2003

We present a theory for analyzing the effects of chain stiffness on the diffusion-influenced quenching kinetics of an excited polymer. We model the polymer as an optimized Rouse–Zimm chain and the quencher molecule as a spherical particle. The excitation is considered to be localized at any one monomer, or to move randomly along the chain backbone. In regard to the dependence on the chain stiffness, we found two distinctive kinetic regimes. When the excitation migration rate is small, the quenching rate decreases as the chain becomes stiffer. On the other hand, when the mobility of excitation is large, the opposite trend is observed. We also investigate the dependence of Stern–Volmer coefficient on the length and stiffness of the chain in the fast excitation migration limit.
The two-phase model for calculating thermodynamic properties of liquids from molecular dynamics: Validation for the phase diagram of Lennard-Jones fluids
Authors: Lin Shiang-Tai , Blanco Mario , Goddard William A.
Published: November 2003

We propose a general approach for determining the entropy and free energy of complex systems as a function of temperature and pressure. In this method the Fourier transform of the velocity autocorrelation function, obtained from a short (20 ps) molecular dynamics trajectory is used to obtain the vibrational density of states (DoS) which is then used to calculate the thermodynamic properties by applying quantum statistics assuming each mode is a harmonic oscillator. This approach is quite accurate for solids, but leads to significant errors for liquids where the DoS at zero frequency, S(0), remains finite. We show that this problem can be resolved for liquids by using a two phase model consisting of a solid phase for which the DoS goes to zero smoothly at zero frequency, as in a Debye solid; and a gas phase (highly fluidic), described as a gas of hard spheres. The gas phase component has a DoS that decreases monotonically from S(0) and can be characterized with two parameters: S(0) and 3Ng, the total number of gas phase modes [3Ng→0 for a solid and 3Ng→3(N−1) for temperatures and pressures for which the system is a gas]. To validate this two phase model for the thermodynamics of liquids, we applied it to pure Lennard-Jones systems for a range of reduced temperatures from 0.9 to 1.8 and reduced densities from 0.05 to 1.10. These conditions cover the gas, liquid, crystal, metastable, and unstable states in the phase diagram. Our results compare quite well with accurate Monte Carlo calculations of the phase diagram for classical Lennard-Jones particles throughout the entire phase diagram. Thus the two-phase thermodynamics approach provides an efficient means for extracting thermodynamic properties of liquids (and gases and solids).
OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules
Authors: Paesani F. , Zillich R. E. , Whaley K. B.
Published: November 2003

We determine the structure and energetics of complexes of the linear OCS molecule with small numbers of para-hydrogen molecules, N=1–8, using zero temperature quantum Monte Carlo methods. Ground state calculations are carried out with importance-sampled rigid body diffusion Monte Carlo and excited state calculations with the projection operator imaginary time spectral evolution methodology. The ground states are found to be highly structured, with a gradual build up of two axial rings as N increases to 8. Analysis of the azimuthal density correlations around the OCS molecule shows that these rings are quite delocalized for small N values, but become strongly localized for N⩾5. Excited state calculations are made for a range of total cluster angular momentum values and the rotational energy levels fitted to obtain effective rotational and distortion constants of the complexed OCS molecule as a function of cluster size N. Detailed analysis of these spectroscopic constants indicates that the complexes of OCS with para-hydrogen have an unusually rich variation in dynamical behavior, with sizes N=1–2 showing near rigid behavior, sizes N=3–4 showing extremely floppy behavior, and the larger sizes N=5–8 showing more rigid behavior again. The large values of the distortion constant D obtained for N=3–4 are rationalized in terms of the coupling between the OCS rotations and the “breathing” mode of the first, partially filled ring of para-hydrogen molecules.
Quantum wave packet study of reactive and inelastic scattering between C(1D) and H2
Authors: Lin Shi Ying , Guo Hua
Published: November 2003

Using a wave packet method, state-to-state inelastic transition probabilities and initial state specified total reaction probabilities are calculated for the title system (J=0) on a recent ab initio potential energy surface. Both the inelastic and reactive scattering probabilities are found to be strongly oscillatory, indicative of the involvement of long-lived resonances that are supported by a deep CH2 well. The oscillation becomes less pronounced at higher collision energies and with internal excitation of the reactant molecule. The reaction from the (νi=0, ji=0) initial state is clearly dominated by the insertion pathway, and this dominance is largely unaffected by the excitation of the reactant rotation or vibration. In addition, low-lying vibrational states of CH2 have been determined and compared with spectroscopic data.
Potential energy surface and rovibrational spectrum of He–N2O dimer
Authors: Chang Ben T. , Akin-Ojo Omololu , Bukowski Robert , Szalewicz Krzysztof
Published: November 2003

Symmetry-adapted perturbation theory (SAPT) calculations were performed to determine a two-dimensional potential for the interaction of the helium atom with the nitrous oxide molecule. For selected geometries, correlated supermolecular calculations were used to verify SAPT data. The ab initio interaction energies were fitted to an analytic function and rovibrational energy levels of He–N2O were computed on the resulting surface. Extensive comparisons were made with a literature ab initio He–CO2 potential and rovibrational states in order to rationalize the counterintuitive observations concerning spectra of N2O and CO2 in superfluid helium nanodroplets. We conjecture that the greater reduction of the N2O rotational constant than that of CO2 is related to the greater potential depth in the former case and the resulting greater probability of attaching helium atoms. An additional factor could be that the secondary minimum on the O side of N2O is 30% deeper than the linear minima in the case of CO2. As a by-product of this work, accurate multipole moments of N2O have been computed. The quadrupole, octupole, and hexadecapole moments are significantly different from experimental values and are probably more accurate than the latter.
The two-dimensional magnetohydrodynamic Kelvin–Helmholtz instability: Compressibility and large-scale coalescence effects
Authors: Baty H. , Keppens R. , Comte P.
Published: November 2003

The Kelvin–Helmholtz (KH) instability occurring in a single shear flow configuration that is embedded in a uniform flow-aligned magnetic field, is revisited by means of high resolution two-dimensional magnetohydrodynamic simulations. First, the calculations extend previous studies of magnetized shear flows to a higher compressibility regime. The nonlinear evolution of an isolated KH billow emerging from the fastest growing linear mode for a convective sonic Mach number Mcs=0.7 layer is in many respects similar to its less compressible counterpart (Mach Mcs=0.5). In particular, the disruptive regime where locally amplified, initially weak magnetic fields, control the nonlinear saturation process is found for Alfvén Mach numbers 4≲MA≲30. The most notable difference between Mcs=0.7 vs Mcs=0.5 layers is that higher density contrasts and fast magnetosonic shocklet structures are observed. Second, the use of adaptive mesh refinement allows to parametrically explore much larger computational domains, including up to 22 wavelengths of the linearly dominant mode. A strong process of large-scale coalescence is found, whatever the magnetic field regime. It proceeds through continuous pairing/merging events between adjacent vortices up to the point where the final large-scale vortical structure reaches the domain dimensions. This pairing/merging process is attributed to the growth of subharmonic modes and is mainly controlled by relative phase differences between them. These grid-adaptive simulations demonstrate that even in very weak magnetic field regimes (MA≃30), the large-scale KH coalescence process can trigger tearing-type reconnection events previously identified in cospatial current–vortex sheets.
Semiclassical nonlinear Schrödinger on the half line
Authors: Kamvissis Spyridon
Published: November 2003

We are studying the semiclassical limit of the 1+1 dimensional integrable nonlinear Schrödinger equation with defocusing cubic nonlinearity on the half line. Our analysis relies on the recent theory of Fokas et al., which reduces boundary value problems for soliton equations to Riemann–Hilbert factorization problems. We employ the method of nonlinear steepest descent to asymptotically deform the given Riemann–Hilbert problem to an explicilty solvable one.
Double structures and double symmetries for the general symplectic gravity models
Authors: Gao Ya-Jun
Published: November 2003

By using the so-called double-complex function method, a doubleness symmetry for each member of the class of stationary axisymmetric general symplectic gravity models is found and exploited so that some double-complex (n+1)×(n+1) matrix Ernst-like potential for any non-negative integer n can be constructed and the associated motion equations can be extended into a double-complex matrix Ernst-like form. Then double symmetry symplectic groups Sp(2(n+1), R(J)) of the theories are given and verified that their actions can be realized concisely by double-complex matrix form generalizations of the fractional linear transformation on the Ernst potential. These results demonstrate that the theories under consideration possess more and richer symmetry structures. The special cases n=0 and n=1 correspond, respectively, to the pure Einstein gravity and the Einstein–Maxwell-dilaton–axion theories. Moreover, as an application, for each n=0,1,2,…, an infinite chain of double-solutions of the general symplectic gravity model is obtained, which shows that the double-complex method is more effective. Some of the results in this paper cannot be obtained by the usual (nondouble) scheme.
Old and new results for superenergy tensors from dimensionally dependent tensor identities
Authors: Edgar S. Brian , Wingbrant Ola
Published: November 2003

It is known that some results for spinors, and in particular for superenergy spinors, are much less transparent and require a lot more effort to establish, when considered from the tensor viewpoint. In this paper we demonstrate how the use of dimensionally dependent tensor identities enables us to derive a number of 4-dimensional identities by straightforward tensor methods in a signature independent manner. In particular, we consider the quadratic identity for the Bel–Robinson tensor TabcxTabcy=δxy TabcdTabcd/4 and also the new conservation law for the Chevreton tensor, both of which have been obtained by spinor means; both of these results are rederived by tensor means for 4-dimensional spaces of any signature, using dimensionally dependent identities, and, moreover, we are able to conclude that there are no direct higher dimensional analogs. In addition we demonstrate a simple way to show the nonexistense of such identities via counter examples; in particular we show that there is no nontrivial Bel tensor analog of this simple Bel–Robinson tensor quadratic identity. On the other hand, as a sample of the power of generalizing dimensionally dependent tensor identities from four to higher dimensions, we show that the symmetry structure, trace-free and divergence-free nature of the 4-dimensional Bel–Robinson tensor does have an analog for a class of tensors in higher dimensions.
Electron beam dynamics in Pasotron microwave sources
Authors: Carmel Y. , Shkvarunets A. , Nusinovich G. S. , Rodgers J. , Bliokh Yu. P. , Goebel D. M.
Published: November 2003

The Pasotron is a high efficiency (∼50%), plasma-assisted microwave generator in which the beam electrons exhibit two-dimensional motion in the slow wave structure. The electron beam propagates in the ion-focusing regime (Bennett pinch regime) because there is no applied magnetic field. Since initially only the neutral gas is present in the vacuum system and the ions in the neutralizing plasma channel are produced only due to the beam impact ionization, the beam dynamics in Pasotrons is inherently a nonstationary process, and important for efficient operation. The present paper contains results of experimental studies of stationary and nonstationary effects in the beam dynamics in Pasotrons and their theoretical interpretation.
Linear theory of nonlocal transport in a magnetized plasma
Authors: Brantov A. V. , Bychenkov V. Yu. , Rozmus W. , Capjack C. E. , Sydora R.
Published: November 2003

A system of nonlocal electron-transport equations for small perturbations in a magnetized plasma is derived using the systematic closure procedure of Bychenkov et al. [Phys. Rev. Lett. 75, 4405 (1995)]. Solution to the linearized kinetic equation with a Landau collision operator is obtained in the diffusive approximation. The Fourier components of the longitudinal, oblique, and transversal electron fluxes are found in an explicit form for quasistatic conditions in terms of the generalized forces: the gradients of density and temperature, and the electric field. The full set of nonlocal transport coefficients is given and discussed. Nonlocality of transport enhances electron fluxes across magnetic field above the values given by strongly collisional local theory. Dispersion and damping of magnetohydrodynamic waves in weakly collisional plasmas is discussed. Nonlocal transport theory is applied to the problem of temperature relaxation across the magnetic field in a laser hot spot.
Dielectric α-relaxation and ionic conductivity in propylene glycol and its oligomers measured at elevated pressure
Authors: Casalini Riccardo , Roland C. Michael
Published: November 2003

Structural dynamics and volume were measured as a function of both temperature and pressure for a propylene glycol and its oligomers (PPG), and the results compared with previous data on higher molecular weight polypropylene glycols. PPG is of special interest because the terminal groups form hydrogen bonds; thus, by studying different molecular weights, the manner in which hydrogen bonding influences the dynamics in the supercooled regime can be systematically investigated. The fragility (Tg-normalized temperature dependence) of the dimer and trimer of PPG increases with pressure, similar to results for other H-bonded liquids, but different from van der Waals glass formers. This behavior is believed to be due to the effect of pressure in decreasing the extent of hydrogen bonding. From the combined temperature and volume dependences of the relaxation times, the relative degree to which thermal energy and volume govern the dynamics was quantified. With decreasing molecular weight, the relative contribution of thermal energy to the dynamics was found to strongly increase, reflecting the role of hydrogen bonding. By comparing the ionic conductivity and the dielectric relaxation times, a decoupling between rotational and translational motions was observed. Interestingly, this decoupling was independent of both pressure and molecular weight, indicating that hydrogen bonds have a negligible effect on the phenomenon.
N
=4
characters in Gepner models, orbits and elliptic genera
Authors: Grünberg Daniel B.
Published: November 2003

We review the properties of characters of the N=4 superconformal algebra in the context of a nonlinear sigma model on K3, how they are used to span the orbits, and how the orbits produce topological invariants like the elliptic genus. We derive the same expression for the K3 elliptic genus using three different Gepner models (16, 24, and 43 theories), detailing the orbits and verifying that their coefficients Fi are given by elementary modular functions. We also reveal the orbits for the 1322, 144, and 1242 theories. We derive relations for cubes of theta functions and study the function (1/η) ∑n∈Z(−1)n(6n+1)k q(6n+1)2/24 for k=1,2,3,4.
Casimir force between surfaces close to each other
Authors: Ahmedov H. , Duru I. H.
Published: November 2003

Casimir interactions (due to the massless scalar field fluctuations) of two surfaces which are close to each other are studied. After a brief general presentation of the technique, explicit calculations are performed for specific geometries.
Estimation of percolation thresholds via percolation in inhomogeneous media
Authors: Zuyev S. , Quintanilla J.
Published: November 2003

This paper mathematically justifies techniques used to estimate the percolation thresholds of fully penetrable disks, or Boolean models of planar disks. Generalizations to systems of other particles in two or more dimensions are also discussed.
SO
2
revisited: Impact of tight d augmented correlation consistent basis sets on structure and energetics
Authors: Wilson Angela K. , Dunning Thom H.
Published: November 2003

The total energies, atomization energies, molecular structures, and harmonic vibrational frequencies of SO2 have been determined using the newly revised correlation consistent basis sets, cc-pV(n+d)Z, with n=2–5. The impact of the additional tight d functions on the energies, structures, and frequencies as well as the convergence of these quantities to the complete basis set (CBS) limit is examined. Overall, the revised sets result in faster convergence to the CBS limit. The effect of the additional tight d function is particularly dramatic for the lower level, double zeta and triple zeta, basis sets.
Advantages and limitations of Kohn–Sham orbitals as single electron basis for multireference configuration interaction and multireference perturbation theory
Authors: Hupp Thomas , Engels Bernd , Görling Andreas
Published: November 2003

In the present work we study the performance of orbitals from an effective exact exchange-only optimized potential method, the localized Hartree–Fock approach, in multireference ab initio methods. In the first part local Hartree–Fock orbitals are used to describe the excited states of carbon monoxide, formaldehyde, and acetone in multireference configuration interaction calculations. The local Hartree–Fock orbitals are compared with standard Hartree–Fock orbitals, and those from a gradient corrected and a hybrid density functional. This part mainly tries to analyze possible deficiencies of the local Hartree–Fock orbitals. In the second part of this work the above-mentioned orbitals are tested in multireference perturbation theory second-order. We test to which extent known deficiencies of single state perturbation theory second-order are eliminated in multireference approaches, and then study how well local Hartree–Fock orbitals perform in multireference perturbation theory second-order computations of excited states.
Full dimensional theoretical study of the torsion-vibration eigenstates and torsional splittings of CH3OH
Authors: Castillo-Chará Jairo , Sibert Edwin L.
Published: November 2003

The vibrations of methanol are studied theoretically via a fully coupled treatment of the small amplitude motions and the large amplitude torsional mode. Van Vleck perturbation theory is used to find a representation in which the coupling between the vibrational states is reduced. The full vibration-torsion eigenstates are obtained subsequently via matrix diagonalization. The convergence of the perturbation theory and variational calculations are discussed. The torsion-vibration energies and the torsional tunneling splittings are compared with available spectroscopic data for all the fundamentals. The unusual inverted E–A torsional tunneling splitting pattern observed spectroscopically for rotationless torsion-vibrational states is predicted by our calculation.
Parallel-tempering Monte Carlo simulations of the finite temperature behavior of (H2O)6−
Authors: Wang F. , Jordan K. D.
Published: November 2003

The parallel-tempering Monte Carlo method is used in combination with a Drude model to characterize the (H2O)6− cluster over the 50–190 K temperature range. Chainlike structures are found to account for about 50% of the population at 190 K, whereas they are unimportant at the temperatures below about 130 K. At the lowest temperature considered, prismlike structures are dominant. Two new low-energy forms of (H2O)6− are identified.
Electronically excited states of sodium–water clusters
Authors: Schulz Claus Peter , Bobbert Christiana , Shimosato Taku , Daigoku Kota , Miura Nobuaki , Hashimoto Kenro
Published: November 2003

The lowest electronically excited state of small Na(H2O)n clusters has been investigated experimentally and theoretically. The excitation energy as determined by the depletion spectroscopy method drops from 16 950 cm−1 for the sodium atom down to 9670 cm−1 when only three water molecules are attached to the Na atom. For larger clusters the absorption band shifts back towards higher energies and reaches 10 880 cm−1 for n=12. The experimental data are compared to quantum-chemical calculations at the Møeller–Plesset second-order perturbation and multireference single and double excitation configuration interaction levels. We found that the observed size dependence of the transition energy is well reproduced by the interior structure where the sodium atom is surrounded by water molecules. The analysis of the radial charge distribution of the unpaired electron in these interior structures gives a new insight into the formation of the “solvated” electron.
Femtosecond time-resolved charged particle imaging studies of the ultraviolet photodissociation of the NO dimer
Authors: Tsubouchi Masaaki , de Lange Cornelis A. , Suzuki Toshinori
Published: November 2003

Ultraviolet photodissociation dynamics of the NO dimer into NO(Α)+NO(Χ) in a supersonic molecular beam is studied by femtosecond pump-probe charged particle imaging. Time-resolved photoelectron imaging reveals that photoionization from the Franck–Condon region of the excited state(s) reaches vibrationally excited states of the NO dimer cation, resulting in spontaneous fragmentation into NO+NO+ even at the shortest pump-probe time delays. The corresponding photoelectron energy distribution extending up to more than 1 eV above the ionization threshold indicates a large structural difference between the dimer cation and the photoexcited neutral state. From the isotropic photoelectron distribution, this state is assigned to a valence state rather than a Rydberg state. The characteristic photoelectron distribution and also the small NO dimer ion signal vanish within 200 fs. In the same time the photoelectron image rapidly evolves from an almost isotropic distribution into an anisotropic one characteristic for ionization from the 3s Rydberg state of the NO dimer that is adiabatically correlated with the NO(Α)+NO(Χ) fragments. Time-resolved photoion imaging used to measure the translational energy release in the NO(Α)+NO(Χ) dissociation channel reveals that at least one of the fragments, most likely NO(Χ), has an inverted vibrational state distribution. The strong vibrational excitation is in qualitative agreement with a large structural change of the NO dimer in the excited state from the neutral ground state, which presumably involves excitation of the NO stretching vibration in the dimer.
Transient ionization in plasmas produced by point-like irradiation of solid Al targets
Authors: Gizzi L. A. , Cecchetti C. A. , Galimberti M. , Giulietti A. , Giulietti D. , Labate L. , Laville S. , Tomassini P.
Published: November 2003

Time-resolved x-ray spectroscopy has been used to investigate ionization dynamics of a micrometer-sized nanosecond laser-plasma during the plasma start-up phase. Experimental results are modeled using two-dimensional hydrodynamic simulations and time-dependent collisional-radiative calculations. The study clearly shows that, due to the rapid expansion cooling, x-ray emission originates predominantly from a well-localized plasma region characterized by rapidly evolving hydrodynamic conditions. In this region, ionization dynamics is found to depart substantially from the steady-state regime. The measurements provide clear evidence of this transient ionization regime showing good agreement with the time-dependent calculations.
Electrostatic ion-cyclotron waves in a currentless, anisotropic plasma with inhomogeneous flow
Authors: Scime Earl E. , Murphy Ryan , Ganguli Gurudas I. , Edlund Eric
Published: November 2003

The linearized dispersion relation describing waves in a plasma having a uniform magnetic field, uniform density, inhomogeneous parallel (to the magnetic field) flow, and thermal anisotropy (Ti⊥/Ti∥) is used to determine the threshold condition for growth of an electrostatic ion cyclotron wave and its harmonics. The inclusion of moderate ion thermal anisotropy (Ti⊥/Ti∥∼3) and parallel-flow shear (dVd/dx∼0.05Ωci), values typical of many space and laboratory plasmas, reduces the critical current necessary for instability growth to nearly zero. That an electrostatic instability conventionally associated with strong field aligned currents can be excited in the absence of any field aligned current and only by moderate parallel-flow shear suggests that ion cyclotron instabilities may play a larger role in many plasma environments than previously believed. The decrease in the instability threshold for increasing thermal anisotropy suggests that ion heating due to ion cyclotron waves may result in a positive feedback process absent in homogeneous plasmas.
X-ray generation in an ion channel
Authors: Kostyukov I. , Kiselev S. , Pukhov A.
Published: November 2003

X-ray generation by relativistic electrons in an ion channel is studied. The emission process is analyzed in the regime of high harmonic generation when the plasma wiggler strength is large. Like for the conventional free electron laser, the synchrotron-like broadband spectrum is generated in this regime. An asymptotic expression for the radiation spectrum of the spontaneous emission is derived. The radiation spectrum emitted from an axisymmetric monoenergetic electron beam is analyzed. The stimulated emission in the ion channel is studied and the gain of the ion-channel synchrotron-radiation laser is calculated. It is shown that the use of laser-produced ion channels leads to a much higher power of x-ray radiation than the one in a self-generated channel. In addition, the mean photon energy, the number of emitted photons and the brilliance of the photon beam increase dramatically. Three-dimensional particle-in-cell simulations of a 25-GeV electron bunch propagating in a laser-produced ion channel are made. Several GeV γ-quants are produced in a good agreement with the analytical results.
Rotational dynamics of nondipolar and dipolar solutes in an isotropic liquid crystal: Comparison with an isotropic liquid
Authors: Dutt G. B.
Published: November 2003

Rotational dynamics of a nondipolar solute, 2,5-dimethyl-1,4-dioxo-3, 6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and a dipolar solute, coumarin 6 (C6) has been studied in the isotropic phase of a liquid crystal, 4′-methoxybenzylidene-4-n-butylaniline (MBBA) to understand the influence of pseudonematic domains on the dynamics of dopant molecules. The reorientation times of both DMDPP and C6 follow the Stokes–Einstein–Debye hydrodynamic model instead of the Landau–de Gennes model, which is used to describe the rotational relaxation of neat isotropic liquid crystals. However, comparison of the data for both DMDPP and C6 in MBBA to that in an isotropic solvent, 1-decanol reveals that the probes are rotating considerably slower in the liquid crystal. These results indicate that ordered local structures of the liquid crystal exert more friction on the rotating solute molecule compared to an isotropic liquid.
Expanded ensemble and replica exchange methods for simulation of protein-like systems
Authors: Fenwick Michael K. , Escobedo Fernando A.
Published: November 2003

Extended state methods are powerful tools for studying the conformational equilibria of proteins. This study focuses on three aspects of their implementation. First, existing approaches for determining importance weights (namely, recursion, random walk, and transition probability schemes) are compared in the context of their use with the method of expanded ensembles (EXE). Second, a combined scheme (REXE) involving EXE and replica exchange (REX) updates is developed for simulating a small number of replicas within a much larger macrostate space. Finally, variants of the extended state methods are considered for accelerating folding, either through special-purpose ensembles which target specific force-field parameters, or through biased sampling of extended macrostates that favor structural fluctuations. All methods are applied to a three-dimensional lattice protein model. Overall, it is found that transition probability approaches employing multiple system replicas perform naturally better than methods that intrinsically require macrostate equilibration by a single replica; the transition probability approaches need about an order of magnitude fewer steps to reach the same degree of convergence in the importance weights. The specific REXE protocol implemented is observed to have an efficiency intermediate to that of EXE and REX schemes at high temperatures, but to outperform them at more glassy conditions. Finally, special-purpose and locally enhanced tempering ensembles are shown to promote faster folding than conventional tempering.
Microphase separation in solutions of associating polyelectrolytes: Strong segregation approximation
Authors: Limberger Roman E. , Potemkin Igor I. , Khokhlov Alexei R.
Published: November 2003

Strong segregation theory of microphase separation in dilute solutions of associating polyelectrolytes is developed. The driving force of the separation is shown to be a competition between a short-range attraction of associating groups and long-range repulsion of charged groups. We predict that with the increase of polymer concentration single chains first aggregate into spherical clusters of optimum size which can be disordered in space or arranged with the symmetry of the bcc lattice. Further increase of polymer concentration can result in the formation of hexagonal and lamellar structures which precede the physical gelation. The hexagonal and lamellar structures are stable at very small fractions of charged groups on the chains.
Effect of particle interactions on sedimentation field-flow fractionation of a binary suspension
Authors: Felderhof B. U. , Martin M.
Published: November 2003

The effect of particle interactions on sedimentation field-flow fractionation of a binary suspension is studied on the basis of a phenomenological model. The solvent fluid is regarded as a background medium mediating the effective interactions between solute particles. A binary suspension with large and small particles is considered. Equilibrium profiles of both species are calculated from an approximate expression for the Helmholtz free energy of a hard sphere mixture. The effect of solute particles on suspension viscosity is taken into account via a semi-empirical relationship. It is found that retention factors of both species depend strongly on the two mean volume fractions. The ratio of the two retention factors is nearly independent of the concentration of the large particles and first decreases, but then increases significantly with the concentration of the small ones.
Population and decay of keto states in conjugated polymers
Authors: Hintschich S. I. , Rothe C. , Sinha S. , Monkman A. P. , Scandiucci de Freitas P. , Scherf U.
Published: November 2003

Using time-resolved and steady-state photoluminescence techniques, fluorene/fluorenone copolymers have been studied to investigate the role of keto defects in degraded polyfluorene. Keto sites can be populated via migration from polyfluorene singlets, thereby quenching the polyfluorene fluorescence, and via direct photon absorption. In the former case, the migration process dominates all thermal and interchain variability in the efficiency of quenching. No annihilation process of fluorenone triplets and no interchain processes such as excimer formation participate in the defect emission itself.
Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. I. Nature of the noncritical adsorption profiles
Authors: Bowers James , Zarbakhsh Ali , Christenson Hugo K. , McLure Ian A. , Cubitt Robert
Published: November 2003

Neutron reflection has been applied to probe the nature and extent of adsorption from a mixture of (1−x)n-hexane+xperfluoro-n-hexane against silicon substrates modified with alkylsilane (fluorophobic) or fluoroalkylsilane (fluorophilic) coupled layers. For an equimolar mixture (x=0.5, 60.7 vol %) in the one-phase region at T=30 °C—removed both in temperature and composition from the upper critical point at 22.65 °C and x=0.36—the structure was resolved at both fluorophobic and fluorophilic surfaces. Liquid mixtures with three different refractive index contrasts were used to reduce model ambiguity in the ensuing analysis. For both surfaces the composition profiles of the adsorbed liquids could be represented using two-layer slab models which included interlayer Gaussian roughness. For the fluorophobic surface, the thickness of the layer closest to the substrate is ∼20 Å and composed of ∼83 vol % n-hexane, and the second, more dilute layer has a composition profile which decays smoothly into the bulk over a range of ∼100 Å. A similar result is found for the fluorophilic surface, but in this case the layer closest to the substrate is ∼15 Å thick and composed of ∼95 vol % perfluoro-n-hexane. Qualitatively similar behavior is found for adsorption from a mixture with x=0.7 against a fluorophobic substrate and for a mixture with x=0.2 against a fluorophilic substrate.