bluuebunny/crossref_metadata_2025 huggingface dataset:
Compression-induced effect on the electronic structure of cyclotrimethylene trinitramine containing an edge dislocation
Authors: Kuklja Maija M. , Kunz A. Barry
Published: July 2002

An effect of a hydrostatic compression on the electronic structure of cyclotrimethylene trinitramine (C3H6N6O6), also known as RDX, with an edge dislocation has been studied by means of the ab initio Hartree–Fock method for a periodic system combined with the many-body perturbation theory. An external pressure causes a significant decrease of the optical gap for both the perfect material and the crystal with dislocations. The edge dislocations produce local electronic states in the optical gap whereas the external pressure moves these states deep within the band gap. This contributes strongly to properties of the RDX crystals creating favorable conditions for the N–NO2 chemical bond rupture due to exciton formation. The relationship between the edge dislocations, hot spot formation, and the sensitivity of RDX to detonation are discussed in detail.
Theoretical study of three dimensional elastic band gaps with the finite-difference time-domain method
Authors: Sigalas M. M. , Garcı́a N.
Published: July 2002

This work presents results on finite-difference time-domain (FDTD) integration of the full three dimensional elastic wave equation in nonhomogenous periodic media, for understanding the propagation and gap existence in these media. Extensive calculations are compared with plane wave expansion (PWE) data for different three dimensional systems consisting of Pb spheres in epoxy matrix forming an fcc lattice. The method of solving the wave equation provides good convergence and the agreement with the PWE method is excellent. The FDTD method, however, can handle cases such as fluids in solids which cannot be treated with the PWE method. Transmission results are presented for the case of Hg spheres in Al with fcc lattice, in order to prove the general use of the FDTD method.
Nanoindentation studies of FeXN (X=Ta, Ti) soft magnetic films
Authors: Papakonstantinou P. , Lemoine P. , McLaughlin J. , MacKay K. , Dodd P. M. , Pollard R. J. , Atkinson R.
Published: July 2002

Nanomechanical characterization of thin FeXN (X=Ta, Ti) films deposited by cosputtering from Fe, Ta and Ti targets on a Si/SiO2 substrate has been conducted using a nanoindenter. To fully understand the influence of the substrate on the measurement of film properties a continuous measure of the hardness and modulus as a function of depth was performed. The hardness of both FeTa and FeTi films was improved with the incorporation of nitrogen due an associated reduction in the grain size. However at increased flow rates of nitrogen a small softening effect was observed. For binary Fe(100−x)Tax a rapid increase in hardness was observed at x>17% which was coincident with a structural transformation from the nanocrystalline to the amorphous state. In the binary Fe(100−x)Tix the addition of 3.2 at. % Ti increased the hardness by a solid solution strengthening mechanism.
Effects of catalyst accelerator on electromagnetic shielding in nonelectrolytic Cu-plated fabrics
Authors: Kim E. A. , Han E. G. , Oh K. W. , Na J. G.
Published: July 2002

The effects of etching and catalyst accelerating conditions on microstructures of copper films and electromagnetic interference (EMI) shielding effectiveness (SE) of nonelectrolytic copper-plated fabrics were investigated. Copper films were coated onto polyester fabrics by a conventional nonelectrolytic copper plating process. Comparison of two etchants had the result where uniform deposition of Cu particles in the smaller size was observed using acidic etchant, which provided the better EMI SE than alkaline etchant. We found that KCl was the better catalyst accelerator than commonly used SnCl2. EMI SEs and conductivities of copper-plated fabrics increased as the concentration of KCl increased up to 0.1 mole/l and then decreased with further addition.
Electron mobility and charge correlation in silicon doped GaAs–AlAs short period superlattices
Authors: Bosc F. , Sicart J. , Robert J. L. , Piotrzkowski R.
Published: July 2002

The silicon deep donor (so-called DX center) is known to exhibit a bistable charged state DX−/d+ in GaAs and related compounds. We investigate the electron mobility as a function of the carrier concentration μ(n) in a silicon doped GaAs–AlAs short period superlattice at the temperature of liquid nitrogen, i.e., in the regime of metastability of the DX center. To vary the electron density, two methods are utilized: gradual photoexcitation of the DX donors or appropriate thermal capture cycles after complete photoionization of the DX donors. We observe the multivalued character of μ(n) and hysteresis effects when tuning of the metastable DX center occupancy is achieved with various procedures. Similar behavior has been previously observed in the silicon doped AlGaAs alloy and has been explained as the result of changing the degree of the donor charge spatial correlation. Our results show the influence of the photon energy when the electron concentration is varied by illuminating the sample with above- or below-bandgap light. This is due to radically different mechanisms of persistent photoconductivity in these two spectral domains. Moreover, the strong mobility enhancement we observed in the interband regime is explained by DX charge transfers which are specific for superlattice structures.
Truncation of Coulombic interactions in computer simulations of liquids
Authors: Linse Per , Andersen Hans C.
Published: July 2002

Several methods are used to take long ranged forces into account in computer simulation of liquids of charged or dipolar particles. These methods include Ewald summation, reaction field methods, and truncation of the potential. Here we investigate various ways of truncating a Coulombic potential to give a finite ranged potential for use in computer simulation. Three truncation schemes, namely the minimum image method, a ‘‘spherical truncation’’ method, and a method we call ‘‘spherical Ewald truncation,’’ are tested by applying them to simulations of an aqueous 2–2 electrolyte solution, liquid KCl, and liquid BeF2. Various ways of calculating the thermodynamic and structural properties for the Coulombic system from the simulation results for the finite ranged truncated potentials are tested and compared. For the dilute electrolyte, the minimum image method and the spherical truncation method (which also includes a shift of the potential to make it continuous) are the most useful truncation schemes. For the two fused salt systems, the spherical Ewald truncated potential is the most useful and gives structures that are very close to those of the full Coulombic potential. The accuracy of the hypernetted chain equation (HNC) for all these systems is tested and found to be remarkably good even for BeF2, which forms a low coordination number random tetrahedral network. The reference hypernetted chain (RHNC) perturbation method for correcting the simulation results for the effect of long ranged forces is also applied to these systems and found to work very well.
Frequency dependent nonlinear optical properties of molecules
Authors: Sekino Hideo , Bartlett Rodney J.
Published: July 2002

Various nonlinear optical polarizabilities are derived and evaluated by time dependent Hartree–Fock theory (TDHF). The recursive nature of the TDHF theory is exploited to develop formulas that are applicable in any order. The theory is applied to evaluate dispersion effects for the series of molecules CH4, CH3F, CH2F2, CHF3, and CF4. Comparisons are made with results obtained from dc-induced, second-Harmonic generation, and third-Harmonic generation experiments. Additional applications are reported for H2 and HF.
Photoelectron spectroscopy of mass-selected metal cluster anions. I. Cu−n, n=1–10
Authors: Leopold Doreen G. , Ho Joe , Lineberger W. C.
Published: July 2002

Negative ion photoelectron spectra of Cu−n (n=1–10) are reported for the 0–2.4 eV region at an instrumental resolution of 10 meV. The cluster anions were prepared in a flowing afterglow ion source incorporating a cold cathode dc discharge. This very simple source provides a convenient, general method to prepare continuous beams of near-thermal metal cluster ions at intensities (up to 10−11 A) sufficient for spectroscopic or chemical studies. Photoelectron spectra of the copper cluster anions yield measurements for vertical electron binding energies and adiabatic electron affinities as a function of cluster size. The overall trend observed is well described by the classical spherical drop electrostatic model. In addition, quantum effects are apparent in the higher electron affinities generally observed for clusters containing odd numbers of atoms. Excited electronic states in the photoelectron spectra show that the transition energy in the neutral molecule decreases rapidly with cluster size. Vibrational structure resolved in the Cu−2 spectrum yields measurements for the vibrational frequency (210±15 cm−1), bond length (2.345±0.010 Å), dissociation energy (1.57±0.06 eV), and vibrational temperature (450±50 K) of the anion.
Does reptation describe the dynamics of entangled, finite length polymer systems? A model simulation
Authors: Kolinski Andrzej , Skolnick Jeffrey , Yaris Robert
Published: July 2002

In order to examine the validity of the reptation model of motion in a dense collection of polymers, dynamic Monte Carlo (MC) simulations of polymer chains composed of n beads confined to a diamond lattice were undertaken as a function of polymer concentration φ and degree of polymerization n. We demonstrate that over a wide density range these systems exhibit the experimentally required molecular weight dependence of the center-of-mass self-diffusion coefficient D∼n−2.1 and the terminal relaxation time of the end-to-end vector τR∼n3.4. Thus, these systems should represent a highly entangled collection of polymers appropriate to look for the existence of reptation. The time dependence of the average single bead mean-square displacement, as well as the dependence of the single bead displacement on position in the chain were examined, along with the time dependence of the center-of-mass displacement. Furthermore, to determine where in fact a well-defined tube exists, the mean-square displacements of a polymer chain down and perpendicular to its primitive path defined at zero time were calculated, and snapshots of the primitive path as a function of time are presented. For an environment where all the chains move, no evidence of a tube, whose existence is central to the validity of the reptation model, was found. However, if a single chain is allowed to move in a partially frozen matrix of chains (where all chains but one are pinned every ne beads, and where between pin points the other chains are free to move), reptation with tube leakage is recovered for the single mobile chain. The dynamics of these chains possesses aspects of Rouse-like motion; however, unlike a Rouse chain, these chains undergo highly cooperative motion that appears to involve a backflow between chains to conserve constant average density. While these simulations cannot preclude the onset of reptation at higher molecular weight, they strongly argue at a minimum for the existence with increasing n of a crossover regime from simple Rouse dynamics in which reptation plays a minor role at best.
An efficient procedure for calculating the evolution of the wave function by fast Fourier transform methods for systems with spatially extended wave function and localized potential
Authors: Heather Robert , Metiu Horia
Published: July 2002

Various methods using fast Fourier transform algorithms or other ‘‘grid’’ methods for solving the time-dependent Schrödinger equation are very efficient if the wave function remains spatially localized throughout its evolution. Here we present and test an extension of these methods which is efficient even if the wave function spreads out, provided that the potential remains localized. The idea is to split the wave function at various times during the propagation into two parts, one localized in the interaction region and the other in the force free region; the first is propagated by a fast Fourier transform method on a grid whose size barely exceeds the interaction region, and the latter by a single application of a free particle propagator. This splitting is performed whenever the interaction region wave function comes close to the end of the grid. The total asymptotic wave function at a given time t is reconstructed by adding coherently all the asymptotic wave function pieces which were split at earlier times, after they have been propagated to the common time t. The method is tested by studying the wave function of a diatomic molecule dissociated by a strong laser field. We compute the rate of energy absorption and dissociation and the momentum distribution of the fragments.
Vibrational frequencies for polyatomic molecules. I. Indole and 2,3-benzofuran spectra and analysis
Authors: Collier William B.
Published: July 2002

A method is presented that can predict the vibrational frequencies of two and three ring polycyclic molecules to an accuracy sufficient for identifying fundamental vibrations. The scaled internal coordinate method of Pulay and co-workers is applied to force constants calculated with the Austin Method 1 semiempirical Hamilitonian. The technique was applied to benzene, furan, and pyrrole to generate a set of scale factors that would allow the a priori prediction of the vibrational spectra for benzofuran and indole. The predicted frequencies had root mean squared deviations of 15–23 cm−1 from the observed frequencies. Using the indole and benzofuran predictions, the vibrational assignments were made and ideal gas entropies were calculated. These spectroscopic entropies agreed with the calorimetrically determined entropies.
Number of distinct sites visited by random walks in lattice gases
Authors: Czech R.
Published: July 2002

The number of distinct sites visited by the correlated random walk of tagged particles in lattice gases is investigated. The lattices are occupied with concentration 0<c<1 and the particles are noninteracting except that double occupancy of lattice sites is forbidden. The distribution of the number of sites visited in one dimension and its mean value in higher dimensions d=2 and d=3 are studied by numerical simulations and analyzed in terms of scaling expressions, partly guided by heuristic models.
Time-dependent treatment of scattering: Integral equation approaches using the time-dependent amplitude density
Authors: Hoffman David K. , Sharafeddin Omar , Judson Richard S. , Kouri Donald J.
Published: July 2002

The time-dependent form of the Lippmann–Schwinger integral equation is used as the basis of several new wave packet propagation schemes. These can be formulated in terms of either the time-dependent wave function or a time-dependent amplitude density. The latter is nonzero only in the region of configuration space for which the potential is nonzero, thereby in principle obviating the necessity of large grids or the use of complex absorbing potentials when resonances cause long collision times (leading, consequently, to long propagation times). Transition amplitudes are obtained in terms of Fourier transforms of the amplitude density from the time to the energy domain. The approach is illustrated by an application to a standard potential scattering model problem where, as in previous studies, the action of the kinetic energy operator is evaluated by fast Fourier transform (FFT) techniques.
Energy equipartitioning in the classical time-dependent Hartree approximation
Authors: Straub John E. , Karplus Martin
Published: July 2002

In the classical time-dependent Hartree approximation (TDH), the dynamics of a single molecule is approximated by that of a ‘‘field’’ (each field being N ‘‘copies’’ of the molecule which are transparent to one another while interacting with the system via a scaled force). It is shown that when some molecules are represented by a field of copies, while other molecules are represented normally, the average kinetic energy of the system increases linearly with the number of copies and diverges in the limit of large N. Nevertheless, the TDH method with appropriate energy scaling can serve as a useful means of enhancing the configurational sampling for problems involving coupled systems with disparate numbers of degrees of freedom.
Molecular point-group symmetry in electronic structure calculations
Authors: Häser Marco
Published: July 2002

A generalization of Pitzer’s equal contribution theorem is put forth. The new theorem specifies which types of matrix elements can be constructed using only symmetry-unique (batches of) integrals. A general and simple algorithm to use molecular point-group symmetry in electronic structure calculations is outlined.
Simulation of the chemical potential and the cavity free energy of dense hard-sphere fluids
Authors: Attard Phil
Published: July 2002

The chemical potential of dense hard-sphere fluids, and also the work of cavity formation, are simulated directly by a force-balance Monte Carlo technique. Here the coupling between a solute and the solvent varies in the presence of an external field. For a hard-sphere fluid the variable is the cavity diameter, and the scaled particle theory proves sufficient for the applied field. The method is shown to be viable for densities as high as the freezing transition. A vectorizable Monte Carlo computer algorithm is also given.
Monte Carlo calculation of the conformational free energy and entropy of a polypeptide chain near a charged surface
Authors: Premilat Samuel , Elhebil Farid
Published: July 2002

The conformational behavior of a molecular model of enkephalin near a charged plane is studied using a Monte Carlo procedure. The variation of the dielectric permittivity near the charged surface is taken into account and the electric image method is used in order to obtain the interaction energy of the molecule with the plane as well as the electrostatic interactions between partial atomic charges. Evaluation of the free energy and entropy of conformation are made as functions of the molecule to plane distance. The molecular chain adopts folded structures near the surface and extended ones far from it, but the folding of the molecule and its attraction or repulsion by the plane depend largely on the charge density of that surface and on the charged ends of the molecule.
Nuclear magnetic resonance studies of homogeneous catalysis using parahydrogen: Analysis of nuclear singlet–triplet mixing as a diagnostic tool to characterize intermediates
Authors: Bargon J. , Kandels J. , Kating P.
Published: July 2002

The 1H–NMR polarization spectra of the products after homogeneous hydrogenations with para- or orthohydrogen (the PHIP spectra) convey significant information about the course of the reaction. Mixing of the nuclear singlet and triplet states of the two protons derived from dihydrogen during the reaction cycle has been found to modify the PHIP signal pattern of the product molecule characteristically. The intermediate coordination of the two dihydrogen protons at the catalyst is proposed to be responsible for this behavior. A parallel to the singlet–triplet mixing of electronic states according to the well-known CIDNP phenomenon is outlined. The homogeneous hydrogenation of 2,5-dihydrofuran to tetrahydrofuran using Wilkinson’s catalyst serves as an example.
Molecular clusters and correlations in liquid methanol at room temperature
Authors: Sarkar S. , Joarder R. N.
Published: July 2002

The analysis of x-ray diffraction data of liquid methanol at room temperature reveals a distinct possibility of the presence of hexamer molecular clusters due to intermolecular hydrogen bonding. On the assumption of hexamer domination in the structure, the center structure is derived, which in turn reproduces closely the available data on neutron diffraction and computer simulation for this system. The center–center correlations show special features of bonded structure.
Gravity and shear flow effects on spinodal decomposition in single component fluids
Authors: Schmitz G. , Klein H. , Woermann D.
Published: July 2002

Results of experiments, carried out in space and on earth, are reported in which spinodal decomposition in a single-component fluid (SF6) is studied by small angle light scattering. Deformed circular light-scattering patterns are observed. They reflect deformation of density fluctuations by gravity effects and shear flow.
Molecular time scale generalized Langevin equation theory for coupled spin systems
Authors: Mensing Glennys , Beyerlein A. L. , McDowell H. K.
Published: July 2002

The MTGLE theory is formulated for spin systems using the exact low temperature limit solution of the one dimensional ferromagnetic Heisenberg model as a guide. The final equations are solved to reproduce the exact time correlation function and GLE response function for the spin 1/2 XY model in the high temperature limit [Sur et al., Phys. Rev. B 12, 3845 (1975)]. The advantage of the method is that it can be applied to complex systems with relative ease and in the case of spin systems provides an avenue for investigating effects of anisotropic dipolar spin interactions. One such application to exchange narrowing of the ESR line shape is discussed.
The relaxation dynamics of spin I=1 nuclei with a static quadrupolar coupling and a radio-frequency field
Authors: van der Maarel J. R. C.
Published: July 2002

The dynamics of spin I=1 nuclei in a radio-frequency (rf) field and with both a static and fluctuating quadrupolar interaction are discussed. Approximate expressions of the relaxation rates are derived under the assumption that the linewidths are much smaller than the characteristic line-splittings. The conventional spin-lock relaxation rate increases with increasing ratio of the spin-lock field strength and static quadrupolar coupling ω1/ωQ and is sensitive to the spectral density at frequency √ω2Q+4ω21. The rate shows a maximum in the presence of low frequency molecular motion. Some experiments are performed on a deuterium spin probe in a low molecular weight liquid crystal. The creation of multipolar states by continuous rf irradiation is demonstrated.
Effects of solvent exclusion on the force between charged surfaces in electrolyte solution
Authors: Tang Zixiang , Scriven L. E. , Davis H. T.
Published: July 2002

Effects of hard-core exclusions of solvent molecules on the force between charged surfaces immersed in electrolyte solution are investigated theoretically. A three-component model of electrolyte solution is used, in which the solvent is modeled as a fluid of neutral hard spheres. A free energy density functional theory, which has been applied previously to study the interaction between primitive electrical double layers, is extended to include ion–solvent and solvent–solvent interactions. Hard-core correlations are incorporated in a nonlocal free energy density functional, while electrostatic correlations are evaluated using the mean spherical approximation. The force between charged surfaces as a function of surface separation is found highly oscillatory, with a period approximately equal to the particle diameter. Because of the dominant role of the hard-core exclusions of the solvent fluid, the force does not depend strongly on either electrolyte concentration or surface charge density.
Diffusion in Lennard-Jones fluids using dual control volume grand canonical molecular dynamics simulation (DCV-GCMD)
Authors: Heffelfinger Grant S. , Swol Frank van
Published: July 2002

A new approach to calculating diffusivities, both transport as well as equilibrium, is presented. The dual control volume grand canonical molecular dynamics (or DCV-GCMD) method employs two local control volumes for chemical potential control via particle creation/destruction as in grand canonical Monte Carlo (GCMC) simulations. The control volumes are inserted in a standard NVT molecular dynamics simulation yielding a simulation with stochastic chemical potential control that may be thought of as a hybrid GCMC-MD approach. The geometrical control of the chemical potential enables a steady state chemical potential gradient to be established in the system. By measuring the density profile and flux, Fick’s law is used to determine the diffusivity. An example calculation is presented for a simple Lennard-Jones system.
Solution of the Redfield equation for the dissipative quantum dynamics of multilevel systems
Authors: Pollard W. Thomas , Friesner Richard A.
Published: July 2002

We present a new method for solving the Redfield equation, which describes the evolution of the reduced density matrix of a multilevel quantum-mechanical system interacting with a thermal bath. The method is based on a new decomposition of the Redfield relaxation tensor that makes possible its direct application to the density matrix without explicit construction of the full tensor. In the resulting expressions, only ordinary matrices are involved and so any quantum system whose Hamiltonian can be diagonalized can be treated with the full Redfield theory. To efficiently solve the equation of motion for the density matrix, we introduce a generalization of the short-iterative-Lanczos propagator. Together, these contributions allow the complete Redfield theory to be applied to significantly larger systems than was previously possible. Several model calculations are presented to illustrate the methodology, including one example with 172 quantum states.
O–H torsional vibrations in the S and S1 states of catechol
Authors: Bürgi Thomas , Leutwyler Samuel
Published: July 2002

A spectroscopic study of supersonic jet-cooled catechol (1,2-dihydroxybenzene) and its d1- and d2-isotopomers, deuterated at the hydroxy groups, was performed by resonant two-photon ionization (R2PI) and fluorescence emission techniques, and supplemented by molecular-beam hole-burning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S0→S1 spectrum from the 000 band to 400 cm−1 above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression associated mainly with the torsion of the hydroxy groups. The torsional bands are very prominent in the R2PI spectra, but are weak in the emission spectra. Detailed analysis of the torsional bands was based on a fit to the S1 and S0 state frequencies and the Franck–Condon factors in absorption and emission, using a double-minimum potential for the S1 state and a harmonic potential for the S0 state. In the S1 state one of the two –O–H torsional mode frequencies is lowered from τ2≊250 to ≊50 cm−1, and the molecule is only quasiplanar with respect to the –O–H torsional coordinates.
An equation of state for fused hard-sphere polyatomic molecules
Authors: Phan S. , Kierlik E. , Rosinberg M. L.
Published: July 2002

An equation of state for a fluid of fused hard sphere (FHS) molecules is developed based on an interpolation scheme which relates the free energy per particle to that of a fluid of tangent hard sphere (THS) molecules at the same packing fraction. Use of Wertheim’s TPT1 (first order thermodynamic perturbation theory) equation for this latter quantity yields an analytical expression for the compressibility factor for any shape of the FHS molecule. Predictions are in good agreement with the simulation results for rigid homonuclear and heteronuclear diatomics, linear and nonlinear triatomics, and tetrahedral pentatomics. For purely repulsive models of n-alkane chains, it is found that the accuracy of the theory deteriorates with increasing chain length. The interpolation procedure is also generalized to the case of chemical association.
Ring opening reaction dynamics in the reaction of hydrogen atoms with ethylene oxide
Authors: Shin S. K. , Jarek R. L. , Böhmer E. , Wittig C.
Published: July 2002

Ethylene oxide, C2H4O, is a three-membered ring with a single oxygen atom bridging the two carbons. Reactions of H and D atoms with ethylene oxide have been studied in the gas phase to provide insight into the dynamics of three-membered ring opening. H atoms were produced by photolyzing HI in the wavelength range 240–266 nm. The channel leading to OH+C2H4 was monitored via laser-induced fluorescence (LIF) of the OH A 2Σ←X 2Π system. The D atom reaction yields OD with no hydrogen scrambling. With an available energy of 23 000 cm−1, the average OH D rotational energy is ∼350 cm−1 for OH(v=0) and OD(v=0) and ∼250 cm−1 for OD(v=1). OH(v=1) was not observed, while the OD(v=1) population was about one-tenth that of OD(v=0). There was no apparent bias in populations between Λ doublets in each of the spin–orbit states for both OH and OD. Doppler broadening of OH(v=0) rotational lines was measured to evaluate the average center-of-mass (c.m.) translational energy, which was found to be ∼2300 cm−1. On average, the ring opening process deposits ∼10% of the available energy into c.m. translation, ∼2% into OH rotation, and ∼88% into ethylene internal energy. Comparison with CH2CH2OH unimolecular dissociation dynamics and theoretical transition state calculations leads to a likely mechanism in which hydrogen abstracts oxygen via sequential C–O bond fission without involving a long-lived CH2CH2OH intermediate.
A simple method for determining approximate static and dynamic vibrational hyperpolarizabilities
Authors: Bishop David M. , Hasan Muhammed , Kirtman Bernard
Published: July 2002

A simple method is presented for calculating approximate static and dynamic vibrational hyperpolarizabilities. It involves determining electrical properties in the presence of a static field with and without geometry optimization. This method is readily applicable to all tensor components of a general polyatomic molecule.
Unexpected diffusion behavior of gas molecules in crystalline poly(4-methyl-1-pentene)
Authors: Müller-Plathe Florian
Published: July 2002

We have studied the behavior of carbon dioxide and methane in crystalline poly(4-methyl-1-pentene) (PMP) by means of molecular simulation. Gas diffusion coefficients in the direction of the crystal axes have been determined by equilibrium and nonequilibrium molecular dynamics techniques. We find that both gases diffuse along the channels of free volume between the PMP helices as well as perpendicular to these channels. The diffusion of CH4 is more isotropic than that of CO2: It diffuses perpendicular to the helices with almost the same rate as parallel. This is attributed to the fact that CH4 dissolves better in the outer perimeters of the PMP helices which are not dissimilar to fluid alkanes. A map of the local relative free energy of sorption within the unit cell is calculated both by test-particle insertion and from the residence periods of gas molecules at certain positions. It also shows that the barriers to perpendicular diffusion are lower for CH4 than for CO2.
A chemical potential equalization method for molecular simulations
Authors: York Darrin M. , Yang Weitao
Published: July 2002

A formulation of the chemical potential (electronegativity) equalization principle is presented from the perspective of density-functional theory. The resulting equations provide a linear-response framework for describing the redistribution of electrons upon perturbation by an applied field. The method has two main advantages over existing electronegativity equalization and charge equilibration methods that allow extension to accurate molecular dynamics simulations. Firstly, the expansion of the energy is taken about the molecular ground state instead of the neutral atom ground states; hence, in the absence of an external field, the molecular charge distribution can be represented by static point charges and dipoles obtained from fitting to high-level ab initio calculations without modification. Secondly, in the presence of applied fields or interactions with other molecules, the density response can be modeled accurately using basis functions. Inclusion of basis functions with dipolar or higher order multipolar character allows molecules or chemical groups to have correct local anisotropic polarizabilities. A modified semiempirical form of the hardness matrix has been introduced that can be evaluated efficiently using Gaussians, and requires only one parameter per basis function. Applications at two basis-set levels demonstrate the method can accurately reproduce induced dipole moments and estimated chemical potentials obtained from density-functional calculations for a variety of molecules. Inclusion of basis functions beyond the conventional spherical-atom type is essential in some instances. The present formulation provides the foundation for a promising semi-empirical model for polarization and charge transfer in molecular simulations.
Binding mean spherical approximation for pairing ions: An exponential approximation and thermodynamics
Authors: Bernard O. , Blum L.
Published: July 2002

The thermodynamics of mean spherical approximation for an arbitrary mixture of charged hard spheres with sticky-point attraction in the Wertheim formalism is derived for the most general case. It is shown that the electrostatic excess thermodynamics can be still computed from relatively simple formulas which depend on the screening parameter ΓB. We introduce also a new exponential approximation, which unlike the simpler mean spherical-like approximation discussed in our previous work, has the correct low density behavior. The restricted case of a binary mixture with a single association constant is discussed explicitly.
Particle simulation of chemical chaos
Authors: Geysermans P. , Baras F.
Published: July 2002

A microscopic computer experiment is set up to investigate the statistical properties of a homogeneous chemical system undergoing chaos at the macroscopic level. A specific model, the Willamowski–Rössler having a well-defined microscopic counterpart is used. Quantitative comparison with both the prediction of the deterministic description based on the rate equations and the results of the stochastic analysis is carried out. Dynamical and static properties obtained from these three procedures are in very good agreement and confirm the robustness of the underlying deterministic attractor even when microscopic aspects are taken into account.
Vibrational predissociation dynamics of the He79Br2 van der Waals molecule: A quantum mechanical study
Authors: González-Lezana Tomás , Hernández Marta I. , Delgado-Barrio Gerardo , Buchachenko Alexei A. , Villarreal Pablo
Published: July 2002

The vibrational predissociation of the HeBr2 van der Waals complex is studied by means of both accurate and approximate three dimensional quantum mechanical calculations. Simple atom–atom potentials have been tested for matching experimental measurements at low Br2 vibrational excitations. The fragmentation dynamics when the bromine subunit is close to its B state dissociation limit is then explored and compared with experiments. For low to intermediate vibrational states v, good agreement with most of the data (spectral shifts, lifetimes, average structures, average product energies) is achieved. The closing of the Δv=−1 channel at v=44 and the binding energy at that position are successfully reproduced, although calculated and experimental blueshifts and linewidths are not in such good accordance in the v≳38 range. For these high v excitations, fragmentation cross sections exhibit complicated structures indicating strong interactions among different quasibound states. In addition, interesting threshold and intramolecular energy redistribution effects are predicted. The closing of the Δv=−1 channel is found to be a gradual process where different dynamical regimes can be investigated in detail.
A low-storage filter diagonalization method for quantum eigenenergy calculation or for spectral analysis of time signals
Authors: Mandelshtam Vladimir A. , Taylor Howard S.
Published: July 2002

A new version of the filter diagonalization method of diagonalizing large real symmetric Hamiltonian matrices is presented. Our previous version would first produce a small set of adapted basis functions by applying the Chebyshev polynomial expansion of the Green’s function on a generic initial vector χ. The small Hamiltonian, H, and overlap, S, matrices would then be evaluated in this adapted basis and the corresponding generalized eigenvalue problem would be solved yielding the desired spectral information. Here in analogy to a recent work by Wall and Neuhauser [J. Chem. Phys. 102, 8011 (1995)] H and S are computed directly using only the Chebyshev coefficients cn=〈χ|Tn(Ĥ)|χ〉, calculation of which requires a minimal storage if the Ĥ matrix is sparse. The expressions for H and S are analytically simple, computationally very inexpensive and stable. The method can be used to obtain all the eigenvalues of Ĥ using the same sequence {cn}. We present an application of the method to a realistic quantum dynamics problem of calculating all bound state energies of H3+ molecule. Since the sequence {cn} is the only input required to obtain all the eigenenergies, the present method can be reformulated for the problem of spectral analysis of a real symmetric time signal defined on an equidistant time grid. The numerical example considers a model signal C(tn)=∑kdk cos(tnωk) generated by a set of N=100 000 frequencies and amplitudes, (ωk,dk). It is demonstrated that all the ωk’s and dk’s can be obtained to very high precision using the minimal information, i.e., 200 000 sampling points.
Theory of solvent effects on electronic absorption spectra of rodlike or disklike solute molecules: Frequency shifts
Authors: Nagae Hiroyoshi
Published: July 2002

A theory, which has been advanced by the author and his co-workers, for frequency shifts of electronic absorption spectra of a nonpolar rodlike molecule in dilute nonpolar and polar solutions is extended so that it is applicable to a polar rodlike solute molecule and a nonpolar or polar disklike solute molecule. The theory takes into account higher-order multipole moments (in addition to a dipole) of the solute molecule, and its solution-phase polarizabilities are properly taken into account. Furthermore, it explains how the dispersive interaction is altered by the presence of permanent dipoles of the solvent.
A new definition of cavities for the computation of solvation free energies by the polarizable continuum model
Authors: Barone Vincenzo , Cossi Maurizio , Tomasi Jacopo
Published: July 2002

A set of rules for determining the atomic radii of spheres used to build the molecular cavities in continuum solvation models are presented. The procedure is applied to compute the hydration free energy for molecules containing H, C, N, O, F, P, S, Cl, Br, and I at a computational level (Hartree–Fock with a medium size basis set) allowing the study of relatively large systems. The optimized radii reduce the mean error with respect to the experimental solvation energies below 0.20 kcal/mol for a set of 43 neutral solutes and around 1 kcal/mol for 27 ions. Moreover the correct trends are observed for the solvation energies of homolog series, like the series ammonia–trimethylamine, that are not correctly reproduced by usual solvation models.
Viscous fingering in periodically heterogeneous porous media. II. Numerical simulations
Authors: De Wit A. , Homsy G. M.
Published: July 2002

We study nonlinear viscous fingering in heterogeneous media through direct numerical simulation. A pseudospectral method is developed and applied to our spatially periodic model introduced in Paper I [J. Chem. Phys. 107, 9609 (1997)]. The problem involves several parameters, including the Peclet number, Pe, the magnitude and wave numbers of the heterogeneity, σ, nx, ny, respectively, and the log of the viscosity ratio R. Progress is made by fixing R at 3.0 and then working first with layered systems nx=0 and finally with “checkerboard” systems in which both wave numbers are nonzero. Strongly nonlinear finger dynamics are compared and contrasted with those occurring in the homogeneous case. For layered systems, it is found that very low levels of heterogeneity leads to an enhancement of the growth rate of the fingered zones, and that both harmonic and subharmonic resonances between the intrinsic scale of nonlinear fingering and those of the heterogeneity occur. We also find that the fingering regime of layered systems can be completely disrupted by modest levels of heterogeneity, leading to a “channeling” regime and dispersive behavior which is identified as a Taylor dispersion mechanism. The effective axial dispersion coefficient in this regime is found to be strongly dependent on the viscosity ratio. The situation becomes more complex for the checkerboard case. The channeling regime can in turn be disrupted by the axial dependence of the heterogeneity, which stimulates tip splitting and a return to complex nonlinear finger dynamics in regions of parameter space, including very large σ, that would otherwise be strongly dispersive. The effectiveness of the axial variation in stimulating tip splitting is studied by a short parametric study in nx, and is found to be maximized for certain axial frequencies in a manner similar to that found in Paper I. All our results are found to be in general qualitative agreement with available (but limited) experimental visualizations.
Benzene-hydrogen halide interactions: Theoretical studies of binding energies, vibrational frequencies, and equilibrium structures
Authors: Tarakeshwar P. , Lee Sang Joo , Lee Jin Yong , Kim Kwang S.
Published: July 2002

High level ab initio calculations have been performed on the benzene-HCl and benzene-HF systems using the second-order Mo/ller-Plesset perturbation theory. In contrast to existing theoretical studies, the calculated binding energies indicate that HCl binds more strongly to benzene than HF. This is in accordance with the limited experimental data available on these systems. An explanation has been forwarded for the above observation by performing a molecular orbital analysis of both C6H6⋯HF and C6H6⋯HCl. In the global minimum of C6H6⋯HF, HF lies inclined to the benzene ring with the hydrogen atom pointing either towards a benzene carbon or the center of carbon-carbon bond. In the C6H6⋯HCl complex, HCl is found to lie along the C6 axis of the benzene ring for smaller basis sets, but it also tends to lie inclined to the benzene ring for a very large basis set. The quantum mechanical probabilistic characterization of the structure of the C6H6⋯HCl complex provides a more realistic description of the experimental equilibrium structure. The van der Waals modes have also been characterized, and the modulation of these modes as one progresses from HF to HCl has also been studied.
Dynamics of n-alkanes: Comparison to Rouse model
Authors: Mondello Maurizio , Grest Gary S. , Webb Edmund B. , Peczak P.
Published: July 2002

The crossover to Rouse-type behavior for the self-diffusion constant D, the viscosity η, and the equilibrium structural statistics of n-alkanes (6⩽n⩽66) is studied numerically. For small n the chains are non-Gaussian and the mean squared end-to-end distance 〈R2〉 is greater than 6〈RG2〉, where 〈RG2〉 is the mean squared radius of gyration. As n increases, 〈R2〉/〈RG2〉→6(1+a/n), where a depends on the interaction model. At constant density, the Rouse model is used to extract the monomeric friction coefficient ζ and the viscosity η independently from the diffusion constant D and the longest relaxation time τR. ζD extracted from D is nearly independent of chain length while ζτ obtained from τR is much larger than ζD for small n. The viscosity measured in a nonequilibrium molecular dynamics simulation is closely approximated by the value of η determined from τR while η inferred from D is smaller for small n. For n≳60, the two estimates for both ζ and η agree as predicted from the Rouse model. D calculated from three interaction models is studied for increasing n and compared to experimental data.
The screening length of charge-asymmetric electrolytes: A hypernetted chain calculation
Authors: McBride Andrew , Kohonen Markus , Attard Phil
Published: July 2002

The screening length of the charge-asymmetric electrolyte is calculated in the hypernetted chain approximation. Results are presented for 1:1, 2:1, 3:1, and 4:1 electrolyte with concentrations ranging from 10−5 to 1 M. A number of analytic approximations for the screening length are compared to the hypernetted chain data. We find that the second moment approach of Attard [Phys. Rev. E 48, 3604 (1993)] gives good results for monovalent but not asymmetric electrolyte. A nonlinear version of the second moment approach is found to give good results for asymmetric electrolytes. We also find that the asymptotic formula of Mitchell and Ninham [Chem. Phys. Lett. 53, 397 (1978)], although giving a null result for monovalent electrolyte, is a satisfactory estimate for asymmetric electrolytes.
Turbidity, light scattering, and coexistence curve data for the ionic binary mixture triethyl n-hexyl ammonium triethyl n-hexyl borate in diphenyl ether
Authors: Wiegand S. , Briggs M. E. , Levelt Sengers J. M. H. , Kleemeier M. , Schröer W.
Published: July 2002

We report turbidity, light scattering, and coexistence curve data for a solution of triethyl n-hexyl ammonium triethyl n-hexyl borate in diphenylether. We recently reported that the present sample shows much higher turbidity than that of K. S. Zhang, M. E. Briggs, R. W. Gammon, and J. M. H. Levelt Sengers [J. Chem. Phys. 109, 4533 (1998)] for an earlier sample. An analysis of the data shows that nonclassical critical behavior is favored in the reduced temperature range from 10−5 to 10−2. At fixed reduced temperature, the correlation length is about twice as large as that of the previous sample. The correlation length amplitude calculated from the fit is 1.4 nm±0.1 nm. A detailed data analysis points out the limitations of turbidity measurements far away from the critical point. The intensity of scattered light was measured at 90°. Multiple scattering is relevant in the wider vicinity of the critical point and was corrected for by a Monte Carlo simulation method. An Ising-type exponent for the correlation length was obtained: ν=0.641±0.003, and the amplitude of the correlation length ξ0=1.34 nm±0.01 nm agrees with that of the turbidity experiment. Mean-field behavior can be ruled out. The refractive indices of coexisting phases were measured in the reduced temperature range from t=10−4 to 0.04. These measurements disagree with results reported by R. R. Singh and K. S. Pitzer [J. Chem. Phys. 92, 6775 (1990)]. The present data lead to an exponent β=0.34±0.01, close to the Ising value. The coexistence curve is much narrower than that of Singh and Pitzer. Crossover could not be detected in any of the experiments. Two-scale-factor universality could be confirmed for this and another ionic system within the experimental uncertainty.
Spatial correlations in polycarbonates: Neutron scattering and simulation
Authors: Eilhard J. , Zirkel A. , Tschöp W. , Hahn O. , Kremer K. , Schärpf O. , Richter D. , Buchenau U.
Published: July 2002

We present the results of a combined experimental (neutron scattering) and theoretical (computer simulation) effort to investigate structural properties of polycarbonate melts and glasses in the wave vector regime of Q⩽2.2 Å−1. The experimental part consists of advanced spin polarized scattering experiments, allowing us to extract the coherent scattering for protonated and deuterated samples. The simulations employ recently developed novel mapping procedures, which allow us to efficiently equilibrate complex polymer melts, thereby reproducing the experiment in much closer detail than earlier attempts.