bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling calcium and strontium clusters with many-body potentials
Authors: Hearn Jason E. , Johnston Roy L.
Published: July 2002

Many-body atomistic potentials, of the Murrell–Mottram (MM) type, obtained by fitting properties of solid phases of calcium and strontium [J. E. Hearn, R. L. Johnston, S. Leoni, and J. N. Murrell, J. Chem. Soc. Faraday Trans. 92, 425 (1996)], have been used to study the structures, stabilities, and growth modes of Ca and Sr clusters. Full structure optimization on small clusters (2–20 atoms) leads to structures involving the fusion of tetrahedral units, and predicts icosahedral cluster growth. Radial relaxation studies on icosahedral, truncated decahedral, cuboctahedral, and rhombic dodecahedral geometric shell clusters, lead to the prediction that icosahedral structures are preferred until around 32 000 (Ca) and 128 000 (Sr), whereupon the fcc-like cuboctahedral clusters become preferred. These results are consistent with experimental findings. A detailed analysis has been performed of the binding energies and radial expansion factors of each set of symmetry equivalent atoms (subshell). As for Lennard–Jones clusters, multishell icosahedral Ca and Sr clusters are predicted to undergo significant core compression, resulting in low binding energies for the central atom and inner shells.
Thermodynamics of the associative mean spherical approximation for the fluid of dimerizing particles
Authors: Kalyuzhnyi Yu. V. , Holovko M. F.
Published: July 2002

Analytical expressions for the thermodynamic properties of the n-component mixture of dimerizing hard spheres with a long-range potential are derived within the associative mean spherical approximation. The derivation is based on the extension of the Ho/ye–Stell scheme [J. Chem. Phys. 67, 439 (1977)] in the framework of the Wertheim’s two-density formalism for associating fluids and the exponential approximation recently suggested by Bernard and Blum [J. Chem. Phys. 104, 4746 (1996)]. In the complete association limit, in which the two-density associative mean spherical approximation reduces to Chandler–Silbey–Ladanyi mean spherical approximation, results of the present study coincide with the corresponding results obtained earlier by the other authors. Closed form expressions for the Helmholtz free energy, chemical potential, and pressure of the complete association version of the electrolyte restricted primitive model with the additional sticky-point attraction are derived.
Tethered chains in theta solvent conditions: An experimental study involving Langmuir diblock copolymer monolayers
Authors: Kent M. S. , Majewski J. , Smith G. S. , Lee L. T. , Satija S.
Published: July 2002

Langmuir monolayers of polydimethylsiloxane-polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at 22 °C are employed as a model tethered chain system in theta solvent conditions. The segmental concentration profile of the tethered PS block is obtained over an order of magnitude in both surface density (σ) and molecular weight (M) by neutron reflection. A depletion layer of PS segments is observed at the air–liquid interface which increases with M and is independent of σ. The variation of the tethered layer height with σ and M is consistent with h∼σ0.18M0.74 over the range of reduced surface density (σπRg2) from 1 to 11. These dependencies, along with the form of the profile, indicate that the asymptotic limit is not achieved for σπRg2⩽11 in theta solvent conditions. The upper limit of surface density is limited by the interaction of the submerged blocks, which leads to a sharp rise in surface pressure (Π). The increase of Π with σ far exceeds theoretical predictions, even in the asymptotic limit, and is attributed to distortion of chain configurations arising from limited lateral interpenetration. Anchoring energies compare well with differential chain energies at the desorption transition when the surface PDMS blocks are noninteracting. Comparisons are made with results obtained previously in good solvent conditions.
In situ SFG spectroscopy of film growth. I. General formulation and the analysis of the signal observed during the deposition of formic acid on Pt(110)-(1×2) surface
Authors: Hirose Chiaki , Ishida Hirokazu , Iwatsu Katsuhiko , Watanabe Nobuyuku , Kubota Jun , Wada Akihide , Domen Kazunari
Published: July 2002

The application of infrared-visible sum-frequency generation (SFG) spectroscopy to the in situ monitoring of film growth was examined by the observation of the growth of the physisorbed layer of formic acid on a reconstructed Pt(110)-(1×2) surface. The signal by the CH stretching band displayed a characteristic interference pattern as deposition proceeded at 158 K to prove that the signal originated from inside the deposited film and the CH bond and molecular plane of formic acid was angled to the surface. The expressions for the electric fields of the SF beams generated inside the film, at the top layer, and at the bottom layer were derived by incorporating the interference effects of the exciting and generated light beams. The formula was used to simulate the observation and to estimate the growth rate.
Experimental and quantum mechanical study of the H+D2 reaction near 0.5 eV: The assessment of the H3 potential energy surfaces
Authors: Bañares Luis , Aoiz F. J. , Herrero Victor J. , D’Mello Michael J. , Niederjohann Britta , Seekamp-Rahn Karen , Wrede Eckart , Schnieder Ludger
Published: July 2002

The hydrogen exchange reaction in its H+D2(v=0,j=0)→HD(v′=0,j′)+D isotopic variant has been investigated theoretically and experimentally at the collision energies 0.52 eV, 0.531 eV and 0.54 eV. A detailed comparison of converged quantum mechanical scattering calculations and state-to-state molecular beam experiments has allowed a direct assessment of the quality of the different ab initio potential energy surfaces used in the calculations, and strongly favors the newly refined version of the Boothroyd–Keogh–Martin–Peterson surface. The differences found in the calculations are traced back to slight differences in the topology of the potential energy surfaces.
Numerical self-consistent field study of tethered chains in Θ solvent
Authors: Baranowski R. , Whitmore M. D.
Published: July 2002

We present a numerical, self-consistent field (SCF) study of tethered polymers in Θ solvent, over the range of anchoring densities found in most experiments. We examine the properties of the dangling block, including the thickness of the brush, the density profile, the depletion layer, the maximum polymer concentration and its location, and the dependence of these properties on surface density and molecular weight. In the calculations, the polymer and solvent are characterized by realistic values of the pure component densities, the statistical segment length, and the Flory interaction parameter, and we make quantitative comparison with recent experiments [M. S. Kent, G. S. Smith, J. Majewski, L. T. Lee, and S. Satija, J. Chem. Phys. 108, 5635 (1998)] and with the asymptotic analytic SCF theory. In agreement with these experiments, our results add to the growing body of evidence that many of these systems are not well described by the asymptotic theories.
Overcoming stability limitations in biomolecular dynamics. I. Combining force splitting via extrapolation with Langevin dynamics in LN
Authors: Barth Eric , Schlick Tamar
Published: July 2002

We present an efficient new method termed LN for propagating biomolecular dynamics according to the Langevin equation that arose fortuitously upon analysis of the range of harmonic validity of our normal-mode scheme LIN. LN combines force linearization with force splitting techniques and disposes of LIN’s computationally intensive minimization (anharmonic correction) component. Unlike the competitive multiple-timestepping (MTS) schemes today—formulated to be symplectic and time-reversible—LN merges the slow and fast forces via extrapolation rather than “impulses;” the Langevin heat bath prevents systematic energy drifts. This combination succeeds in achieving more significant speedups than these MTS methods which are limited by resonance artifacts to an outer timestep less than some integer multiple of half the period of the fastest motion (around 4–5 fs for biomolecules). We show that LN achieves very good agreement with small-timestep solutions of the Langevin equation in terms of thermodynamics (energy means and variances), geometry, and dynamics (spectral densities) for two proteins in vacuum and a large water system. Significantly, the frequency of updating the slow forces extends to 48 fs or more, resulting in speedup factors exceeding 10. The implementation of LN in any program that employs force-splitting computations is straightforward, with only partial second-derivative information required, as well as sparse Hessian/vector multiplication routines. The linearization part of LN could even be replaced by direct evaluation of the fast components. The application of LN to biomolecular dynamics is well suited for configurational sampling, thermodynamic, and structural questions.
On the relationship between C-C and C-D order parameters and its use for studying the conformation of lipid acyl chains in biomembranes
Authors: Douliez J.-P. , Ferrarini A. , Dufourc E.-J.
Published: July 2002

We have used the order tensor method to establish relationships between carbon-carbon and carbon-deuterium order parameters (SCC and SCD, respectively) in saturated and unsaturated lipid acyl chains in biomembranes. We thus confirm the existence of a recursion relation between these order parameters (−2SkCD=SkCC+Sk+1CC) as was previously shown using Wigner matrices formalism [Douliez, Léonard, and Dufourc, Biophys. J. 68, 1727 (1995)]. This leads to the determination of SkCC as a function of the position k along the chain, having experimentally measured the corresponding SkCD. Making use of a mean field approach for computing the energy of chain conformers in the frame of the rotational isomeric model (RIS) we have also calculated SCC and SCD order profile for the sn-1 chain of dipalmitoylphosphatidylcholine. A very good agreement is obtained between computational and predicted SkCC values, reinforcing the validity of the recursion relation. A marked odd-even effect is evidenced on the SCC order profile for positions near the polar head, as was previously experimentally reported for dimyristoylphosphatidylcholine. We propose that this effect reveals an average bent orientation of the beginning of the chain with respect to the normal of the bilayer. Finally, it is shown that our formalism can be applied to some extent to cis or trans unsaturated lipid.
Kinetic theory and hydrodynamics of rigid body fluids
Authors: Grmela Miroslav , Lafleur Pierre G.
Published: July 2002

Macroscopic systems composed of particles whose time evolution is governed by the rigid body dynamics are called rigid body fluids. Such fluids could be, for example, macromolecular fluids composed of rigid macromolecules. Kinetic theory and hydrodynamics of these systems are introduced by using the method of Hamiltonian modeling. The physical basis of this method is the requirement that solutions to the governing equations agree with results of the experimental observations that constitute the empirical basis of equilibrium thermodynamics. The advantages of the Hamiltonian method are best displayed on the level of extended hydrodynamics, i.e., hydrodynamics with state variables composed of classical hydrodynamics fields and some extra fields characterizing the rigid body motion of the particles. Both the time evolution equations and the formula for the extra stress tensor are obtained without the need of closure approximations. Moreover, the formula for the extra stress tensor is guaranteed to be compatible with the rest of the time evolution equations. Qualitative properties of solutions of the governing equations derived in the paper lead to the conclusion that rigid body fluids possess a nonsymmetric stress tensor and exhibit elastic behavior. Both of these features have been seen in simulations.
Hydrogen-bond-induced clustering in the fragile glass-forming liquid m-toluidine: Experiments and simulations
Authors: Morineau Denis , Alba-Simionesco Christiane
Published: July 2002

We study the “prepeak” appearing in the static structure factor of the molecular glass-former m-toluidine by means of neutron scattering experiments and Monte Carlo simulations. The occurrence of this prepeak is interpreted as resulting from spatial organization of the molecules that goes beyond the usual short range liquid order and has a typical length scale of several molecular diameters. The origin of this phenomenon, as well as its specific temperature and density dependence, is explained by the competition between hydrogen-bonding interactions that tend to favor clustering and steric hindrance between aromatic rings that limits the extension of the H-bond network. Finally, effects of such clustering on the relaxational properties of the liquid and on the glass transition are discussed.
Quantum state-resolved reactive scattering of F+H2 in supersonic jets: Nascent HF(v,J) rovibrational distributions via IR laser direct absorption methods
Authors: Chapman William B. , Blackmon Brad W. , Nizkorodov Sergey , Nesbitt David J.
Published: July 2002

Supersonically cooled discharge radical atom sources are combined with high-sensitivity IR absorption methods to investigate state-to-state reactive scattering of F+n-H2→HF(v,J)+H in low-density crossed supersonic jets at center-of-mass collision energies of 2.4(6) kcal/mole. The product HF(v,J) is probed with full vibrational and rotational quantum state selectivity via direct absorption of a single mode (Δν≈0.0001 cm−1), tunable F-center laser in the Δv=1 fundamental manifold with near shot noise limited detection levels of 108 molecules/cm3/quantum state per pulse. The high absorption sensitivity, long mean free path lengths, and low-density conditions in the intersection region permit collision-free HF(v,J) rovibrational product state distributions to be extracted for the first time. Summed over all rotational levels, the HF vibrational branching ratios are 27.0(5)%, 54.2(23)%, 18.8(32)%, and <2(2)%, respectively, into vHF=3:2:1:0. The nascent vibrational distributions are in good agreement with rotationally unresolved crossed-beam studies of Neumark et al. [J. Chem. Phys. 82, 3045 (1985)], as well as with full quantum close-coupled calculations of Castillo and Manolopoulos [J. Chem. Phys. 104, 6531 (1996)] on the lowest adiabatic F+H2 potential surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)]. At a finer level of quantum state resolution, the nascent rotational distributions match reasonably well with full quantum theoretical predictions, improving on the level of agreement between theory and experiment from early arrested relaxation studies. Nevertheless, significant discrepancies still exist between the fully quantum state-resolved experiment and theory, especially for the highest energetically allowed rotational levels.
Membrane stability under electrical stress: A nonlocal electroelastic treatment
Authors: Partenskii Michael B. , Dorman Vladimir L. , Jordan Peter C.
Published: July 2002

Existing models of membrane instability and breakdown under an applied voltage are critically examined. An alternative, speculative treatment of the electroelastic model is suggested, based on the assumption that spatial dispersion of the elastic moduli leads to their effective softening at short wave lengths. The model parameters that account for these effects are chosen to ensure that short wave length thickness fluctuations become unstable at moderate applied voltages, ∼1–1.5 V. With these parameters we treat the membrane stretching diagram and membrane thickness fluctuations. The stretching diagram agrees with experimental findings and earlier calculations. Computed thickness fluctuations are consistent with previous investigations.
Focusing of DCl and HCl dimers by an electrostatic hexapole field: The role of the tunneling motion
Authors: Imura K. , Kasai T. , Ohoyama H. , Naaman R.
Published: July 2002

The focusing of HCl and DCl dimers was observed using a 2-m-long electrostatic hexapole field. The results indicate the existence of two types of species. The first is the homodimers, either the H35Cl–H35Cl or the D35Cl–D35Cl, for which the data indicate a fast tunneling motion. The second is the heterodimers, H35Cl–H37Cl or D35Cl–D37Cl, that do not show evidence for significant tunneling motion on the time scale of the experiment. In the case of HCl dimers, even at relatively high fields, only one species could be focused, the heterodimer. The electric dipole moments for both (DCl)2 isotopomers were determined to be 1.5±0.2 D, which is the same value as observed for (HCl)2.
The potential energy surface and near-dissociation states of He-H2+
Authors: Meuwly Markus , Hutson Jeremy M.
Published: July 2002

The potential energy surface for the ground state of He-H2+ is calculated using ab initio QCISD(T) calculations and a correlation-consistent basis set. The geometries chosen include all combinations of 21 intermolecular distances R, three H-H distances r, and seven Jacobi angles θ. The final potential is fitted to a functional form that incorporates the correct long-range behavior. Close-coupling calculations of both low-lying and near-dissociation vibration-rotation states are carried out. The results are expected to be of assistance in assigning the microwave spectra of He-H2+ in near-dissociation states [Carrington et al., Chem. Phys. Lett. 260, 395 (1996)].
Free energy calculations for peptides via deterministic global optimization
Authors: Klepeis J. L. , Floudas C. A.
Published: July 2002

The αBB algorithm is a deterministically based global optimization method that has been successfully used to locate the global minimum energy conformations of peptide systems. The goal of this procedure is to identify the native conformation of a given peptide by identifying the structure possessing the global minimum potential energy. However, a rigorous conformational search should locate the structure exhibiting the global minimum free energy. In this work, novel methods are developed for locating free energy global minimum conformations and clusters of peptides. These methods are based on an harmonic approximation for entropic effects, which requires the ability to generate a dense ensemble of distinct low energy local minima. Two approaches, both based on the general concepts of the αBB branch and bound framework, are used to generate these ensembles. In performing these calculations, potential-energy contributions were modeled using an all-atom force field. In addition, hydration effects were also considered by utilizing a solvent-accessible volume of hydration shell model. The free energy analysis was applied to both the unsolvated and solvated forms of met- and leu-enkephalin. It was found that both methods produce dense, Boltzmann-type, distributions of low-energy metastable states. The inclusion of entropic effects was also found to influence the prediction of free energy global minima. In addition, a statistical treatment of the thermodynamics of folding showed that the transition temperature, which signified a collapse from high energy, extended structures to a ground-statelike ensemble, could be identified.
State-to-state rate coefficients for H+H2
Authors: Mandy M. E. , Martin P. G.
Published: July 2002

We have used the quasiclassical trajectory (QCT) method to determine state-to-state rate coefficients for all transitions among the 348 (v,j) states of the ground electronic state (Σg+) of the hydrogen molecule caused by collision with a hydrogen atom. Direct and indirect results were combined to give best possible rate coefficients within the quasiclassical approximation. Because energy transfer behavior differs for reactive and nonreactive channels and these channels are distinguishable classically, it is appropriate to consider separately the rate coefficients for each channel. Where available, quantum cross sections were incorporated into our calculation. The temperature dependence of the rate coefficients is parametrized over the temperature range 600 to 10 000 K. We use this complete set of state-to-state rate coefficients to test several scaling laws.
Morphing ab initio potentials: A systematic study of Ne–HF
Authors: Meuwly Markus , Hutson Jeremy M.
Published: July 2002

A procedure for “morphing” an ab initio potential energy surface to obtain agreement with experimental data is presented. The method involves scaling functions for both the energy and the intermolecular distance. In the present work, the scaling functions are parametrized and determined by least-squares fitting to the experimental data. The method is tested on the system Ne–HF, for which high-resolution infrared spectra are available. It is shown to work well even with relatively low-level ab initio calculations. Several basis sets are investigated at the CCSD(T) correlation level, including various aug-cc-pVnZ basis sets and the specially-tailored Ne–HF basis set of ONeil et al. All give good results after morphing, but the changes needed to match experiment are much smaller for the ONeil basis set. The use of MP2 calculations is also investigated: again, the MP2 potential is quite satisfactory after morphing, but requires much more modification than the CCSD(T) potential.
Potential curves for the ground states and some excited states of MgNe, Mg+Ne, and Mg+2Ne van der Waals complexes
Authors: Leung Allen W. K. , Julian Ryan R. , Breckenridge W. H.
Published: July 2002

We have estimated the potential curves of the Mg(3s2)⋅Ne(1Σ+), Mg(3s3p)⋅Ne(3Π,3Σ+), Mg(3p2)⋅Ne(3Σ−), Mg+(3s)⋅Ne(2Σ+), Mg+(3p)⋅Ne(2Π), and Mg+2(2p6)⋅Ne(1Σ+) van der Waals states by means of ab initio calculations. Similar to the analogous doubly-excited states of MgAr and MgKr, the Mg(3pπ3pπ)⋅Ne(3Σ−) state is found to be unusually strongly bound, De=548 cm−1, a bond strength which is more than 20 times that of the singly-excited Mg(3s3pπ)⋅Ne(3Π) state and even more than three times that of the Mg+(3s)⋅Ne ion. The strong bonding is attributed primarily to the lack of a Mg(3s) electron, so that all the attractive van der Waals forces can extend to smaller internuclear distances because there is no Mg(3sσ)/Ne(2pσ) exchange repulsion.
Adsorption of human serum albumin: Dependence on molecular architecture of the oppositely charged surface
Authors: Sukhishvili Svetlana A. , Granick Steve
Published: July 2002

We contrast the adsorption of human serum albumin (HSA) onto two solid substrates previously primed with the same polyelectrolyte of net opposite charge to form one of two alternative structures: randomly adsorbed polymer and the “brush” configuration. These structures were formed either by the adsorption of quaternized poly-4-vinylpyridine (QPVP) or by end-grafting QPVP chains of the same chemical makeup and the same molecular weight to surfaces onto which QPVP segments did not adsorb. The adsorption of HSA was quantified by using Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). The two substrates showed striking differences with regard to HSA adsorption. First, the brush substrate induced lesser perturbations in the secondary structure of the adsorbed HSA, reflecting easier conformational adjustment for longer free segments of polyelectrolyte upon binding with the protein. Second, the penetration of HSA into the brush substrate was kinetically retarded relative to the randomly adsorbed polymer, probably due to both pore size restriction and electrostatic sticking between charged groups of HSA and QPVP molecules. Third, release of HSA from the adsorbed layer, as the ionic strength was increased from a low level up to the high level of 1 M NaCl, was largely inhibited for the brush substrate, but occurred easily and rapidly for the substrate with statistically adsorbed QPVP chains. Finally, even after addition of a strong polymeric adsorption competitor (sodium polystyrene sulfonate), HSA remained trapped within a brush substrate though it desorbed slowly from the preadsorbed QPVP layer. This method to produce irreversible trapping of the protein within a brush substrate without major conformational change may find application in biosensor design.
Rate constants for the reaction of O2+ with NO from 300 to 1400 K
Authors: Midey Anthony J. , Viggiano A. A.
Published: July 2002

The rate constants for the charge transfer reaction of O2+ with NO have been measured from 300 to 1400 K using a high temperature flowing afterglow. The current results agree well with the previous flowing afterglow studies made at temperatures up to 900 K. The rate constants have no significant temperature dependence over the entire temperature range. The rate constants are in good agreement with the previous flow drift tube measurements at the same translational temperatures. Any dependence of the rate constant on internal energy is therefore small.
Photodissociation dynamics of H2S on new coupledab initiopotential energy surfaces
Authors: Simah David , Hartke Bernd , Werner Hans-Joachim
Published: July 2002

Three-dimensional potential energy surfaces for the electronic ground state as well as the lowest three 1A″ states have been computed using highly correlated CASSCF-MRCI wave functions and a large basis set. An approximate diabatization scheme has been employed to generate quasidiabatic potential energy surfaces for the lowest two 1A″ states. The diabatization is based on the condition that both the orbitals as well as the configuration coefficients of the diabatic wave functions change as little as possible as function of geometry. The diabatic potential energy surfaces are used in time-dependent simulations of the absorption spectrum as well as the vibrational and rotational product distributions. Excellent agreement between the computed and experimental absorption spectra and product distributions is obtained, indicating that the ab initio potentials as well as the diabatization scheme are accurate.
Application of an equation-of-motion coupled cluster method including higher-order corrections to potential energy surfaces of radicals
Authors: Saeh Jamal C. , Stanton John F.
Published: July 2002

A method proposed some time ago to correct for residual correlation effects within the framework of equation-of-motion coupled-cluster theory for ionized states in the singles and doubles approximation (EOMIP-CCSD) is discussed, and a minor modification is suggested that provides a more balanced theoretical treatment. The resulting approach is used to calculate properties of diatomic radicals, where comparison with experiment and other high-level calculations is straightforward. In addition, two electronic states of NO2 and the ground state of the cyclic C3H molecule are studied, cases in which symmetry-breaking phenomena play an important role. Results indicate that EOMIP-CCSD* generally provides a systematic improvement relative to the uncorrected EOMIP-CCSD approach for predicting properties of radicals.
Molecular dynamics simulation of fluorination effects on a phospholipid bilayer
Authors: Smondyrev A. M. , Berkowitz M. L.
Published: July 2002

To study the effect of fluorination of the core of a phospholipid bilayer we performed two molecular dynamics computer simulations. The first simulation was performed on a regular dimyristoylphosphatidylcholine (DMPC) bilayer, the second simulation was performed on its fluorinated counterpart FDMPC. In FDMPC, bilayer hydrogen atoms belonging to the last four hydrocarbon groups in both chains of phospholipid molecules were replaced by fluorines. From our simulations we observed that as a result of fluorination the core of the bilayer represented a tightly packed structure, while the structure of the head groups and of the region in the beginning of the tails remained similar to the one observed in the ordinary bilayer. A simple model for charge distribution in the phospholipid tails was proposed which explained the change in the sign of the dipole potential due to fluorination.
Vibrationally resolved threshold photoelectron–photoion coincidence spectra of KrXe
Authors: Yoshii Hiroshi , Tanaka Tomoaki , Morioka Yumio , Hayaishi Tatsuji , Ito Kenji
Published: July 2002

High resolution threshold photoelectron–photoion coincidence spectra of KrXe were measured in the photon energy region 11.7–14.7 eV using the penetrating field technique and synchrotron radiation. Six vibrational progressions were observed and ascribed to the production of the six electronic states of KrXe+ ions (X 1/2, A1 3/2, A2 1/2, C1 3/2, B 1/2, and C2 1/2) via photoionization of the ground state KrXe dimer. The vibrational structures for the X 1/2, A2 1/2, B 1/2, and C2 1/2 states were observed for the first time in the present measurements. The molecular constants for vibration were obtained from the analysis of the deduced vibrational energy levels. The potential curves of each state were drawn by using the Morse function.
Influence of the cross-link density and the filler content on segment dynamics in dry and swollen natural rubber studied by the NMR dipolar-correlation effect
Authors: Grinberg Farida , Garbarczyk Maciej , Kuhn Winfried
Published: July 2002

Chain dynamics in a series of natural rubbers with different cross-link densities was studied using the dipolar correlation effect (DCE) on the stimulated echo. In dry samples, amplitudes of the stimulated echo were shown to be affected by attenuation mechanisms due to the DCE and spin exchange which occurs between the protons of CH and CH3 (and/or CH2) groups of polyisoprene chains. The estimated spin exchange time was 0.04 s. The DCE free of undulations due to spin exchange was examined using a special modification of the radio frequency pulse sequence with additional π-pulses inserted in the free-evolution intervals. Modulations of stimulated echo amplitudes could completely be suppressed. A strong dependence of the DCE on the cross-link density was observed. Attenuation curves of the dipolar-correlation quotient, that is, the quotient of the stimulated and the primary echo amplitudes, were fitted using analytical expressions. The mean-squared fluctuation of the dipolar coupling constant, 〈δΩd2〉, and correlation times were determined. In dry samples, values of 〈δΩd2〉 were shown to scale with the number of segments between cross-links (N) as ∝N−1.3±0.2. The attenuation of echo amplitudes due to the DCE was insensitive to the increase of a carbon black content by approximately a factor of two. In swollen samples, attenuations of the stimulated echo were free of modulations by spin exchange thus permitting measurements of the DCE with a standard three-90° pulse sequence. Contrary to dry samples, the attenuation curves contained slowly decaying components with relative intensities ⩽ 10%. These components were subtracted from the total signal before the dipolar-correlation quotient was evaluated. 〈δΩd2〉 scales with N with nearly the same exponent as in dry rubber. The DCE of dry and swollen rubber is suggested as a method for the determination of cross-link densities.
Molecular dynamics simulations with constrained roto-translational motions: Theoretical basis and statistical mechanical consistency
Authors: Amadei A. , Chillemi G. , Ceruso M. A. , Grottesi A. , Di Nola A.
Published: July 2002

From a specific definition of the roto-translational (external) and intramolecular (internal) coordinates, a constrained dynamics algorithm is derived for removing the roto-translational motions during molecular dynamics simulations, within the leap-frog integration scheme. In the paper the theoretical basis of this new method and its statistical mechanical consistency are reported, together with two applications.
Reentrant condensation of DNA induced by multivalent counterions
Authors: Nguyen T. T. , Rouzina I. , Shklovskii B. I.
Published: July 2002

A theory of condensation and resolubilization of a dilute DNA solution with growing concentration of multivalent cations, N, is suggested. It is based on a new theory of screening of a macroion by multivalent cations, which shows that due to strong cation correlations at the surface of DNA the net charge of DNA changes sign at some small concentration of cations N0. DNA condensation takes place in the vicinity of N0, where absolute value of the DNA net charge is small and the correlation induced short range attraction dominates the Coulomb repulsion. At N>N0 positive DNA should move in the opposite direction in an electrophoresis experiment. From comparison of our theory with experimental values of condensation and resolubilization thresholds for DNA solution containing Spe4+, we obtain that N0=3.2 mM and that the energy of DNA condensation per nucleotide is 0.07 kBT.
Diagrammatic complete active space perturbation theory: Calculations on benzene, N2, and LiF
Authors: Finley James P. , Witek Henryk A.
Published: July 2002

Second-order diagrammatic complete active space perturbation theory (D-CASPT2) is designed to have the best qualities from multireference Mo/ller–Plesset (MRMP) theory, complete active space perturbation theory CASPT2, and the effective valence shell Hamiltonian Hv method, including possessing a linked diagram theorem (LDT) and an efficient second-order formalism. The accuracy of this method is illustrated through calculations of the electronic excitation energies of N2 and benzene, the spectroscopic constants of the N2 ground state, and the ionic-neutral avoided crossing in the potential curves of LiF.
Nonadiabatic dynamics via the classical limit Schrödinger equation
Authors: Burant John C. , Tully John C.
Published: July 2002

The coupled Schrödinger equations that describe nonadiabatic dynamics are recast using the Bohm formulation of quantum mechanics. The resulting coupled Bohm equations are solved numerically for two scattering models, giving results that are essentially identical to wave-packet solution of the original coupled Schrödinger equations. The classical limit of the set of coupled Bohm equations is then described, producing a mixed quantum-classical theory incorporating classical-like motion on each potential energy surface accompanied by quantum transitions between the quantum states. Numerical tests of the mixed quantum-classical method are in excellent agreement with the accurate full-quantum results for the model problems. The method is contrasted with the related surface-hopping approach. It is shown that computing the dynamics of a distribution of classical particles is more consistent and more accurate than computing the motion of independent point particles as with surface hopping.
Applying molecular theory to steady-state diffusing systems
Authors: Frink Laura J. Douglas , Thompson Aidan , Salinger Andrew G.
Published: July 2002

Predicting the properties of nonequilibrium systems from molecular simulations is a growing area of interest. One important class of problems involves steady-state diffusion. To study these cases, a grand canonical molecular dynamics approach has been developed by Heffelfinger and van Swol [J. Chem. Phys. 101, 5274 (1994)]. With this method, the flux of particles, the chemical potential gradients, and density gradients can all be measured in the simulation. In this paper, we present a complementary approach that couples a nonlocal density functional theory (DFT) with a transport equation describing steady-state flux of the particles. We compare transport-DFT predictions to GCMD results for a variety of ideal (color diffusion), and nonideal (uphill diffusion and convective transport) systems. In all cases, excellent agreement between transport-DFT and GCMD calculations is obtained with diffusion coefficients that are invariant with respect to density and external fields.
Extrapolation and perturbation schemes for accelerating the convergence of quantum mechanical free energy calculations via the Fourier path-integral Monte Carlo method
Authors: Mielke Steven L. , Srinivasan Jay , Truhlar Donald G.
Published: July 2002

We present two simple but effective techniques designed to improve the rate of convergence of the Fourier path-integral Monte Carlo method for quantum partition functions with respect to the Fourier space expansion length, K, especially at low temperatures. The first method treats the high Fourier components as a perturbation, and the second method involves an extrapolation of the partition function (or perturbative correction to the partition function) with respect to the parameter K. We perform a sequence of calculations at several values of K such that the statistical errors for the set of results are correlated, and this permits extremely accurate extrapolations. We demonstrate the high accuracy and efficiency of these new approaches by computing partition functions for H2O from 296 to 4000 K and comparing to the accurate results of Partridge and Schwenke.
Geometry of hyperbolic monopoles
Authors: Nash C.
Published: July 2002

The hyperbolic monopoles of Atiyah [M. F. Atiyah, Commun. Math. Phys. 93, 471 (1984); ‘‘Magnetic monopoles in hyperbolic space,’’ in Proceedings of the International Colloquium on Vector Bundles (Tata Institute, Bombay, 1984)] and Chakrabarti [A. Chakrabarti, J. Math. Phys. 27, 340 (1986)] are introduced and their geometric properties and relations to instantons and ordinary monopoles clarified. A key tool is the use of the ball model of hyperbolic space to construct and examine solutions.
Multipole moments of axisymmetric systems in relativity
Authors: Fodor G. , Hoenselaers C. , Perjés Z.
Published: July 2002

An algorithm is developed for computing the nth gravitational multipole moment of an asymptotically flat, empty, stationary axisymmetric space-time. The moments are expressed in terms of the expansion coefficients of the Ernst potential on the axis of symmetry. The values of the first ten multipole moments are given.
Infinite-dimensional algebras and a trigonometric basis for the classical Lie algebras
Authors: Fairlie D. B. , Fletcher P. , Zachos C. K.
Published: July 2002

This paper explores features of the infinite-dimensional algebras that have been previously introduced. In particular, it is shown that the classical simple Lie algebras (AN, BN, CN, DN) may be expressed in an ‘‘egalitarian’’ basis with trigonometric structure constants. The transformation to the standard Cartan–Weyl basis, and the particularly transparent N→∞ limit that this formulation allows is provided.
Averaging principle and systems of singularly perturbed stochastic differential equations
Authors: Golec J. , Ladde G.
Published: July 2002

By developing certain auxiliary results, a modified version of the stochastic averaging principle is developed to investigate dynamical systems consisting of fast and slow phenomena. Moreover, an attempt is made to establish a relationship between the averaging assumption and certain ergodic-type properties of the random process determined by an auxiliary system of stochastic differential equations. Finally, an example is given to illustrate the scope of the results.
Completeness of the eigenfunctions for the Caudrey–Beals–Coifman system
Authors: Gerdjikov V. S. , Yanovski A. B.
Published: July 2002

The direct and the inverse scattering problem for the first order linear systems of the type Lψ(x,λ)≡(i(d/dx)+q(x)−λJ)ψ(x,λ)=0, J∈𝔥, q(x)∈𝔤J , which generalizes the Zakharov–Shabat system and the systems studied by Caudrey, Beals, and Coifman (CBC) is analyzed herein. Here J is a regular complex constant element of the Cartan subalgebra 𝔥⊆𝔤 of the simple Lie algebra 𝔤 and the potential q(x) vanishes fast enough for ‖x‖ → ∞ taking values in the image 𝔤J of adJ. The CBC results are generalized and the fundamental analytic solution m(x,λ) for any choice of the irreducible finite-dimensional representation V of 𝔤 is constructed. Four pairwise equivalent minimal sets of scattering data for L, invariant with respect to the choice of the representation of 𝔤, are extracted from the asymptotics of m(x,λ) for x → ±∞. From m(x,λ) the resolvent of L is constructed in the adjoint representation Vad and the completeness relation is proven for the eigenfunctions of L in Vad. It is also proven that the discrete spectrum of L consists of the sets of zeroes of certain spectral invariants D+j(λ) of L.
Quantum affine Lie algebras, Casimir invariants, and diagonalization of the braid generator
Authors: Gould Mark D. , Zhang Yao-Zhong
Published: July 2002

Let Uq(𝒢̂) be an infinite-dimensional quantum affine Lie algebra. A family of central elements or Casimir invariants are constructed and their eigenvalues computed in any integrable irreducible highest weight representation. These eigenvalue formulas are shown to be absolutely convergent when the deformation parameter q is such that ‖q‖≳1. It is proven that the universal R-matrix R of Uq(𝒢̂) satisfies the celebrated conjugation relation R°=TR with T the usual twist map. As applications, the braid generator is shown to be diagonalizable on arbitrary tensor product modules of integrable irreducible highest weight Uq(𝒢̂)-module and a spectral decomposition formula for the braid generator is obtained which is the generalization of Reshetikhin’s and Gould’s forms to the present affine case. Casimir invariants acting on a specified module are also constructed and their eigenvalues, again absolutely convergent for ‖q‖≳1, computed by means of the spectral decomposition formula.
The Adler–Shiota–van Moerbeke formula for the BKP hierarchy
Authors: van de Leur Johan
Published: July 2002

We study the BKP hierarchy and prove the existence of an Adler–Shiota–van Moerbeke formula. This formula relates the action of the BW1+∞ algebra on tau functions to the action of the ‘‘additional symmetries’’ on wave functions.
Lattice topological field theory on nonorientable surfaces
Authors: Karimipour V. , Mostafazadeh A.
Published: July 2002

The lattice definition of the two-dimensional topological quantum field theory [Fukuma et al., Commun. Math. Phys. 161, 157 (1994)] is generalized to arbitrary (not necessarily orientable) compact surfaces. It is shown that there is a one-to-one correspondence between real associative *-algebras and the topological state sum invariants defined on such surfaces. The partition and n-point functions on all two-dimensional surfaces (connected sums of the Klein bottle or projective plane and g-tori) are defined and computed for arbitrary *-algebras in general, and for the group ring A=↛[G] of discrete groups G, in particular.
Asymptotic of the density of states for the Schrödinger operator with periodic electromagnetic potential
Authors: Mohamed Abderemane
Published: July 2002

For the Schrödinger operator in L2(Rn), n>1, with C∞ periodic electromagnetic potential, we give an asymptotic formula of the integrate density of states of the form N(μ)=anμn/2+O(μ(n−2+ε)/2), ∀ε>0. When n=2, this estimate enables us to prove the finiteness of gaps in the spectrum.
A general theorem on square-integrability: Vector coherent states
Authors: Twareque Ali S.
Published: July 2002

We derive a generalization of the well-known theorem for the square integrability of a unitary irreducible representation of a locally compact group. The generalization covers the case of representations admitting vector coherent states. The result is illustrated by an example drawn from the isochronous Galilei group. The construction yields a wide variety of coherent states, labeled by phase space points, which satisfy a resolution of the identity condition, and incorporate spin degrees of freedom.
Analytic-bilinear approach to integrable hierarchies. II. Multicomponent KP and 2D Toda lattice hierarchies
Authors: Bogdanov L. V. , Konopelchenko B. G.
Published: July 2002

An analytic-bilinear approach for the construction and study of integrable hierarchies is discussed. Generalized multicomponent KP and 2D Toda lattice hierarchies are considered. This approach allows us to represent generalized hierarchies of integrable equations in a condensed form of finite functional equations. A generalized hierarchy incorporates basic hierarchy, modified hierarchy, singularity manifold equation hierarchy, and corresponding linear problems. Different levels of generalized hierarchy are connected via invariants of Combescure symmetry transformation. The resolution of functional equations also leads to the τ function and addition formulas to it.
Analytic-bilinear approach to integrable hierarchies. I. Generalized KP hierarchy
Authors: Bogdanov L. V. , Konopelchenko B. G.
Published: July 2002

An analytic-bilinear approach for construction and study of integrable hierarchies, in particular, the KP hierarchy is proposed. It starts with the generalized Hirota identity for the Cauchy–Baker–Akhiezer (CBA) function and leads to a generalized KP hierarchy in the form of compact functional equations containing a special shift operator. A generalized KP hierarchy incorporates the basic KP hierarchy, modified KP hierarchy, KP singularity manifold equation hierarchy, and corresponding hierarchies of linear problems. Different “vertical” levels of the generalized KP hierarchy are connected via invariants of the Combescure symmetry group. The resolution of functional equations also gives rise to the τ function and the addition formulas for it.
Monopoles and harmonic maps
Authors: Ioannidou Theodora , Sutcliffe Paul M.
Published: July 2002

Recently Jarvis has proved a correspondence between SU(N) monopoles and rational maps of the Riemann sphere into flag manifolds. Furthermore, he has outlined a construction to obtain the monopole fields from the rational map. In this paper we examine this construction in some detail and provide explicit examples for spherically symmetric SU(N) monopoles with various symmetry breakings. In particular we show how to obtain these monopoles from harmonic maps into complex projective spaces. The approach extends in a natural way to monopoles in hyperbolic space and we use it to construct new spherically symmetric SU(N) hyperbolic monopoles.
Simulation of quantum field theory and gravity in superfluid He3
Authors: Volovik G. E.
Published: July 2002

Superfluid phases of He3 are quantum liquids with the interacting fermionic and bosonic fields. In many respects they can simulate the interacting quantum fields in the physical vacuum. One can observe analogs of such phenomena as axial anomaly, vacuum polarization, zero-charge effect, fermionic charge of the vacuum, baryogenesis, ergoregion, vacuum instability, etc. We discuss some topics using as an example several linear defects in He3-A: (1) disgyration, which simulates the extremely massive cosmic string, (2) singular vortex, which is analogous to the spinning cosmic string, and (3) continuous vortex, whose motion causes the «momentogenesis» which is the analog of baryogenesis in early Universe. The production of the fermionic momentum by the vortex motion (the counterpart of the electroweak baryogenesis) has been recently measured in Manchester experiments on rotating superfluid He3-A and He3-B. To simulate the other phenomena, one needs a rather low temperature and high homogeneity, which probably can be attained under microgravity conditions.
On sonoluminescence of an oscillating gas bubble
Authors: Greenspan Harvey P. , Nadim Ali
Published: July 2002

It is proposed that shock dynamics within the gas of a small bubble explains sonoluminescence—the emission of visible radiation in response to spherically symmetric, ultrasonic excitation of a gas bubble in a liquid. As the bubble radius oscillates, shock waves develop from spherical sound waves created inside the gas bubble. As any such shock propagates toward the center, it strengthens and, upon convergence and subsequent reflection, dramatically increases the temperature of the gas behind it. Sufficiently high temperatures are predicted to explain the emission of light by the gas molecules.