bluuebunny/crossref_metadata_2025 huggingface dataset:
Pair connectedness and mean cluster size for continuum-percolation models: Computer-simulation results
Authors: Lee Sang Bub , Torquato S.
Published: July 2002

We devise a new algorithm to obtain the pair-connectedness function P(r) for continuum-percolation models from computer simulations. It is shown to converge rapidly to the infinite-system limit, even near the percolation threshold, thus providing accurate estimates of P(r) for a wide range of densities. We specifically consider an interpenetrable-particle model (referred to as the penetrable-concentric-shell model) in which D-dimensional spheres (D=2 or 3) of diameter σ are distributed with an arbitrary degree of impenetrability parameter λ, 0≤λ≤1. Pairs of particles are taken to be ‘‘connected’’ when the interparticle separation is less than σ. The theoretical results of Xu and Stell for P(r) in the case of fully penetrable spheres (λ=0) are shown to be in excellent agreement with our simulations. We also compute the mean cluster size as a function of density and λ for the case of 2D, and, from these data, estimate the respective percolation thresholds.
The Wei–Norman Lie-algebraic technique applied to field modulation in nuclear magnetic resonance
Authors: Campolieti G. , Sanctuary B. C.
Published: July 2002

By implementing the Wei–Norman Lie algebra approach, this paper focuses on the development of a new method of solution to the quantum Liouville equation for NMR field modulation. First, the general method is reviewed and discussed for systems of any finite dimensional Lie algebra. The theory is then applied to arbitrary time-dependent Zeeman interactions [or SU(2) Hamiltonians] in which the complete arbitrary spin I≥1/2 density operator problem reduces to a Riccati equation. Novel exact analytical solutions of the complete spin density operator for a class of temporal field modulations are obtained. In particular, a spherical tensor operator basis is used to expand the density operator, and the solutions retain the physically appealing form of Wigner rotations with time-modulated rotation angles which are special functions with well known analytical properties. The exact solutions include the frequency swept hyperbolic secant pulse shapes, as well as any exponentially modulated amplitude pulse. In contrast to all other existing formulations, the present treatment provides the first known examples of exactly soluble nonrectangular pulse shapes for all resonance off-set, all field amplitudes, all time and valid for all spin I≥1/2. In addition, from the underlying Riccati equation, a new series solution for the complete spin density matrix is given which is valid for any well-behaved field modulations. Spin inversion profiles are also calculated for exponentially decaying pulses. These show applications to selective excitation.
Dynamics of boron cluster ion reactions with deuterium: Adduct formation and decay
Authors: Ruatta Stephen A. , Hanley Luke , Anderson Scott L.
Published: July 2002

Product cross sections for reaction of size-selected boron cluster ions with deuterium are reported for collision energies ranging from 0.05 to 10 eV. The major reactions are those in which one or two deuterium atoms add to the intact cluster. Other, higher energy channels involve fragmentation of the clusters, with and without addition of deuterium atoms. Cluster size dependent thresholds are observed for many reactions, and we have used these to estimate the cluster–D atom binding energy as a function of cluster size. Metastable adducts of the form BnD+2 are observed for reagent clusters as small as the tetramer. For small clusters, adducts form without activation energy, but starting at B+8 activation barriers are observed. Unimolecular decay rates of the adducts have been estimated, and the binding energy of D2 to various size cluster ions was extracted using RRKM modeling. Comparison is made with related results for boron surfaces and for aluminum cluster ions.
A simple measure of electron localization in atomic and molecular systems
Authors: Becke A. D. , Edgecombe K. E.
Published: July 2002

We introduce in this work a new approach to the identification of localized electronic groups in atomic and molecular systems. Our approach is based on local behavior of the Hartree–Fock parallel-spin pair probability and is completely independent of unitary orbital transformations. We derive a simple ‘‘electron localization function’’ (ELF) which easily reveals atomic shell structure and core, binding, and lone electron pairs in simple molecular systems as well.
Density-functional thermochemistry. I. The effect of the exchange-only gradient correction
Authors: Becke Axel D.
Published: July 2002

Previous work by the author on diatomic molecules and by others on polyatomic systems has revealed that Kohn–Sham density-functional theory with ‘‘gradient corrected’’ exchange-correlation approximations gives remarkably good molecular bond and atomization energies. In the present communication, we report the results of an extensive survey of density-functional atomization energies on the 55 molecules of the Gaussian-1 thermochemical data base of Pople and co-workers [J. Chem. Phys. 90, 5622 (1989); 93, 2537 (1990)]. These calculations have been performed by the fully numerical molecules (NUMOL) program of Becke and Dickson [J. Chem. Phys. 92, 3610 (1990)] and are therefore free of basis-set uncertainties. We find an average absolute error in the total atomization energies of our 55 test molecules of 3.7 kcal/mol, compared to 1.6 kcal/mol for the Gaussian-1 procedure and 1.2 kcal/mol for Gaussian-2.
Phase diagrams of Lennard-Jones fluids
Authors: Smit B.
Published: July 2002

Gibbs ensemble simulations are used to calculate part of the phase diagram of the Lennard-Jones fluid. The phase diagram turned out to be very sensitive to the details of how the tail of the Lennard-Jones potential is taken into account in the simulations.
Perturbations in the infrared spectrum of the NH3 umbrella mode of HOH—NH3
Authors: Fraser G. T. , Suenram R. D.
Published: July 2002

A tunable microwave-sideband CO2 laser has been used with a molecular-beam electric-resonance optothermal spectrometer to observe the infrared spectrum of the NH3 umbrella fundamental vibration (ν5 in Cs ) of HOH––NH3 at a resolution of ∼3 MHz. Ground- and excited-state assignments were verified and extended using microwave–infrared double-resonance spectroscopy, with microwave transitions observed in both the ground and the excited states. The spectrum exhibits numerous perturbations, as evidenced by the observation of a minimum of 13 subbands originating from the (K,m)=(0,0) ground NH3 internal-rotor state and the (K,m)=(±1,±1) first excited NH3 internal-rotor state. For an unperturbed spectrum, only four such subbands are expected, two for the symmetric H2O tunneling state and two for the antisymmetric H2O tunneling state. The rotational progressions within the excited states are poorly fit to polynomial series in J(J+1), in contrast to the ground-state progressions which are well characterized by such series. The B rotational constants in the excited states are smaller than in the ground state, indicating an extension of the hydrogen-bonding interaction distance upon vibrational excitation. This is consistent with the observed infrared band origin for the (K,m)=(0,0) state of ∼1021 cm−1, which is blue shifted by 71 cm−1 from the hypothetical inversion-free 950 cm−1 ν2 band origin of uncomplexed NH3. The observed ν5 band origin is also in good agreement with matrix-isolation results scaled to correct for the matrix shift of the NH3 umbrella frequency found in the recently studied NH3–HCN complex. The complex does not dissociate upon vibrational excitation, implying that the binding energy is greater than the laser frequency of ∼1021 cm−1.
Molecular dynamics of the surface tension of a drop
Authors: Nijmeijer M. J. P. , Bruin C. , van Woerkom A. B. , Bakker A. F. , van Leeuwen J. M. J.
Published: July 2002

The curvature dependence of the liquid–vapor surface tension is described in the limit of small curvatures by Tolman’s length. Measurements of it, either experimentally or in a simulation, have not yet given a clear idea of its magnitude, even its sign is being debated. Previous attempts to relate Tolman’s length to a pressure tensor have led to ill-defined expressions. From an analysis of the pressure difference over the interface of a liquid drop, a pressure tensor expression is obtained for Tolman’s length that does not suffer from the previously encountered inconsistencies. This pressure difference is studied in a simulation of liquid drops, leading to an estimate of Tolman’s length. It appears to be small and bounds are given on it.
Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions
Authors: Kendall Rick A. , Dunning Thom H. , Harrison Robert J.
Published: July 2002

The calculation of accurate electron affinities (EAs) of atomic or molecular species is one of the most challenging tasks in quantum chemistry. We describe a reliable procedure for calculating the electron affinity of an atom and present results for hydrogen, boron, carbon, oxygen, and fluorine (hydrogen is included for completeness). This procedure involves the use of the recently proposed correlation-consistent basis sets augmented with functions to describe the more diffuse character of the atomic anion coupled with a straightforward, uniform expansion of the reference space for multireference singles and doubles configuration-interaction (MRSD-CI) calculations. Comparison with previous results and with corresponding full CI calculations are given. The most accurate EAs obtained from the MRSD-CI calculations are (with experimental values in parentheses) hydrogen 0.740 eV (0.754), boron 0.258 (0.277), carbon 1.245 (1.263), oxygen 1.384 (1.461), and fluorine 3.337 (3.401). The EAs obtained from the MR-SDCI calculations differ by less than 0.03 eV from those predicted by the full CI calculations.
Problems in the comparison of theoretical and experimental hyperpolarizabilities
Authors: Willetts Andrew , Rice Julia E. , Burland Donald M. , Shelton David P.
Published: July 2002

Frequently it is useful to compare experimental values of the hyperpolarizabilities β and γ with calculated values. It is also often helpful to compare experimental values of β obtained from dc-electric field induced second harmonic generation (dc-SHG) experiments, e.g., with values obtained using the solvatochromism method. In order to do this the hyperpolarizabilities must be defined using consistent conventions. In this paper, four commonly used conventions are discussed and simple factors for converting between them presented. In addition, the sum-over-states expression for the calculation of β and γ is described and its correct use in comparing with hyperpolarizabilities obtained using other experimental and theoretical techniques discussed. As an illustration of the consistent use of conventions, ab initio and semiempirical calculations on para-nitroaniline are compared with experimental dc-SHG values. This comparison highlights the difference between theoretical values of the hyperpolarizability with the molecule in a gas phase environment and experimental values obtained in polar solvents−a difference that has in the past been obscured by inconsistent choice of conventions.
Preliminary results on the performance of a family of density functional methods
Authors: Johnson Benny G. , Gill Peter M. W. , Pople John A.
Published: July 2002

The performance of six density functional methods is examined for a set of 32 small neutral molecular systems. The Kohn–Sham orbitals were obtained using the 6-31G* basis set, without employing any auxiliary fitting procedures. Atomization energies were calculated by each density functional method, as well as by Hartree–Fock, MP2, and QCISD using the same basis. Comparisons with experimental results are summarized for each method. Two of the density functional methods give excellent agreement with experiment, while none of the other density functional or ab initio methods give acceptable results.
Effective Hamiltonians for motions with disparate time scales: The quantum shell model and the classical statistical shell model
Authors: Kiang Ching-Hwa , Goddard William A.
Published: July 2002

The shell model of atomic polarizability is generalized to include a statistical distribution of shell states for each instantaneous configuration of the cores. Effective Hamiltonians for motions of the cores are derived for both quantum and classical descriptions of the shells. This extends the shell model into a formalism useful for molecular dynamics. The fluctuations of the shell positions account for London dispersion interactions. The resulting equations may also have applications to other systems containing motions with disparate time scales.
Determining the contributions of constraints in free energy calculations: Development, characterization, and recommendations
Authors: Pearlman David A.
Published: July 2002

We develop and/or characterize three methods for determining the contributions of constraints in free energy calculations. A new method for determining such contributions in thermodynamic integration (TI) calculations, the potential forces (PF) method, is developed and compared to a second method, the constraint forces (CF) method. Both methods are also compared to a previously described technique for calculating such contributions in free energy perturbation (FEP) simulations. We find that the TI/PF protocol is considerably more efficient than the TI/CF method, and is preferred except in cases where the constraints contributing to the free energy are part of a closed ring. Compared to TI/PF, the FEP method is shown to be relatively poor for generating potential of mean force (PMF) curves, though the FEP method is suitable for determining the ‘‘PMF bond contribution’’ in compositional free energy changes. PMF curves for a system of two neon atoms in a periodic box of water have been derived. The convergence behavior of the free energy derivative ∂G/∂R, where R is a distance constraint, has been examined in detail for this system. As much as a nanosecond of molecular dynamics sampling can be required to derive a fully converged value for this derivative at a single (λ) point. We have determined the sampling ratio for ∂G/∂R as a function of Ne–Ne distance for a 295 water (21 Å/side) periodic water box, and conclude that for free energy changes where long-distance constraint contributions are being determined, a minimum of 0.7 ps of sampling should be performed per window. When constraint contributions arise from short distances (such as when the PMF bond contribution is being calculated), correlation in the constraint derivative series dies out relatively quickly and the minimum sampling—then dictated by correlation in the nonbonded series—should be about 0.6 ps.
Indeterminacy of molecular constants for a symmetric top molecule in an electric field
Authors: Tanaka Takehiko
Published: July 2002

Indeterminacy of molecular constants for a symmetric top molecule placed in an electric field is studied by a group-theoretical method. For an E vibronic state of a C3v molecule, the three constants τxxxz, θ xxx, and λ cannot be simultaneously determined. Simultaneous determination is also impossible for the set of θ zxx, θ xxz, and α⊥, as well as for the set of ηJ, ηK, λ, h19, r, and μr.
The equation of motion coupled-cluster method. A systematic biorthogonal approach to molecular excitation energies, transition probabilities, and excited state properties
Authors: Stanton John F. , Bartlett Rodney J.
Published: July 2002

A comprehensive overview of the equation of motion coupled-cluster (EOM-CC) method and its application to molecular systems is presented. By exploiting the biorthogonal nature of the theory, it is shown that excited state properties and transition strengths can be evaluated via a generalized expectation value approach that incorporates both the bra and ket state wave functions. Reduced density matrices defined by this procedure are given by closed form expressions. For the root of the EOM-CC effective Hamiltonian that corresponds to the ground state, the resulting equations are equivalent to the usual expressions for normal single-reference CC density matrices. Thus, the method described in this paper provides a universal definition of coupled-cluster density matrices, providing a link between EOM-CC and traditional ground state CC theory. Excitation energy, oscillator strength, and property calculations are illustrated by means of several numerical examples, including comparisons with full configuration interaction calculations and a detailed study of the ten lowest electronically excited states of the cyclic isomer of C4.
Structural disorder and spin-lattice relaxation
Authors: Kaiser G. , Pierron H. , Friedrich J.
Published: July 2002

We measured the spin-lattice relaxation of the photoexcited triplet state of quinoxaline in variety of organic glasses and polymers as a function of temperature. The experiment covered the temperature range from a few K close to the glass transition. The remarkable thing is that the temperature dependence of the spin-lattice relaxation seems to be uniform for the various glassy and polymeric materials within the whole temperature range and seems to follow a quadratic temperature dependence. The only exception in polyethylene. There, the temperature dependence is much steeper but levels off above 40 K.
Velocity dependence of associative ion production in K (np)–C6H6 collisions
Authors: Popple R. A. , Smith K. A. , Dunning F. B.
Published: July 2002

The velocity dependence of the formation of stable KC6H6+ ions through associative ionization in K(np)/C6H6 collisions has been investigated for values of n in the range 13–25. The data show that the associative ionization rate increases rapidly with decreasing collision velocity. Possible reasons for this are discussed.
Nonexponential relaxations in strong and fragile glass formers
Authors: Böhmer R. , Ngai K. L. , Angell C. A. , Plazek D. J.
Published: July 2002

Deviations from thermally activated and from exponential response are typical features of the vitrification phenomenon and previously have been studied using viscoelastic, dielectric, calorimetric, optical, and other techniques. Linear response data from literature on about 70 covalent glass formers, ionic melts, supercooled liquids, amorphous polymers, and glassy crystals are surveyed. Except for orientational glasses and monohydric aliphatic alcohols a distinct but broad correlation of non-Debye behavior with non-Arrhenius relaxations is found. Within the broad trend several groups of materials, distinguished by their respective molecular complexity, can be identified and are shown to exhibit narrow correlations. At a given degree of deviation from Arrhenius behavior externally imposed stresses are relaxed with a departure from exponential behavior which is stronger the more the molecular or atomic subunits of the glassforming material are interconnected with each other.
How the unit cell surface charge distribution affects the energetics of ion–solvent interactions in simulations
Authors: Roberts James E. , Schnitker Jurgen
Published: July 2002

The evaluation of the electrostatic potential in condensed phase simulations normally includes an ‘‘extrinsic’’ contribution that manifests natural imbalances in the surface charge distribution of the microscopic unit cell. Most directly affected are ion–solvent interaction energies, and depending on whether the specific simulation conditions eliminate the extrinsic potential or not, these energies can vary by a considerable amount. This is illustrated by examining simulations of dilute aqueous solutions of Cl− and Fe2+ that use either a cutoff scheme or Ewald summation. It is found that the ion–water potential energy can vary with the type of boundary condition by as much as ≊60 kJ mol−1 for Cl− and ≊800 kJ mol−1 for Fe2+. The difference is exclusively due to the extrinsic potential effect and it is easy to calculate an appropriate correction term.
Equations of state valid continuously from zero to extreme pressures for H2O and CO2
Authors: Pitzer Kenneth S. , Sterner S. Michael
Published: July 2002

A new form of equation of state is proposed for use over extremely wide ranges of pressure where conventional equations fail. In particular, fluids including H2O and CO2 as well as argon, etc., remain more compressible at very high densities than can be represented by typical equations with van der Waals or Carnahan and Starling repulsive terms. The new equation is fitted to the data for H2O and CO2 over the entire range from the vapor and liquid below the critical temperature to at least 2000 K and from zero pressure to more than 105 bar (10 GPa) with good agreement. The extension of the equation for mixed fluids is discussed.
Condensation of a supersaturated vapor IX. Nucleation on ions
Authors: Katz Joseph L. , Fisk Jeffery A. , Chakarov Vasil M.
Published: July 2002

A new method for measuring steady-state nucleation rates on ions in an upward thermal diffusion cloud chamber is described. It provides quantitative measurement of all relevant variables, i.e., temperature, supersaturation, and ion density. Each of these variables, as well as the ion sign and the chemical makeup of the ions, can be readily changed and investigated. Using this technique, it is shown that the nucleation rate is directly proportional to the ion density. It is also shown that there is no dependence of the nucleation rate on electric field strength nor is there a dependence on the ion’s chemical makeup.
Theoretical study on the ground and excited states of the chromate anion CrO2−4
Authors: Jitsuhiro Shinya , Nakai Hiromi , Hada Masahiko , Nakatsuji Hiroshi
Published: July 2002

The symmetry adapted cluster (SAC) and SAC-configuration interaction (SAC-CI) theories are applied to the calculations of the ground and excited states of the chromate ion CrO2−4. Electron correlations are very large for this molecule and work to relax the charge polarization of the Cr–O bonds in the ground state. The experimental spectrum of CrO2−4 is well reproduced by the present calculations, which is the first ab initio study of the excited states including electron correlations. All of the observed peaks are assigned to the dipole allowed transitions to the 1T2 excited states. Furthermore, many kinds of forbidden transitions are calculated in the lower energy region. Both allowed and forbidden transitions are characterized as the electron-transfer excitations from oxygen to metal. In comparison with the previous theoretical studies, the present SAC-CI results are in good agreement with experiment and give reliable assignments of the spectrum. We also compare the electronic structures and spectra of CrO2−4, MoO2−4, MnO−4, RuO4, and OsO4, which have been studied by the SAC and SAC-CI methods.
Dynamics of glass-forming liquids. I. Temperature-derivative analysis of dielectric relaxation data
Authors: Stickel F. , Fischer E. W. , Richert R.
Published: July 2002

We have measured the dielectric relaxation times of the α process of phenyl salicylate (salol) covering 11 decades in frequency. Being representative for the class of low molecular weight organic glass forming materials, the highly resolved temperature dependence of the dynamics in salol does not follow a particular function like the Vogel–Fulcher–Tammann (VFT) law over the entire accessible range of temperatures. In order to conduct a detailed and unambiguous analysis of the temperature dependence, we take advantage of the derivatives of the experimental log(fmax) values with respect to temperature, which allow us to either linearize the frequently used temperature laws or to resolve subtle changes in fmax(T) by decreasing the number of free parameters. In this manner we observe that none of the common routes for rationalizing the dynamics like Arrhenius, VFT, Souletie scaling, and idealized mode-coupling theory account for the experimental findings properly. However, we do observe a VFT behavior within the limits 265 K≤T≤320 K, i.e., for temperatures ranging from significantly above the glass transition at Tg=220 K to far above the melting point.
Quasiclassical trajectory studies of N+OH, O+NH, and H+NO collisions using global ab initio potential energy surfaces
Authors: Guadagnini Renee , Schatz George C. , Walch Stephen P.
Published: July 2002

We present a quasiclassical trajectory study of the collisions which occur on the 1A′, 3A″, and 1A″ potential surfaces of HNO using recently developed global potential surfaces that were derived from ab initio calculations. Each of these surfaces was assumed to be uncoupled from the other surfaces for the purposes of the calculations, and the appropriate statistical average of electronic states for each process was calculated. For N+OH and O+NH, we specifically studied reactive collisions which give H+NO as products, and we also studied the production of N+OH from O+NH. Overall rate constants calculated for either N+OH or O+NH are in good agreement with most experiments, and in addition, the product NO vibrational distribution from the N+OH reaction is in good correspondence with recent measurements, revealing modest excitation that is close to what would be expected from a statistical distribution. For O+NH, the calculated NO vibrational distributions are much hotter than statistical, in apparent disagreement with recent measurements. However, a careful analysis of limitations on the measurements due to spectral interference and to collisional relaxation indicates that it is not possible to say if theory and experiment are at odds for this reaction. We find a significant cross section for O+NH→N+OH on the 3A″ surface (roughly 5% of the total reactive cross section, independent of energy), and this leads to rate constants at low temperatures that are orders of magnitude higher at low temperature than estimates made earlier based on H atom abstraction on the 5Π surface.

The mechanism of this reaction is found to involve three steps: addition to form HNO, isomerization to HON, and dissociation to produce N+OH. We have also studied nonreactive vibrational and rotational excitation in H+NO collisions, and we obtain distributions that are somewhat closer to experiment than obtained in previous theoretical studies, although there are still points of disagreement. We find that reactive H+NO→N+OH collisions have absolute cross sections and average NO rotational excitation that are in excellent agreement with recent fast H atom measurements.
Accuracy and efficiency of the particle mesh Ewald method
Authors: Petersen Henrik G.
Published: July 2002

In this article, a recently proposed method called the particle mesh Ewald (PME) method for computing the long ranged Coulomb interactions in for example molecular dynamics simulations is studied. The PME method has a complexity 𝒪(N log N), where N is the total number of charges. This complexity should in particular be compared with the complexity 𝒪(N3/2) for the well known Ewald method and 𝒪(N) for the rather new (but already famous) fast multipole method (FMM). However, these complexities say nothing about which method is fastest at some finite N. The purpose of this article is thus to study the PME method and compare its efficiency with the Ewald method and the fast multipole method. To enable this, a theoretical estimate for the accuracy of the PME method as function of its truncation parameters is derived. It is shown that this estimate is very precise by comparing it with results obtained from molecular dynamics simulations of a molten NaCl. Based on this estimate and very careful time experiments, the overall necessary time overhead for the PME method as function of N and a required accuracy is predicted. By a direct comparison with a similar prediction for the Ewald method and by studying existing Ewald-FMM comparisons, it is found that the PME method is significantly faster than both the Ewald method and the fast multipole method in the important decades N≂104–105.
Dynamics of glass-forming liquids. II. Detailed comparison of dielectric relaxation, dc-conductivity, and viscosity data
Authors: Stickel F. , Fischer E. W. , Richert R.
Published: July 2002

We have studied the temperature dependence of dielectric relaxation times in terms of the peak frequency fmax(T) of dielectric loss ε″(ω) and the dc-conductivity σdc(T) of several glass-forming liquids, covering 12 decades in the peak frequency fmax and 9 decades in σdc. Although dc-conductivity samples the mobility of ionic tracers, its variation with temperature is similar to that of fmax(T). The fmax(T) and σdc(T) are analyzed using the temperature-derivative method and compared to the viscosity data η−1(T). While most liquids reveal a common Vogel–Fulcher–Tammann (VFT) behavior for fmax, σdc and η−1 in an extended temperature range T≥Tm, some liquids deviate from this behavior by displaying a crossover at T=TA to an Arrhenius regime. In these cases the quantity fmax(T) decouples from the common curves for σdc(T) and η−1(T) and attains activation energies in excess (∼40% for alcohols) of those related to translational processes. For many samples a departure from the VFT behavior occurs at lower temperatures TB<Tm which tends to retard the glass transition. The onset of this qualitative change in the temperature dependence at TB turns out to be a characteristic temperature also in other experiments.
Enhanced translation of probe molecules in supercooled o-terphenyl: Signature of spatially heterogeneous dynamics?
Authors: Cicerone Marcus T. , Ediger M. D.
Published: July 2002

A holographic fluorescence recovery after photobleaching technique has been used to measure translational diffusion coefficients DT for four probes in supercooled o-terphenyl (OTP). DT values from 10−6 to 10−14 cm2/s were observed in the temperature range from Tg+8 K to Tg+90 K (Tg=243 K). In agreement with previous reports, the translational diffusion of probe molecules which are the same size as OTP molecules has a significantly weaker temperature dependence than T/η. However, as the size of the probe molecule is increased the temperature dependence of DT tracks T/η increasingly well. The transition between a weak temperature dependence to that of T/η occurs over only a factor of 3 in probe size. Previous work established that the rotational correlation times τc of these four probes in OTP tracks η/T. The product DTτc is independent of temperature for the largest probe but increases almost 2 orders of magnitude for the smaller probes as Tg is approached. A strong correlation is observed between this enhancement of translational diffusion and the KWW β values obtained from rotation measurements; probes with small β values show enhanced translation. These observations are qualitatively explained by spatially heterogeneous dynamics. The probes studied are rubrene, 9,10-bis(phenylethynyl)anthracene, tetracene, and anthracene.
The universal repulsive-core singularity: A temperature-dependent example
Authors: Baram Asher
Published: July 2002

It is shown that the critical exponents characterizing the dominant singularity of the Mayer series of a two-dimensional lattice gas model with a square barrier interaction are temperature independent. This result suggests that the universal repulsive core singularity extends to repulsive models with temperature-dependence cluster coefficients.
On the effects of an internal barrier on fast four-atom ion–molecule reactions
Authors: Dubernet M. L. , Rebentrost F. , Kompa K. L. , Levine R. D.
Published: July 2002

We investigate the influence of an internal barrier on an exothermic adiabatic reaction model between diatomic ions and molecules. Reaction cross-sections are calculated from quasi-classical trajectories for different initial vibrational and rotational states of the reactants and for relative collision energies in the range from 0.01 to 3 eV. It is shown that the height of a late internal barrier strongly influences both the characteristics of the state-selected cross-sections and the energy distributions of the products. In contrast to complex formation in the entrance region according to the Langevin model our analysis emphasizes the role of the full potential energy surface for an understanding of the dynamics of ion–molecule reactions.
Rationale for mixing exact exchange with density functional approximations
Authors: Perdew John P. , Ernzerhof Matthias , Burke Kieron
Published: July 2002

Density functional approximations for the exchange-correlation energy EDFAxc of an electronic system are often improved by admixing some exact exchange Ex: Exc≊EDFAxc+(1/n)(Ex−EDFAx). This procedure is justified when the error in EDFAxc arises from the λ=0 or exchange end of the coupling-constant integral ∫10 dλ EDFAxc,λ. We argue that the optimum integer n is approximately the lowest order of Görling–Levy perturbation theory which provides a realistic description of the coupling-constant dependence Exc,λ in the range 0≤λ≤1, whence n≊4 for atomization energies of typical molecules. We also propose a continuous generalization of n as an index of correlation strength, and a possible mixing of second-order perturbation theory with the generalized gradient approximation.
Block copolymer microstructures in the intermediate-segregation regime
Authors: Matsen M. W. , Bates F. S.
Published: July 2002

A detailed examination of the intermediate-segregation regime of diblock copolymer melts is presented using the incompressible Gaussian chain model and self-consistent field theory (SCFT). We find that the competition between interfacial tension and chain stretching used to describe behavior in the strong-segregation regime also explains behavior in this regime. Phase transitions from lamellae (L) to cylinders (C) to spheres (S) occur due to the spontaneous curvature produced as the asymmetry in the diblock composition increases. Complex phases, gyroid (G), perforated lamellar (PL), and double diamond (D), have curvatures between those of L and C, and therefore they compete for stability along the L/C boundary. Nevertheless, only G exhibits a region of stability. To explain why, we recognize that interfacial tension prefers the formation of constant mean curvature (CMC) surfaces to reduce interfacial area, and chain stretching favors domains of uniform thickness so as to avoid packing frustration. While the classical structures, L, C, and S, are successful at doing both simultaneously, the complex phases are not. Of the complex phases, G is the least frustrated and consequently is stable at intermediate degrees of segregation. However, G becomes unstable in the strong-segregation regime because the relative penalty for packing frustration increases with segregation. The PL and D structures are simply too frustrated, and therefore are never stable.
Comment on “Concerning the applicability of density functional methods to atomic and molecular negative ions” [J. Chem. Phys. 105, 862 (1996)]
Authors: Rösch Notker , Trickey S. B.
Published: July 2002

We systematize and clarify the significance and relationship of recently published numerical findings regarding atomic and molecular anions to both density functional theory fundamentals and approximations. Calculations for F− with all-numerical codes are included as brief examples.
Orthorhombic phase of crystalline polyethylene: A Monte Carlo study
Authors: Martoňák R. , Paul W. , Binder K.
Published: July 2002

In this paper we present a classical Monte Carlo simulation of the orthorhombic phase of crystalline polyethylene, using an explicit atom force field with unconstrained bond lengths and angles and periodic boundary conditions. We used a recently developed algorithm which apart from standard Metropolis local moves employs also global moves consisting of displacements of the center of mass of the whole chains in all three spatial directions as well as rotations of the chains around an axis parallel to the crystallographic c-direction. Our simulations are performed in the NpT ensemble, at zero pressure, and extend over the whole range of temperatures in which the orthorhombic phase is experimentally known to be stable (10–450 K). In order to investigate the finite-size effects in this extremely anisotropic crystal, we used different system sizes and different chain lengths, ranging from C12 to C96 chains, the total number of atoms in the super-cell being between 432 and 3456. We show here the results for structural parameters, such as the orthorhombic cell parameters a, b, c, and the setting angle of the chains, as well as internal parameters of the chains, such as the bond lengths and angles. Among thermodynamic quantities, we present results for thermal expansion coefficients, elastic constants and specific heat. We discuss the temperature dependence of the measured quantities as well as the related finite-size effects. In the case of lattice parameters and thermal expansion coefficients, we compare our results to those obtained from other theoretical approaches as well as to some available experimental data. We also suggest some possible ways of extending this study.
The multielectron, hidden crossings method for inelastic processes in slow ion/atom–atom collisions
Authors: Bent G. , Krstić P. S. , Schultz D. R.
Published: July 2002

A new method is described for studying collision dynamics in slow ion/atom–atom collisions. It is a generalization of the single-electron, two-center hidden crossings method to multielectron systems. This approach derives from the analytic properties of energy surfaces and wave functions of the adiabatic electronic Hamiltonian when the internuclear distance is extended into the complex plane. The collision dynamics in the adiabatic limit is determined by the topology of the unique multivalued electronic energy surface, particularly by its singular points, the square-root branch points. The surfaces described here have been studied using a complex version of the unrestricted Hartree–Fock and configuration interaction method with all single electron excitations, based on a bivariational principle. Although various inelastic processes can be calculated, the method is especially useful for the description of ionization. We have illustrated this through the calculation of cross sections for ionization of helium by proton and antiproton impact, as well as for the collision of two hydrogen atoms.
On the potential surface of the locally enhanced sampling approximation
Authors: Stultz Collin M. , Karplus Martin
Published: July 2002

The locally enhanced sampling (LES) methodology is used for simulating molecules consisting of a small subsystem that is of interest and a larger subsystem that interacts with the smaller one. The small subsystem is replicated to enhance its sampling and the large subsystem sees the mean field of the replicated components while the replicas are energetically transparent to one another. In this work we investigate the properties of the LES potential surface. It is shown that every local energy minimum on the potential surface of the real system is also a local energy minimum on the LES potential surface. It is also demonstrated that the converse of this is not true; i.e., there are minima on the LES potential energy surface that do not correspond to any minimum on the potential surface of the real system. A quantitative measure is derived to estimate how far minima on the LES potential surface are from minima on the potential surface of the real system. The implications and potential applications of this work are discussed.
Cumulant expansion of the reduced density matrices
Authors: Kutzelnigg Werner , Mukherjee Debashis
Published: July 2002

k -particle cumulants λk (for 2⩽k⩽n) corresponding to the k-particle reduced density matrices γk for an n-fermion system are defined via a generating function. The two-particle cumulant λ2 describes two-particle correlations (excluding exchange), λ3 genuine three-particle correlations etc. The properties of these cumulants are analyzed. Conditions for vanishing of certain λk are formulated. Necessary and sufficient for λ2=0 is the well-known idempotency condition γ2=γ for γ≡γ1. For λ3=0 to hold, a general necessary condition is Tr{2γ3−3γ2+γ}=0, for three special forms of the wave function (arbitrary two-electron state, antisymmetrized product of strongly orthogonal geminals on antisymmetrized geminal power wave function of extreme type) 2γ3−3γ2+γ=0 turns out to be necessary and sufficient. For a multiconfiguration self-consistent field wave function the only nonvanishing matrix elements of the cumulants are those where all labels refer to active (partially occupied) spin orbitals. Spin-free cumulants Λk corresponding to the spin-free reduced density matrices Γk are also defined and analyzed. The main interest in the density cumulants is in connection with the recently formulated normal ordering and the corresponding Wick theorem for arbitrary reference functions, but they are also useful for an analysis of electron correlation.
Intrazeolite anchoring of [HRuCo3(CO)9(μ2-CO)3]: Synthesis and reversible decarbonylation
Authors: Shen James G. C. , Ichikawa Masaru
Published: July 2002

The synthesis of intrazeolite anchoring of [HRuCo3(CO)9(μ2-CO)3] involves the adsorption of metal carbonyl species into zeolite cages followed by reductive carbonylation under a mixed CO and H2 atmosphere. The physicochemical characterization of the samples was based on a multianalytical approach, including Fourier transform-infrared, extended x-ray absorption fine structure spectroscopies, and CO/H2 gas chemisorption. The research encomassed several key points as follows: (a) The generation process of bimetallic carbonyl clusters was considered to occur through the interaction of [Ru–Co], Co(CO)4− and RuI carbonyls, and conversion into [HRuCo3(CO)9(μ3-CO)3] and further to [HRuCo3(CO)9(μ2-CO)3] isomer. (b) There is significant bimetallic interaction. (c) Internal and external confinement of [HRuCo3(CO)9(μ2-CO)3] were compared. (d) The intrazeolite anchoring of [RuCo3] carbonyl clusters shows a strong reaction with the extraframework Na+ α-cage cations, through involvement of the oxygen end of the bridging carbonyl ligands.
Ro-vibrational Stark effect on H2 and D2 molecules adsorbed in NaA zeolite
Authors: Bras N.
Published: July 2002

In order to explain the induced infrared bands of H2 and D2 adsorbed in NaA zeolites the Stark effect on the ro-vibrational levels of these molecules is considered for electric fields created by various charge distributions. The shift and intensity of the induced ro-vibration transitions are calculated.
A patching model for surface tension and the Tolman length
Authors: Bykov T. V. , Zeng X. C.
Published: July 2002

In the framework of density functional theory (DFT), a patching model for the density profile of the liquid–vapor interface is developed. The patching is based on analytical expressions of the asymptote of the density profiles. Derived from the model the surface tension of planar liquid–vapor interface as well as the Tolman length can be computed from analytic expressions. Two prototype systems are considered; the Yukawa and the Lennard-Jones. As a result, the temperature dependence of the surface tension as well as the Tolman length are obtained. The results are compared with numerical DFT calculations.
Computer simulations of liquid/vapor interface in Lennard-Jones fluids: Some questions and answers
Authors: Trokhymchuk Andrij , Alejandre José
Published: July 2002

Canonical molecular dynamics (MD) and Monte Carlo (MC) simulations for liquid/vapor equilibrium in truncated Lennard-Jones fluid have been carried out. Different results for coexistence properties (orthobaric densities, normal and tangential pressure profiles, and surface tension) have been reported in each method. These differences are attributed in literature to different set up conditions, e.g., size of simulation cell, number of particles, cut-off radius, time of simulations, etc., applied by different authors. In the present study we show that observed disagreement between simulation results is due to the fact that different authors inadvertently simulated different model fluids. The origin of the problem lies in details of truncation procedure used in simulation studies. Care has to be exercised in doing the comparison between both methods because in MC calculations one deals with the truncated potential, while in MD calculations one uses the truncated forces, i.e., derivative of the potential. The truncated force does not uniquely define the primordial potential. It results in MD and MC simulations being performed for different potential models. No differences in the coexistence properties obtained from MD and MC simulations are found when the same potential model is used. An additional force due to the discontinuity of the truncated potential at cut-off distance becomes crucial for inhomogeneous fluids and has to be included into the virial calculations in MC and MD, and into the computation of trajectories in MD simulations. The normal pressure profile for the truncated potential is constant through the interface and both vapor and liquid regions only when this contribution is taken into account, and ignoring it results in incorrect value of surface tension.
Interchain interactions in conjugated materials: The exciton model versus the supermolecular approach
Authors: Beljonne D. , Cornil J. , Silbey R. , Millié P. , Brédas J. L.
Published: July 2002

Correlated quantum-chemical techniques are applied to the description of electronic excitations in interacting conjugated chains. The focus is on the magnitude and conjugation-length dependence of the splitting of the lowest optically allowed excitonic state, which is induced by interchain interactions. We first examine cofacial dimers formed by linear polyene chains of various lengths and use two strategies to compute the exciton coupling energy. One is based on molecular exciton theory, which assumes that the excited-state wave functions of the isolated chains remain unperturbed by the intermolecular forces; in the other, the supermolecular approach, the wave functions are obtained from molecular orbital calculations performed for the whole system and are therefore not constrained to a single chain. We find that the two techniques lead to consistent results, provided an appropriate form for the interchain Coulomb interactions is adopted in the excitonic model. In particular, both formalisms indicate a peak behavior for the evolution of the exciton splitting energy with the length of the interacting conjugated chains. As an illustration, the chain-length dependence of the Davydov splitting is evaluated in the case of oligothiophenes on the basis of the experimental x-ray crystal structures; the results are compared to recent polarized absorption data.
Temperature-accelerated dynamics for simulation of infrequent events
Authors: So/rensen Mads R. , Voter Arthur F.
Published: July 2002

We present a method for accelerating dynamic simulations of activated processes in solids. By raising the temperature, but allowing only those events that should occur at the original temperature, the time scale of a simulation is extended by orders of magnitude compared to ordinary molecular dynamics, while preserving the correct dynamics at the original temperature. The main assumption behind the method is harmonic transition state theory. Importantly, the method does not require any prior knowledge about the transition mechanisms. As an example, the method is applied to a study of surface diffusion, where concerted processes play a key role. In the example, times of hours are achieved at a temperature of 150 K.