bluuebunny/crossref_metadata_2025 huggingface dataset:
Raman intensity study of scaling properties of Na+, K+, Ag+, and Pb2+ doped NH4NO3 crystals during phase transitions
Authors: Ma Shuguo , Wu Guozhen , Wang Huanru
Published: July 2002

It is observed that, for molecular crystals, during phase transitions the Raman intensities of the vibrational modes obey the power law |T−Tc|β. The exponent β contains much critical information and is mode dependent. The scaling properties of the doped materials are imbedded in β which is found to be very sensitive to very small degrees of doping. One of the eminent phenomena is the so-called crossover point where β of the totally symmetric ν1 mode equals the nonsymmetric ν4 mode of the nitrate ion. The scaling is expressed through the scaled coordinate d/M1/2 where d is the averaged interdoped-cationic distance and M is the mass of the cation. β of the ν1 mode behaves as (d/M1/2)γ with exponent γ rather independent of the transitions and the kinds of doping. Resonant modes due to doped cations are employed to explain the origin of the scaling.
Direct simulation of hydrodynamic relaxation in microchannels
Authors: Palmer Bruce J.
Published: July 2002

Simulations were performed on a fluid confined between two parallel walls. The fluid is modeled by a Lennard-Jones potential and the walls by a simple cubic lattice of harmonically bonded sites. A Lennard-Jones potential is also used to model the interactions between the wall and the fluid. The simulation consisted of over 30 000 sites arranged to form a liquid film approximately 35 Lennard-Jones diameters in thickness. This is large enough to begin approximating the range where classical hydrodynamics is expected to be applicable. Both equilibrium simulations and simulations of velocity transients were performed on the system. Two values of the wall–fluid interaction strength were examined, which appear to correspond to a wetting and nonwetting surface. Results from equilibrium simulations show that both the density and the stress tensor relax to their bulk values within a short distance of the wall. Furthermore, examination of the relaxation of spontaneous momentum fluctuations indicates that there is little change in the value of transport coefficients near the boundary compared to the bulk fluid. Nonequilibrium simulations on the decay of a parabolic velocity profile, however, suggest that the decay of the profile is faster than would be predicted from classical hydrodynamics and that the type of boundary conditions that should be used in a hydrodynamic analysis may depend on the details of the wall–fluid interaction.
Dispersions of rodlike particles in shear flow by Brownian dynamics simulations
Authors: Brańka A. C. , Heyes D. M.
Published: July 2002

A model 3D colloidal fluid of moderately nonspherical axisymmetric molecules interacting through a two-site Yukawa potential has been studied in planar Couette flow by the Brownian dynamics simulation method. It is shown that these fluids exhibit nonlinear rheological behavior. Shear thinning, normal stress effects and shear-induced alignment are reported and analyzed. The influence of the one-particle diffusion tensor on the rheological and orientational properties is discussed. The shear viscosities are lower for rods that diffuse with an anisotropic diffusion tensor.
Two-photon spectroscopy of the (n,3s) Rydberg transition in acetaldehyde: The torsional sequence
Authors: Kim Y. , Fleniken J. , Meyer H.
Published: July 2002

Eigenvalues and wave functions for acetaldehyde in its electronic ground state and the (n,3s) Rydberg state are calculated using the rho-axis method in combination with the high barrier approximation. The rotational structure of the first members of the torsional sequence in the two-photon spectrum is analyzed in terms of a zeroth-order rotation–torsional Hamiltonian. Combining experimental information on band positions and rotational contours, we determine the range of internal rotation constants and barrier heights, which is compatible with the experimental data. The analysis yields the following parameters: F=8.45±0.15 cm−1, V3=809±10 cm−1, ρ=0.33±0.03, ΘRAM=4.5°±0.8°, and APA=1.990 cm−1, BPA=0.330 cm−1, CPA=0.298 cm−1.
The generator coordinate method for a reaction coordinate coupled to a harmonic oscillator bath
Authors: de Souza Cruz Frederico F. , Ruzzi Maurizio , Kersten Schmidt André C.
Published: July 2002

This paper investigates the usefulness of the generator coordinate method (GCM) for treating the dynamics of a reaction coordinate coupled to a bath of harmonic degrees of freedom. Models for the unimolecular dissociation and isomerization process (proton transfer) are analyzed. The GCM results, presented in analytical form, provide a very good description and are compared to other methods like the basis set method and multiconfiguration time dependent self-consistent field.
Clustering of water in polyethylene: A molecular-dynamics simulation
Authors: Fukuda Mitsuhiro
Published: July 2002

Molecular-dynamics (MD) simulation of the diffusion of water in amorphous polyethylene has been performed under a constant particle number, constant pressure, and constant temperature (NPT) conditions. The three simulation systems with the different water concentration from 0.120% (g/g) to 0.324% (g/g) were tested. A time duration beyond 10 ns at 298 K and at 0.1 MPa revealed that the system retained the amorphous structure and the water molecules came together forming water clusters of various sizes. The amount of water clustering depended on the water concentration and we observed irreversible clustering of a large cluster over 18 ns duration in the system with higher concentration. The oxygen–oxygen, hydrogen–hydrogen, and oxygen–hydrogen radial distribution function in a ten-water cluster showed that the position of the first and second peaks were quite similar to those of liquid TIP4P water, indicating the liquidlike structure of clustered water. As the self-diffusion coefficients, Dself, greatly depended on the time range used, the mutual-diffusion coefficients, Dmutual were also evaluated from the mean-square displacement of the center of mass of all water molecules. The Dmutual values were not so time dependent, and the values around 1×10−6 cm2/s were qualitatively comparable to the experimental values.
Kinetics of crystallization preceded by metastable-phase formation
Authors: Kashchiev Dimo , Sato Kiyotaka
Published: July 2002

A theoretical description is proposed of the kinetics of overall crystallization when the appearance of the stable crystalline (SC) phase is preceded by the formation of a metastable phase. A general formula is derived for the fraction αs of volume of the SC phase crystallized at time t. This formula is used for obtaining the αs(t) function in the case of Kolmogorov–Johnson–Mehl–Avrami kinetics of overall crystallization. The time t1/2 at which half of the transformation of the parent phase into the SC one is accomplished is determined and it is found that this time is the sum of the times for half completion of the two successive phase transformations of the parent phase into the metastable one and of the latter into the SC phase. A general condition for formation of a long living metastable phase is formulated in terms of the nucleation and growth rates of the metastable and the SC phases. The general results are applied to melt crystallization and an expression for the temperature dependence of t1/2 is obtained. It is found that the presence of a metastable phase in the melt is easy to observe only below a certain critical temperature, because then the transformation of the metastable phase into the SC one is sufficiently delayed. In a supplementary analysis, an answer is given to the classical question about the possibility for formation of a metastable phase prior to the stable one in accordance with Ostwald’s Rule of Stages.
ν
1
and ν1+ν5 of DCCN: Determination of the ν5 energy and the quasilinearity of DCCN
Authors: Sun Fuge , Kosterev Anatoliy , Scott Graham , Litosh Vladislav , Curl R. F.
Published: July 2002

The high resolution infrared spectrum of DCCN in the region of 2385–2530 cm−1 has been investigated using infrared kinetic spectroscopy. The CD stretching fundamental, ν1,together with the closely related hot bands, ν1+ν5−ν5,ν1+2ν5±2−2ν5±2, and ν1+2ν50−2ν50 have been observed and assigned. In addition the combination band, ν1+ν5, was observed. When the information provided by this perpendicular band is combined with that provided by ν1+ν5−ν5, the vibrational energy of ν5 is accurately determined to be 74.845(2) cm−1. This measurement, when combined with the ab initio and semirigid bender calculations of Malmquist et al., gives a barrier to linearity of 280 cm−1. Generally, fine structure splittings are not resolved for any levels except those belonging to the upper l- or K-type doubling components of ν1+ν5. This fine structure splitting is most strongly exhibited in the Q-branch of the ν1+ν5 band where it is 0.039 cm−1.
Potential energy curves for the dissociation of the Rydberg NH4 radical into (NH2+H2)
Authors: Park Jong Keun
Published: July 2002

Potential energy curves of the ground and excited states for the dissociation of the Rydberg NH4 radical into (NH2+H2) have been calculated using ab initio Hartree–Fock and singly and doubly excited configuration interaction methods with a large basis set including Rydberg basis functions. The ground potential curve (2A1) of the (NH4+)(e−)3s radical adiabatically correlates to the [NH2*(Ã 2A1)+H2(X̃ 1Σg+)] asymptote, while the first excited state (2T2) of (NH4+)(e−)3p correlates to [NH2(X̃ 2B1)+H2(X̃ 1Σg+)]. Two diabatic valence curves emerging from the [NH2*(Ã 2A1)+H2(X̃ 1Σg+)] and [NH2(X̃ 2B1)+H2(X̃ 1Σg+)] asymptotes are repulsively represented, while two diabatic curves from [NH2+(Ã 1A1)+H2−(X̃ 2Σu+)] and [NH2+(X̃ 3B1)+H2−(X̃2 Σu+)] are attractively represented. At shorter than R(NH)≃2.0 Å, the avoided curve crossings between the dissociative diabatic states of the [(NH4+)(e−)Rydberg] radical and the repulsive diabatic states emerging from the antibonding interactions of the [NH2+H2(X̃ 1Σg+)] asymptote are found mainly. While, at larger than R(NH)≃2.0 Å, the avoided curve crossings between the repulsive diabatic states emerging from H2 and the Rydberg states of NH2 and the attractive diabatic states from [NH2+(Ã 1A1)+H2−(X̃ 2Σu+)] and [NH2+(X̃ 3B1)+H2−(X̃ 2Σu+)] are found.
Statistical theory of cluster cooling in rare gas. I. Energy transfer analysis for palladium clusters in helium
Authors: Westergren Jan , Grönbeck Henrik , Rosén Arne , Nordholm Sture
Published: July 2002

The cooling and heating of palladium clusters Pd13 and Pd55 by binary collisions with atoms of a surrounding helium gas are studied by means of molecular dynamics simulation. The efficiency of the collisional energy transfer is determined as a function of cluster and gas temperature and of cluster phase, the cluster being in either a solid or a liquid phase. A simple statistical analysis is presented for the energy transfer between a cluster and a rare gas atom. The analysis is based on an ergodic collision assumption of microcanonical relaxation in each collision. The deviation from this limiting law is collected in a collision efficiency factor which reflects incomplete energy redistribution during the lifetime of the collision complex. The thermal energy and change in heat capacity observed for the clusters at the freezing (melting) transition is accounted for by a parametrized density of states reflecting separate contributions from a solid and a molten structure. The same density of states is then used in the ergodic collision theory for the analysis of energy transfer.
Linear response in theory of electron transfer reactions as an alternative to the molecular harmonic oscillator model
Authors: Georgievskii Yuri , Hsu Chao-Ping , Marcus R. A.
Published: July 2002

The effect of solvent fluctuations on the rate of electron transfer reactions is considered using linear response theory and a second-order cumulant expansion. An expression is obtained for the rate constant in terms of the dielectric response function of the solvent. It is shown thereby that this expression, which is usually derived using a molecular harmonic oscillator (“spin-boson”) model, is valid not only for approximately harmonic systems such as solids but also for strongly molecularly anharmonic systems such as polar solvents. The derivation is a relatively simple alternative to one based on quantum field theoretic techniques. The effect of system inhomogeneity due to the presence of the solute molecule is also now included. An expression is given generalizing to frequency space and quantum mechanically the analogue of an electrostatic result relating the reorganization free energy to the free energy difference of two hypothetical systems [J. Chem. Phys. 39, 1734 (1963)]. The latter expression has been useful in adapting specific electrostatic models in the literature to electron transfer problems, and the present extension can be expected to have a similar utility.
Characterization of the I3 radical by anion photoelectron spectroscopy
Authors: Taylor Travis R. , Asmis Knut R. , Zanni Martin T. , Neumark Daniel M.
Published: July 2002

The ground and first excited states of the I3 radical are characterized by photoelectron spectroscopy of I3− and Ar⋅I3− at 266 nm. The electron affinity of I3 is 4.226±0.013 eV. Based on the recently determined bond dissociation energy of I3−, the I3 ground state is bound by 0.143±0.06 eV. The first excited state of I3 lies 0.27 eV above the ground state. A vibrational progression is seen in the ground state band of the I3− photoelectron spectrum. The addition of an argon atom to I3− reduces the contribution of hot bands to the photoelectron spectrum, facilitating the interpretation of the vibrational structure. Simulations indicate that the I3 ground state is linear with a symmetric stretch frequency of 115±5 cm−1 and is likely to be centrosymmetric.
Observation of D2H+ (or H2D+) products in coincidence with excited H* (or D*) atoms in the reaction of D2+ with H2
Authors: Jugi B. , Dhuicq D. , Benoit C. , Sidis V.
Published: July 2002

The reaction D2++H2→D2H++H*(n>1) or H2D++D*(n>1) is studied experimentally at a relative collision energy of 20 eV. The ionic products are angle and energy analyzed in coincidence with the excited atomic products. The latter are detected by the radiation they emit, possibly after quenching in case of metastable atoms. The contour diagrams obtained for the intensity of the D2H+ or H2D+ products as functions of their center-of-mass scattering angle and velocity show the presence of these two products, with a predominance of the former. These products are essentially found in low vibrational levels and the correlated atoms are observed predominantly in n=2 states; the 2s to 2p cross section ratio is found to be nearly equal to 0.35. That the considered reaction exists was actually predicted on the basis of its analogy with the reaction He++H2→HeH++H*(n>1). The main features of the two reactions are compared. The mechanism of the D2++H2 reaction is explained using an adaptation of the interpretation framework proposed by Dhuicq et al. [Chem. Phys. 206, 139 (1996)] for the He++H2 reaction. It involves energy level promotion, diabatic potential energy surface crossings and related electron transitions, charge transfer, and proton transfer. The discussion shows that the occurrence of this reaction and some of its characteristics stem from the Franck–Condon vibrational distribution of the D2+ reactant ions and especially to the corresponding distribution of bond distances.
Control of reaction rate by asymmetric two-state noise
Authors: Schmid G. J. , Reimann P. , Hänggi P.
Published: July 2002

We revisit the far from equilibrium escape problem across a fluctuating potential barrier that is driven by asymmetric, unbiased dichotomous noise. Our closed analytical solution for arbitrary noise strengths reveals new aspects of the so-called “resonant-activation” effect and leads to interesting implications regarding far from equilibrium or externally controlled chemical reaction processes. Specifically, a genuine asymmetry-induced variant of resonant activation within the constant intensity scaling scheme is discovered, and a new possibility to manipulate reaction rates and yields, as well as the balance between reactants and products, is put forward.
Unimolecular decay of the thiomethoxy cation, CH3S+: A computational study on the detailed mechanistic aspects
Authors: Aschi Massimiliano , Grandinetti Felice
Published: July 2002

The unimolecular decay of the triplet thiomethoxy cation CH3S+, ion 1, has been investigated by density functional theory, ab initio, and Phase–space/Rice Ramsperger Kassel Marcus (PST/RRKM) calculations. We have first located on the singlet and triplet B3LYP/6-311+G(d,p) [C,H3,S]+ potential energy surfaces the energy minima and transition structures involved in the lowest energy decompositions of 1, including the loss of H, H2, and S. We have subsequently located the minimum energy points lying on the B3LYP/6-311+G(d,p) hyperline of intersection between the singlet and triplet surfaces, using a recently described steepest descent-based method [Theor. Chem. Acc. 99, 95 (1998)]. The total energies of all these species were refined by CCSD(T)/cc-pVTZ single-point calculations. The obtained potential energy surface has been used to outline the full kinetic scheme for the unimolecular decay of ion 1. The rate constants of the various elementary steps have been calculated by the PST and the RRKM theory. We used a nonadiabatic version of the latter to evaluate the rate constants of the elementary steps which involve a change in the total spin multiplicity. We found that the two kinetically favored decomposition channels are the loss of atomic hydrogen, with formation of 2CH2S+⋅, and molecular hydrogen, with formation of 1HCS+. The former process is predicted to prevail for ions 1 in the lowest rotational states and with an internal energy content of at least 60 kcal mol−1. The loss of H2 was found to be by far the prevailing process in the time scale of ca. 10−5 to ca. 10−6 s from the formation of 1. This is fully consistent with the experimentally observed exclusive loss of H2 by the CH3S+ ions which decompose in the “metastable” time window of the mass spectrometer. The loss of H2 from ion 1 with formation of 1HCS+ may occur by two distinct “spin-forbidden” paths, i.e., a simple concerted 1,1 H2 elimination or a 1,2 H shift followed by a 1,2 H2 elimination from the singlet mercaptomethyl ion 2. In the metastable time window, these two mechanisms may occur alternatively, depending on the degree of rotational excitation of 1.
Fast computation of analytical second derivatives with effective core potentials: Application to Si8C12, Ge8C12, and Sn8C12
Authors: Bode Brett M. , Gordon Mark S.
Published: July 2002

An improved method is described for the computation of integrals involving effective core potentials. The improved method provides better scalability to higher angular momenta as well as improved speed. The new method is also applied to the determination of the minimum energy structures of Si8C12, Ge8C12, and Sn8C12, main group analogs of the Ti8C12 compounds (known as metcars). Relative energies, geometries, and vibrational frequencies are reported for several novel structures.
Solvent effects in positronium complex formation and the bubble model
Authors: Gangopadhyay Debarshi , Ganguly Bichitra Nandi , Mukherjee Tapas , Dutta-Roy Binayak
Published: July 2002

The formation of positronium–acceptor complexes (Ps–Ac) with nitro aromatics and quinone has attracted much attention. The dependence of such reactions on temperature and on the nature of the solvent is sometimes discussed within the ambit of the bubble model of Ps in liquids. However, the description generally adopted is that of a sharp bubble boundary which is not only unrealistic as a liquid interface but has also been shown recently, through a series of papers by the present authors, to be in contradiction with experimental observations. An explanation is sought for the observed occurrence of both Arrhenius and anti-Arrhenius temperature dependence in the reaction rate for Ps–Ac complex formation seen over different temperature regimes in terms of the Kramers’ turnover in the context of the improved version of the bubble model.
Fine structure, alignment, and orientation of 32S16O and 16O18O molecules in congruent electric and magnetic fields
Authors: Boca Andreea , Friedrich Bretislav
Published: July 2002

We analyze the eigenproperties of the SO and 16O18O ground-state (3Σ) molecules in congruent electric and magnetic fields. The energy levels are found to exhibit avoided crossings for states with the same projection M of the total angular momentum on the common axis of the fields. The avoided crossings are due to a first-order Stark effect which connects intersecting levels of aligned Zeeman states of opposite parity and makes them strongly oriented. We exemplify the enhancement of orientation and other generic features of the combined electric and magnetic dipole interaction in polar 3Σ molecules by evaluating the expectation values of the orientation and alignment cosines as a function of field-strength parameters for a selection of states with |M|⩽5. The molecular states created by the congruent fields can be used to state-select and focus molecules, to orient the molecular axis in studies of collision stereodynamics, to facilitate assignments of spectral transitions, and in molecular trapping. In particular, we found that mixed-isotope O2 and 17O2 are the only chalcogen molecules which are suitable for spatial trapping in a static trap, owing to their rotational ground state which correlates with a low-field seeking Zeeman state.
The role of autoionization in (1+2′)-photon above-threshold ionization of H2 molecules: Study of photoelectron energy spectrum
Authors: Dastidar Krishna Rai , Das Ratan Kumar
Published: July 2002

We have theoretically studied the effect of autoionization through doubly-excited autoionizing states on the photoelectron energy spectrum in (1+2′)-photon above-threshold ionization (ATI) of H2 molecules. We have considered excitation from the ground X 1Σg(v″=0 ,j″) state to the dissociation continuum of autoionizing states of 1Σu and 1Πu symmetry, via the intermediate resonant B 1Σu(v′=5, j′) state. These autoionizing states decay to form molecular ions in the dissociation continuum (dissociative autoionization) and in the bound vibrational levels (nondissociative autoionization) of the ionic ground state (X 2Σg), as well as free electrons carrying extra energy. We have found that the photoelectron spectrum (PES) is mainly determined by the autoionization from the doubly-excited autoionizing states of ungerade symmetry, and that it is oscillatory in nature and shows a structure similar to that observed in a recent experiment by Rottke et al. [J. Phys. B 30, 4049 (1997).]
Time-resolved laser magnetic resonance study of deactivation of Cl(2P1/2)
Authors: Chichinin A. I.
Published: July 2002

Rate constants for the collisional deactivation of spin–orbitally excited Cl* (≡Cl(2P1/2)) atoms by some selected gases have been determined using time-resolved laser magnetic resonance (LMR) techniques. Cl* atoms were produced by photodissociation of ICl at 248 nm, and the relative quantum yield of Cl* atoms is determined to be 0.47±0.06. The rate constants for the relaxation of Cl* (×10−11 cm3/molecule⋅s, ±2σ) by H2(5.5±1.6), HF(1.4±0.4), DF(0.7±0.2), DCl(0.7±0.2), HBr(1.2±0.36), DBr(1.4±0.4), NO(0.66±0.2), SO2 (1.9±0.5), N2O (0.63±0.2), NF3 (22±6), CH4 (1.9±0.6), CD4 (13±4), CF4 (2.4±0.7), SiF4 (14±4), and SF6 (18±5) at 298 K are reported. The rate constants of the reactions of ground state Cl(2P3/2) atoms with HBr and DBr were found to be, in units of ×10−11 cm3/molecule⋅s, 0.75±0.15 and 0.50±0.10, respectively. The study of the deactivation process I(2P1/2)+ICl→Cl*+I2 (a), Cl+I2 (b), I+ICl (c) yielded the ratios ka/(ka+kb)=0.63±0.17, ka/(ka+kb+kc)⩾0.70.
A nonorthogonal approach to perfect pairing
Authors: Voorhis Troy Van , Head-Gordon Martin
Published: July 2002

We present an alternative formulation of perfect pairing (PP) aimed at giving a more faithful representation of the valence correlation energy of an arbitrary molecule. In the new theory, the occupied and virtual orbitals are nonorthogonal amongst themselves but orthogonal to each other. Whereas for the fully orthogonal version of PP one has the number of pairs equal to the number of occupied orbitals, the current formulation allows for an arbitrary number of pairs built from redundant orbitals. We propose setting the number of pairs equal to the number of valence orbitals in the molecule. Preliminary results indicate that the redundant formulation gives qualitatively improved results for delocalized systems such as benzene, while maintaining the attractive features of PP for localized systems.
Theoretical studies of the fullerene growth mechanism: Ring-collapse model to C28 and cascade bond formation
Authors: Mishra Rama K. , Lin Ying-Ting , Lee Shyi-Long
Published: July 2002

The ring-collapse mechanism that suggests the reactions among the mono- to polycyclic carbon clusters has been analyzed using semiempirical AM1 and HF/6-31G* methods. The two cage structures D2 (chiral) and Td (achiral) for the C28 clusters are considered. Basing on the ring-stacking/circumscribing model and the ring-collapse mechanism various precursors are selected along with some appropriate carbon belts. Reactions between the precursors and the belts are found to be endoergic and lead to stable intermediates. All these stacking processes follow gradual and sequential paths. Various possible transition states structures (TSs) have been located and the barrier heights are found to be well within the earlier prescribed limits. Further, stacking the stable intermediates by suitable carbon belts generate the desired cage structures. The second step of the stacking resembles the annealing mechanism for the formation of the cage structures that is essentially an exoergic process. In this annealing process cascade-type bond formation is visualized. Finally, basing on the deformation energies of the precursors and the barrier heights, it is observed that monocyclic precursors are more suitable for the fullerene growth mechanism.
Topological repulsion between polymer globules
Authors: Chuang Jeffrey , Grosberg Alexander Yu. , Tanaka Toyoichi
Published: July 2002

The work is motivated by the experimental observation of B. Chu et al. [Macromolecules 28, 180 (1995)] which suggests that, while polymer globules in dilute solution in poor solvent are supposed to be very sticky, in actuality they collide many hundreds of times before merging and before aggregation starts. We argue that this slow-down is caused by an “entanglement force” which is operational on the prereptational time scale. This force arises from the fact that two touching globules cannot enjoy the mixing entropy gain expected in equilibrium until after they explore all conformations, including entangled ones. We report a molecular dynamics simulation in which we were able to measure the entanglement force as a function of distance. The important conclusion we can formulate so far is qualitative; the entanglement force exists and is sufficient to explain the observed slow-down of aggregation.
Inertial stochastic dynamics. I. Long-time-step methods for Langevin dynamics
Authors: Beard Daniel A. , Schlick Tamar
Published: July 2002

Two algorithms are presented for integrating the Langevin dynamics equation with long numerical time steps while treating the mass terms as finite. The development of these methods is motivated by the need for accurate methods for simulating slow processes in polymer systems such as two-site intermolecular distances in supercoiled DNA, which evolve over the time scale of milliseconds. Our new approaches refine the common Brownian dynamics (BD) scheme, which approximates the Langevin equation in the highly damped diffusive limit. Our LTID (“long-time-step inertial dynamics”) method is based on an eigenmode decomposition of the friction tensor. The less costly integrator IBD (“inertial Brownian dynamics”) modifies the usual BD algorithm by the addition of a mass-dependent correction term. To validate the methods, we evaluate the accuracy of LTID and IBD and compare their behavior to that of BD for the simple example of a harmonic oscillator. We find that the LTID method produces the expected correlation structure for Langevin dynamics regardless of the level of damping. In fact, LTID is the only consistent method among the three, with error vanishing as the time step approaches zero. In contrast, BD is accurate only for highly overdamped systems. For cases of moderate overdamping, and for the appropriate choice of time step, IBD is significantly more accurate than BD. IBD is also less computationally expensive than LTID (though both are the same order of complexity as BD), and thus can be applied to simulate systems of size and time scale ranges previously accessible to only the usual BD approach. Such simulations are discussed in our companion paper, for long DNA molecules modeled as wormlike chains.
Adsorption of acetone molecules on proton ordered ice. A molecular dynamics study
Authors: Picaud S. , Hoang P. N. M.
Published: July 2002

The adsorption of acetone molecules on a proton ordered ice Ih(0001) surface was studied using classical molecular dynamics simulations between 50 and 150 K. At low coverage, we show that acetone molecules form an ordered monolayer on this ice surface, which is stable for T⩽100 K. At higher temperature, it exhibits orientational disordering, though local translational order remains. Preliminary simulations at higher coverage indicates the formation of additional ordered layers above the first monolayer, which are also stable below 100 K. These results support previous conclusions on the acetone/ice interactions based on the interpretation of experimental data.
Localized excess negative charges in surface states of the clean Ga-rich GaAs(100)c(8×2)/4×2 reconstruction as imaged by scanning tunneling microscopy
Authors: Kruse Peter , McLean James G. , Kummel Andrew C.
Published: July 2002

Scanning tunneling microscopy images of the Ga-rich GaAs(100)c(8×2)/(4×2) surface exhibit vivid long-range patterns consisting of bright spots (“ghosts”) which are attributed to localized excess charge rather than atomic clusters. The nearly planar geometry of the sp2-hybridized gallium dimer atoms results in localized π states made up of a combination of the Ga pz orbitals. These states in the upper half of the band gap form the lowest unoccupied band. Surface or bulk defects lead to excess negative charge flowing into these localized states. Repulsion between the trapped negative excess charges leads to the observed “ghost” pattern.
Model of hydrophobic attraction in two and three dimensions
Authors: Barkema G. T. , Widom B.
Published: July 2002

An earlier one-dimensional lattice model of hydrophobic attraction is extended to two and three dimensions and studied by Monte Carlo simulation. The solvent-mediated contribution to the potential of mean force between hydrophobic solute molecules and the solubility of the solute are determined. As in the earlier model, an inverse relation is observed between the strength and range of the hydrophobic attraction. The mean force no longer varies monotonically with distance, as it does in one dimension, but has some oscillations, reflecting the greater geometrical complexity of the lattice in the higher dimensions. In addition to the strong attraction at short distances, there is now also a local minimum in the potential of depth about kT at a distance of three lattice spacings in two dimensions and one of depth about 2kT at a distance of two lattice spacings in three dimensions. The solubility of the solute is found to decrease with increasing temperature at low temperatures, which is another signature of the hydrophobic effect and also agrees with what had been found in the one-dimensional model.
Application of density functional perturbation theory to pure fluid liquid–vapor interfaces
Authors: Wadewitz T. , Winkelmann J.
Published: July 2002

Common density functional theories often use the so-called mean-field approximation to the attractive term of the free energy, which is not accurate when compared with molecular dynamics simulations. We applied a modification of this attractive free energy term of a local density functional approach where an analytical representation of the radial distribution function is taken into account. A cutoff Lennard-Jones-12-6 potential, divided according to the Weeks–Chandler–Andersen prescription, is used as the intermolecular interaction force model. This density functional perturbation approach gives the correct phase quilibrium and predicts a surface tension in excellent agreement with recent molecular dynamics simulations. The surface tension of some pure simple fluids is predicted from an intrinsic contribution, obtained by the modified density functional perturbation theory, plus contributions due to capillary wave roughening of the surface. The respective Lennard-Jones potential parameters were obtained from fitting to saturated liquid densities of the pure fluids.
Nonlinear multigroup neutron-flux systems: Blowup, decay, and steady states
Authors: Chen Gen-Shun , Leung Anthony W.
Published: July 2002

This article investigates a nonlinear system of partial differential equations describing multigroup neutron-flux reaction-diffusion inside a nuclear fission reactor. The neutrons are divided into n energy groups, with fission and scattering rates dependent on temperature, which gives rise to the (n+1)th equation. Cases concerning directly coupled and down scattering, when group transfers only occur from higher to lower energy groups, are considered. Quantitative conditions on the various fission and scattering rates are found for eventual ‘‘blowup’’ and ‘‘decay’’ of neutron concentrations. Finally, a system with fission and scattering rates dependent on neutron density is also investigated. Sufficient conditions for positive nontrivial steady state are found.
Transverse nearest neighbor degeneracies on a 2×N lattice
Authors: Fuller William Roger
Published: July 2002

An exact 15-term recursion relation and the associated generating function are derived for the number of arrangements of q particles on a 2×N lattice such that s occupied nearest neighbor pairs, v of which are transverse, and t unoccupied nearest neighbor pairs are formed.
A notion of duality for twistor functions
Authors: Eastwood Michael , Jozsa Richard
Published: July 2002

A class of functions on the primed spin bundle of complexified Minkowski space, defined by ∇AA′∂f/∂πA′=0, is introduced and it is shown that these functions bear a close relationship to twistor functions, especially through their use in describing massless fields.
Moment-equation and path-integral techniques for wave propagation in random media
Authors: Codona Johanan L. , Creamer Dennis B. , Flatté Stanley M. , Frehlich R. G. , Henyey Frank S.
Published: July 2002

Differential equations for all moments of the field of a wave propagating through a random medium are derived under the parabolic approximation and the Markov approximation, but including anisotropy in the random medium and a deterministic background refractive index. Mathematical equivalence is demonstrated between these moment equations and path-integral expressions for the moments obtained under the same approximations. A discussion of approximations that are weaker than Markov is given.
Charged particles with a short-range force: Perturbation theory with respect to the range and to additional effects
Authors: Gesztesy F. , Karner G. , Streit L.
Published: July 2002

Previously derived Blatt–Jackson type formulas for Coulomb corrected scattering lengths are generalized to include the effect of an additional strong interaction potential in first Born approximation. It is shown that the strong model dependence on the hadronic interaction is considerably reduced as soon as the finite extent of the charge is taken into account.
Weyl group and tensor operators for hadron-type SU(n) representations
Authors: Cieciura Grzegorz , Szczyrba Igor
Published: July 2002

Let a particle symmetry be described by a simple Lie algrebra 𝔤 of the type An−1, i.e., 𝔤=sl(n,C) or 𝔤 is a real form of sl(n,C). For 𝔤 representations describing three-particle or particle–antiparticle states, relationships between two actions, the action of 𝔤 and the action of the corresponding Weyl group Sn, on observables are analyzed. It is shown, in particular, how impossible physical relations depend on these two actions. The results enable one to verify quickly if given experimental data can be fitted by means of a 𝔤-symmetry theory.
Lorentz invariance and the equivalence of metric, spinor, and frame field concomitants
Authors: McKellar R. J.
Published: July 2002

Any theory constructed out of the metric tensor and its derivatives can also be regarded as a theory constructed out of a frame field and its derivatives. In general, the converse is not true. It is shown that with the additional assumption that the theories are Lorentz invariant the converse is true. Theories involving the metric spinor and its derivatives are then included in this equivalence and the various interrelationships are studied.
The stochastic Wigner–Weisskopf atom
Authors: Applebaum David
Published: July 2002

Using techniques of quantum stochastic calculus, a dilation scheme is developed to describe the evolution of a two-level atom, which is initially in a (nontracial) Gibbs state, through a radiation field. The Wigner–Weisskopf approximation is assumed but not the rotating wave approximation, the latter being shown to be equivalent to the requirement that the dilation satisfies the quantum detailed balance condition with respect to the initial state.
A folk theorem revisited: Degenerate representations
Authors: Giroux Y. , Sharp R. T.
Published: July 2002

It is known that when bases for generic irreducible representations of a semisimple group are reduced according to a semisimple subgroup the number of functionally independent missing label operators is just twice the number of missing labels. It is shown that the relation continues to hold when degenerate irreducible representations are considered.
Indices of representations of simple superalgebras
Authors: Morel B. , Patera J. , Sharp R. T. , Van der Jeugt J.
Published: July 2002

Indices and anomaly numbers for representations of basic classical Lie superalgebras are defined, and their explicit expressions are derived in terms of Kac–Dynkin labels. Useful properties of indices and anomalies are determined, and several examples are given. A similar analysis is made for superindices and superanomalies, and it is demonstrated how all these objects form a helpful tool in decomposing tensor products or in constructing branching rules for representations.
On the prolongation approach in three dimensions for the conservation laws and Lax pair of the Benjamin–Ono equation
Authors: Roy Chowdhury A. , Ahmad Siraj
Published: July 2002

The prolongation structure approach of Wahlquist and Estabrook [J. Math. Phys. 16, 1 (1975)] is used effectively in a new situation in relation to the integrodifferential type BO equation (the Benjamin–Ono equation). The main clue lies in the possible differential equation representation of such equations in three dimensions. Here it is shown how the usual analysis of prolongation structure can be utilized to deduce a Lax pair for a BO type equation in three dimensions. Effectiveness of the present approach is further demonstrated by an independent derivation of some conservation laws associated with the equation. Last, the whole formalism is reduced to two dimensions to make contact with known results.
Waves, bifurcations, and solitons in a model with sixfold symmetry
Authors: Augusteijn Marijke F. , Breitenberger Ernst
Published: July 2002

The traveling-wave solutions of a model Lagrangian density for a complex-valued scalar field with possible application to charge density waves in systems with sixfold symmetry are explored by means of the slow-fluctuation technique. There are five main wave types separated by three bifurcations with respect to integration constants. In addition, there are several classes of degenerate waves, including some with purely harmonic, stable, straight-line or circular motions of the amplitude vector. Beyond the range of the slow-fluctuation approximation there are several unstable soliton solutions, three of which have unexpectedly curved paths of the amplitude vector.
The anharmonic oscillator at a finite temperature. Comparison of quantum and classical stochastic calculations
Authors: Blanco R. , Pesquera L. , Santos E.
Published: July 2002

An oscillator with a small, but otherwise arbitrary, perturbing potential is considered immersed in a random cavity radiation. Classical (stochastic) calculations are done when the radiation has a Rayleigh–Jeans spectrum and a complete Planck spectrum (i.e., with zero point). These are compared with the results obtained by a quantum calculation. First, a comparison is made of stationary values, in particular, the energy. Then the emission and the absorption spectra are calculated, in particular, the absorption spectrum for an arbitrary incoming radiation. Finally, a detailed comparison is made of the absorption bands when the perturbing potential has the form λx2K (K=2,3,...). In all cases, it is explicitly shown that the quantum and the classical behavior agree in the limit of high temperatures. It is also shown that the classical system immersed in a radiation with complete Planck spectrum is much closer to the quantum system than the fully classical system (with a Rayleigh–Jeans spectrum).
Commutants and bicommutants of algebras of unbounded operators
Authors: Inoue Atsushi , Ueda Hideyuki , Yamauchi Toshiyuki
Published: July 2002

The first purpose of this paper is to show that for each Op*-algebra (ℳ,𝒟) whose weak commutant ℳ′w is an algebra, there exists a closed Op*-algebra (ℳ̂,𝒟̂), which is the smallest extension of (ℳ,𝒟) satisfying ℳ̂w =ℳ′w and ℳ̂w 𝒟̂ =𝒟̂. The second purpose is to characterize an unbounded bicommutant ℳ″wσ of an Op*-algebra ℳ. The third purpose is to generalize the well-known Radon–Nikodym theorem for von Neumann algebras to Op*-algebras ℳ satisfying the von Neumann density type theorem ℳ̄ t*s =ℳ″wσ.
Hamiltonian dynamics of higher-order theories of gravity
Authors: Szczyrba Victor
Published: July 2002

The Hamiltonian dynamics of gravitational theories with general Lagrangians quadratic in components of the curvature tensor is investigated. It is shown that the Noether generator corresponding to the action of the diffeomorphism group of space-time naturally defines the energy-momentum function E. The analysis of the differential of E gives rise to a formula for the symplectic two-form Ω and thereby defines symplectic (canonical) variables of the system. This construction is four-covariant, that is, independent of a chosen slicing of space-time. A more thorough analysis is performed in the (3+1) decomposition. In this scheme the canonical classification of quadratic Lagrangians is presented. It singles out the class of canonically regular Lagrangians as well as four classes of degenerate ones. The Cauchy–Kowalewska problem for all these classes is formulated.
Conservation laws for nonlinear evolution equations
Authors: Cavalcante J. A. , Tenenblat K.
Published: July 2002

A method to derive conservation laws for evolution equations that describe pseudospherical surfaces is introduced based on a geometrical property of these surfaces. A new third-order evolution equation is obtained as a first example for a nongeneric case in the classification given by Chern and Tenenblat [Stud. Appl. Math. 74, 1 (1986)].
The Duistermaat–Heckman integration formula on flag manifolds
Authors: Picken R. F.
Published: July 2002

An exposition is given of various geometrical properties of flag manifolds and of the Duistermaat–Heckman integration formula as applied to flag manifolds.
Star quantization on the cylinder
Authors: Alcalde Carlos
Published: July 2002

Star-product quantization as an autonomous quantum theory is reviewed with emphasis on its relation to path integral and Hilbert space formulations. Quantization of the usual Heisenberg group is used as an example. When phase space is the cylinder, this group is replaced by E(2), the Euclidean group in two dimensions. This case is worked out in detail. In doing so, polarizations on the cylinder are discussed and a phase space path integral is constructed.
δ expansion for a quantum field theory in the nonperturbative regime
Authors: Bender Carl M. , Milton Kimball A. , Pinsky Stephen S. , Simmons L. M.
Published: July 2002

The δ expansion, a recently proposed nonperturbative technique in quantum field theory, is used to calculate the dimensionless renormalized coupling constant of a λ(φ2)1+δ quantum field theory in d-dimensional space-time at the critical point defined by λ→∞ with the renormalized mass held fixed. The calculation is performed to leading order in δ and compared with previous lattice strong-coupling calculations. The numerical results are good and provide new evidence that the theory in four dimensions is free for all δ.
Integrable heterogeneous nonlinear Schrödinger equations with dielectric loss: Lie–Bäcklund symmetries
Authors: Broadbridge P. , Godfrey S. E.
Published: July 2002

The general cubic nonlinear Schrödinger equation, with heterogeneity and dielectric loss, is iψt=− (1)/(2) ψxx+V(x,t)ψ+iU(x,t)ψ+R (x,t)‖ψ‖2ψ. All such equations are found that possess a third-order Lie–Bäcklund symmetry. Each of these equations are then transformed to the standard completely integrable cubic Schrödinger equation (U=V=0, R=±1). In addition, some useful structure results are established for the time-dependent symmetries of the linear 2+1-dimensional Schrödinger equation with arbitrary real potential.
Contraction of information in systems far from equilibrium
Authors: Kishida Kuniharu
Published: July 2002

The relationship between a physical model and a time series model (or an innovation model) is discussed from the viewpoint of contraction of information due to the way of observation. In this framework, measurable correlation functions are expressed in terms of both models. By using the Riccati type equation and the singular value decomposition of the Hankel matrix, a data-oriented model is derived from measurable correlation functions. There occurs the contraction in a noise source space (or random forces). From the mechanism of contraction, random forces of systems can be identified by using two innovation models in different states.
Twistor representation of Yang–Mills fields on Minkowski space
Authors: Fioravanti Mario A. , Porter John R.
Published: July 2002

A correspondence between (not necessarily self-dual nor anti-self-dual) Yang–Mills fields on Minkowski space, and pairs of cohomology classes γ, φ, on null twistor space PN■ is established; γ∈H1CR (PN■,g) defines a deformed Cauchy–Riemman (C–R)(γ) structure on a principal bundle over PN, and φ∈H1CR(γ) (PN■,O(−4)⊗g). The correspondence depends on the choice of a spacelike hyperplane in Minkowski space. Here, γ and φ provide initial values in the spin bundle over this hyperplane for a system of evolution equations, along the null geodesic spray congruence in the spin bundle over Minkowski space.
Junction conditions on null hypersurfaces for spherically symmetric metrics
Authors: Fan Zu-hui , Kuang Zhi-quan , Liang Can-bin , Yu Yun-qiang
Published: July 2002

A necessary condition for the junction of two spherically symmetric metrics on a null hypersurface to satisfy Lichnerowicz’s condition is derived. It is shown using this necessary condition that the junction of the Schwarzschild metric with the Gonzalez metric can never be c1, and the junction of the Reissner–Nordström metric with the metric suggested by Shen and Zhu can never be c1 either.