bluuebunny/crossref_metadata_2025 huggingface dataset:
The effects of internal constraints on the configurations of chain molecules
Authors: Chan Hue Sun , Dill Ken A.
Published: July 2002

We explore the three-dimensional configurations of chain molecules containing one or more self-contacts (constraints). We focus predominantly on the 1-, 2-, and 3-constraint ensembles. We take into account excluded volume by exhaustive computer enumeration of the conformational spaces of short chains on three-dimensional simple cubic lattices, and through use of the path integral approach of Edwards and Freed. We develop topological correlation functions to describe how the cyclization probability of one loop affects cyclization of another. There are two rather striking findings. (i) Considerable amounts of internal architecture (helices and antiparallel and parallel sheets) are predicted to arise in compact polymers due simply to steric restrictions. This appears to account for why there is so much internal organization in globular proteins. (ii) Several cyclization properties are remarkably ideal for chains which are relatively or highly compact in three dimensions. For example, in relatively compact molecules the correlation functions of loop pairs are well predicted by the random-flight model of Jacobson and Stockmayer; the number of configurations of maximally compact chains is predicted relatively well by the Flory theory of excluded volume, which is found to be better than the Huggins theory in three dimensions; and the probability of cyclization within globular chains is well predicted by the Bragg–Williams approximation.
On the determination of rate constants from equilibrium molecular dynamics simulations
Authors: Brown David , Clarke Julian H. R.
Published: July 2002

In this article we consider the calculation of rate constants from equilibrium molecular dynamics simulations using the conformational isomerization in liquid butane as an example. In particular we discuss the method by which transition state theory estmates of the rate constant may be calculated from simulation data. Care has to be taken when calculating the average velocity along the reaction coordinate at the barrier. If this is computed just from actual barrier crossings we show that there is a systematic overestimation by a factor of exactly π/2. Further analysis has allowed the connection between the barrier crossing rate and the transition state theory value for the rate constant to be established. We compare rate constants determined by the ‘‘reactive flux’’ method with values obtained using alternative approaches applied to liquid n-butane [D. Brown and J. H. R. Clarke, J. Chem. Phys. 92, 3062 (1990)]. The results of this comparison have implications regarding the identification of the plateau value when using the reactive flux method. A straightforward procedure is described which can establish confidence limits for rate constants calculated in this way.
The calculation of excited states with quantum Monte Carlo. II. Vibrational excited states
Authors: Bernu B. , Ceperley D. M. , Lester W. A.
Published: July 2002

Using a new Monte Carlo method for computing properties of excited quantum states, correlation function quantum Monte Carlo, we calculate the lowest ten vibrational excited state energies of H2O and H2CO in the Born–Oppenheimer approximation. The statistical errors for H2O are 0.1 cm−1 for the ground state and 15 cm−1 for the tenth excited state while for H2CO they are 2 cm−1 for the ground state and 30 cm−1 for the eighth excited state. The algorithm presented here is easily extensible to larger systems.
Endohedral complexes: Atoms and ions inside the C60 cage
Authors: Cioslowski Jerzy , Fleischmann Eugene D.
Published: July 2002

The results of ab initio electronic structure calculations on the C60 cage and its endohedral (‘‘inside-the-cage’’) complexes with F−, Ne, Na+, Mg2+, and Al3+ are presented. Placing the ions at the center of the cage results in a net stabilization and screening of the charges. The ionic guests either decrease (F−) or increase (Na+, Mg2+, and Al3+ ) the cage radii. The complexes with the ions at the cage center are local maxima with respect to the displacement of the guests. The C60⋅Ne complex, which is destabilized by ca. 0.4 kcal/mol relative to the separated components, is an energy minimum. In the C60⋅Na+ complex, the energy minimum (which lies only 0.8 kcal/mol below the maximum) corresponds to the Na atom displaced by 0.66 Å from the cage center. The calculated properties of the endohedral complexes are easily rationalized with a model involving a double-layer polarizable C60 cage affected by the electrostatic potential produced by the enclosed guests.
The microwave spectrum, structure, and tunneling motion of the sulfur dioxide dimer
Authors: Taleb-Bendiab Amine , Hillig Kurt W. , Kuczkowski Robert L.
Published: July 2002

The microwave spectrum of (SO2)2 has been reinvestigated using a pulsed beam Fourier-transform microwave spectrometer. Several new a-type transitions for the normal species and the a-type spectra of eight isotopically substituted species were measured. The spectra indicate that the SO2 dimer undergoes a high-barrier tunneling motion. Based on the analysis used for (H2O)2 by Coudert and Hougen [J. Mol. Spectrosc. 130, 86 (1988)], the internal motion is identified as a geared interconversion motion similar to that displayed by (H2O)2. From the analysis of the moments of inertia of the various isotopic species, an ac plane of symmetry is established for the dimer and the tilt angles of the C2 axes of each subunit relative to the line joining their centers of mass were determined. From Stark effect measurements, μa was redetermined and μc was shown to be nearly zero. Electrostatic calculations using distributed multipoles were carried out to explore the structure of this dimer.
Dynamics of reversible electron transfer reactions
Authors: Zhu Jianjun , Rasaiah Jayendran C.
Published: July 2002

The dynamics of reversible electron transfer reactions in Debye solvents are studied by employing two coupled diffusion–reaction equations with the rate constants depending on the reaction coordinate. The equations are solved analytically in four limiting cases: fast and slow reactions as well as wide and narrow reaction windows. A general solution for the survival probabilities is obtained by employing a decoupling approximation similar to the one used by Sumi and Marcus [J. Chem. Phys. 84, 4896 (1986)] for nonreversible reactions; our solution verifies the existence of four limiting cases and also predicts the behavior between these limits. Interpolation between long and short time approximations to the general solution, leads to survival probabilities with a single exponential time dependence and rate constants ki satisfying the relation k1/k2=exp(−βΔG0), where ΔG0 is the standard free energy change for the reaction. Multiexponential behavior of the survival probabilities is exhibited when higher order terms are included in the evaluation of the general solution, but this deteriorates to a single exponential, governed by a first order rate constant, at long times. In the narrow reaction window limit the multiexponential solution is exact when both the forward and reverse reactions are barrierless, and the behavior at long times is determined by a rate constant k=0.83 τ−1L where τL is the longitudinal relaxation time. Similar behavior is found when the forward reaction alone is barrierless and the barrier for the reverse reaction is large (βΔG*1=0, βΔG*2≫1), except that the forward rate constant k1≊τ−1L [0.6+(π/βΔG*2)1/2]−1 depends on the barrier height for the reverse reaction which has a small rate constant. Our solutions reduce to those of Sumi and Marcus when the reverse reaction is ignored. They are also compared with numerical solutions to the diffusion reaction equations. The extension to non-Debye solvents is briefly discussed.
X-ray grazing incidence diffraction from alkylsiloxane monolayers on silicon wafers
Authors: Tidswell I. M. , Rabedeau T. A. , Pershan P. S. , Kosowsky S. D. , Folkers J. P. , Whitesides George M.
Published: July 2002

X-ray reflection (both specular and off-specular) and grazing incidence diffraction (GID) have been used to study the structure of alkylsiloxane monolayers (n-C18H37SiO1.5) formed by self-assembly from solution on silicon wafers. GID studies of complete monolayers reveal a single ring of scattering associated with the monolayer. The Lorentzian line shape of this ring indicates that the film is characterized by liquidlike order, with a typical translational correlation length of about 45 Å. The thermal coefficient of expansion of the monolayer, as determined from the GID peak position, is approximately equal to the value for liquid n-alkanes. Upon either heating or cooling, the monolayer correlation lengths decrease, suggesting that the differential thermal-expansion coefficients of the film and substrate figure prominently in thermal changes of the molecular ordering. GID data for incomplete monolayers also reveal a single ring of scattering associated with the monolayer. While both the translational correlation lengths and integrated peak areas are significantly reduced relative to complete monolayers, the peak positions of the incomplete monolayers are comparable to those of complete monolayers. Given the lower average areal density of incomplete monolayers, this finding implies that incomplete monolayers are inhomogeneous.
The crystallization of alkali halides from aqueous solution: An application of density-functional theory
Authors: Ursenbach C. P. , Patey G. N.
Published: July 2002

Density-functional theory is applied to the problem of salt crystallization from solution and explicit results are given for model aqueous alkali-halide systems. Both direct- and Fourier-space methods of calculation are considered and it is found that only the direct (i.e., r space) method converges sufficiently rapidly to provide reliable results for ionic crystals at 25 °C. It is shown that the density-functional method is capable of predicting crystallization, but that the solid-state parameters and, for some salts, the crystal structures obtained are in poor agreement with experiment or computer simulations. The calculated crystal/solution coexistence concentrations are found to be extremely sensitive to the short-range part of the interionic pair potentials. This is consistent with earlier observations that the activity coefficients of model aqueous alkali-halide solutions are very strongly dependent upon the short-range ion–ion interactions. Therefore, we do not believe that this sensitivity to details of the short-range interionic potentials is an artifact of theoretical approximations, but rather a real effect significantly influencing crystallization.
Wide amplitude motion in the water–carbon dioxide and water–acetylene complexes
Authors: Block P. A. , Marshall Mark D. , Pedersen L. G. , Miller R. E.
Published: July 2002

The optothermal detection method has been used to obtain near-infrared spectra of the H2O–CO2 and H2O –HCCH van der Waals molecules in the 3 μm region. For H2O–CO2, the observed vibrational band correlates with the asymmetric O–H stretching vibration of the water monomer and gives a direct measurement of the internal rotation tunneling splitting, from which the height of the associated barrier is estimated. This experimental estimate is compared with results obtained from both a distributed multipole analysis and an ab initio calculation. In the case of H2O –HCCH, three different vibrational bands are observed. Although these spectra are not directly sensitive to the wide amplitude motion in this molecule, they are consistent with the quasiplanar picture suggested by the ab initio calculations. These calculations show the molecule to have a bent equilibrium structure, with a small barrier located at the planar geometry.
Towards elucidating the interplay of structure and dynamics in clusters: Small KCl clusters as models
Authors: Rose John P. , Berry R. Stephen
Published: July 2002

An investigation of melting, freezing, and coexistence phenomena in binary clusters is presented with the salt clusters (KCl)4 and (KCl)5 as specific examples. The results of molecular dynamics simulations are combined with analyses of the potential surfaces, notably the energies and geometries at the minima and saddle points. On the basis of this knowledge, the molecular dynamics results are interpreted in terms of melting, freezing, and phase coexistence, and the ways a nonrigid system may explore its potential surface. A comparison is made of isomerization rates derived from molecular dynamics and Rice–Ramsberger–Kassel–Marcus (RRKM) theory.
Rates of convergence of the partial-wave expansions of atomic correlation energies
Authors: Kutzelnigg Werner , Morgan John D.
Published: July 2002

The coefficients of the leading terms of the partial-wave expansion of atomic correlation energies in powers of (l+1/2)−1 are derived for the second- and third-order perturbed energies in the 1/Z expansion for all possible states of two-electron atoms, and for second-order Mo/ller–Plesset (many-body perturbation) theory for arbitrary n-electron atoms. The expressions for these coefficients given in Table I involve simple integrals over the zeroth-order wave functions (for the third order energies first-order wave functions are also involved). The leading term of E(2) goes as (l+1/2)−4 for natural parity singlet states, as (l+1/2)−6 for triplet states, and as (l+1/2)−8 for unnatural parity singlet states. There are no odd powers of (l+1/2)−1 present in E(2), and the coefficient of the (l+1/2)−6 term for natural parity singlet states of two-electron systems in the 1/Z expansion is generally −5/4 times the coefficient of the (l+1/2)−4 term. In E(3) there are terms that go as odd powers of (l+1/2)−1; the leading term is expressible in terms of the zeroth-order wave function and goes as (l+1/2)−5 for natural parity singlet states, as (l+1/2)−7 for triplet states, and as (l+1/2)−9 for unnatural parity singlet states. Numerical values of the coefficients for E(2) in two-electron atoms are given in Table II. The relation of our results to those obtained by Hill for variational calculations on the He ground state is discussed and generalizations of Hill’s formulas are conjectured. The unexpected results for unnatural parity singlet states are traced back to the behavior of their wave functions at the point of coalescence of two electrons. In terms of the relative coordinate of the two electrons they represent d waves, in contrast to s waves for natural parity singlet states and p waves for triplet states of either parity.
On the molecular theory of diffusion, heat conduction, and momentum transfer in multicomponent solutions
Authors: Kim Won-Tae , Altenberger Andrzej R. , Dahler John S.
Published: July 2002

A projection operator procedure is used to analyze the particle currents and the fluxes of momentum and energy in a multicomponent classical fluid. The projection is onto the fluctuations from equilibrium of the temperature, of the concentrations of all species present in the solution, and of the total density of momentum. Each of the flow rates is resolved into a sum of convective, dissipative, and random parts. Correlation function expressions are derived for the wave vector and frequency-dependent Onsager kinetic coefficients appropriate to the laboratory frame of reference and to several flow reference frames as well. Special attention is devoted to the frame associated with the mass-averaged, barycentric velocity and connections are considered between the results of the present investigation and those obtained in an earlier, closely related study which used a projection onto a subspace of that used here.
Dissociative charge-transfer reactions of Ar+ with fluoromethanes at thermal energy
Authors: Tsuji Masaharu , Funatsu Tsuyoshi , Kouno Hiroyuki , Nishimura Yukio
Published: July 2002

A flowing-afterglow apparatus coupled with a low pressure chamber has been used to measure product ion distributions and rate constants in the charge-transfer reactions of Ar+ with CHnF4−n (n=1–3) at thermal energy. Near-resonant dissociative charge transfer followed by loss of H and/or F are major product channels observed. In the Ar+/CH3F reaction, CHF+ and CH2+ ions, which are either absent or very weak in He(i) photoionization, are also found as minor products. The product ion distributions suggest that the Ar+/CHnF4−n (n=1–3) reactions proceed dominantly through near-resonant charge transfer. The total rate constants are 1.7±0.6, 1.9±0.7, and 2.0±0.6×10−9 cm3 s−1 for CH3F, CH2F2, and CHF3, respectively. These values are in reasonable agreement with those predicted from the average dipole orientation (ADO) theory, being independent of the existence of ionic states with favorable Franck–Condon factors for ionization.
Ab initio study of geometrically metastable multiprotonated species: MHk+n
Authors: Boldyrev Alexander I. , Simons Jack
Published: July 2002

The geometries and stabilities relative to fragmentation of H4O2+, H4S2+, H3F2+, H3Cl2+, H2Ne2+, H2Ar2+; H4F3+, H4Cl3+, H3Ne3+, H3Ar3+; H4Ne4+, and H4Ar4+ have been studied using the quadratic configuration interaction (with single and double excitations plus approximate triple excitations included) QCISD(T)/6–311G(2df,2p) method at second-order Mo/ller–Plesset optimized geometries MP2(full)/TZP+ZPE/6–31G**. All of the triply charged H4F3+, H4Cl3+, H3Ne3+, and H3Ar3+ and more highly charged H4Ne4+ and H4Ar4+ ions, as well as doubly charged H2Ne2+ were not found to possess local minima on their zero-point corrected ground state surfaces, although H4Cl3+ is only slightly unstable. Tetrahedral (Td) structures for H4O2+ and H4S2+, planar triangular (D3h) H3F2+, triangular pyramidal (C3v) H3Cl2+ and bent (C2v) H2Ar2+ were found to be local minima on the respective ground state surfaces. The latter five species lie above their respective ion-plus-ion dissociation products by 61, 91, 111, 67, and 116 kcal/mol, and have barriers to dissociation of 38, 20, 12, 34, and 5 kcal/mol (all energies being zero-point corrected). Such multiply charged cations store a great deal of energy, which may be released by the addition of a single extra electron to form the corresponding cation of one less charge.
The random spheres model as a representation of a random solid: A study using a one-dimensional system of penetrable rods
Authors: Sturgeon Kathy S. , Reiss Howard , Talbot Julian
Published: July 2002

Many real porous solids and possibly composite complex liquid systems such as microemulsions can be modeled as a random porous medium with given void fraction and specific interface area, thereby providing a means for estimation of many physical properties of the real systems. Another, even simpler model consists of a random array of mutually penetrable spheres [the random spheres model (RSM)], the void fraction and specific interface of which may be chosen. We have augmented this model to include penetrable spheres having a ‘‘random’’ distribution of sizes. If the RSM and the random porous solid model exhibited similar behavior, the RSM could then be applied in studies of real porous solids and microemulsions, specifically in computation of the ‘‘entropy of mixing’’ of oil and water domains in a microemulsion in the continuum rather than on a lattice. Recently it has been demonstrated that the entropy of mixing on a lattice may be appreciably less than the more accurate corresponding quantity in the continuum. We compare the RSM and the random solid model by means of their respective void–void correlation functions in one dimension where all results are exact.
Gradient extremals and steepest descent lines on potential energy surfaces
Authors: Sun Jun-Qiang , Ruedenberg Klaus
Published: July 2002

Relationships between steepest descent lines and gradient extremals on potential surfaces are elucidated. It is shown that gradient extremals are the curves which connect those points where the steepest descent lines have zero curvature. This condition gives rise to a direct method for the global determination of gradient extremals which is illustrated on the Müller–Brown surface. Furthermore, explicit expressions are obtained for the derivatives of the steepest-descent-line curvatures and, from them, for the gradient extremal tangents. With the help of these formulas, a new gradient extremal following algorithm is formulated.
On analytic potential functions for reactions on metal surfaces: The case of H2→2H on the liquid mercury surface
Authors: Sellers Harrell
Published: July 2002

The energy surface for the interaction between atomic hydrogen and the close packed Hg surface was computed at the relativistic effective core potential (RECP) Hartree–Fock+correlation (second order perturbation theory-MBPT2) level of theory and fitted to two modified forms of the Morse potential, which are designed to describe interactions between adsorbates and surfaces. The ab initio dissociation potential for H2 on the close packed Hg surface was computed and fit to an energy expression of the bond order conservation-Morse potential (BOC-MP) type. The BOC-MP-type energy expression was found to give an excellent representation of this surface molecular dissociation potential. The bond order conservation constraint introduces into the energy expression 2, 3, and 4 body terms that describe bond order correlation during the reaction.
Angular momentum in unimolecular reactions
Authors: Klots Cornelius E.
Published: July 2002

Several formulations may be noted for the role of angular momentum in unimolecular reactions. We use here a generalization of the Rice–Gershinowitz model to exemplify their inter-relations. The equations which emerge are simple and equally applicable to loose and tight transition states. Their predictions are compared with experiment and with more tedious calculations.
Raman linewidths and rotationally inelastic collision rates in nitrogen
Authors: Green Sheldon
Published: July 2002

Using the N2–N2 interaction potential of van der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)] rotationally inelastic collision cross sections have been computed within the infinite order sudden (IOS) approximation, assuming the molecules are distinguishable. Methods for enforcing detailed balance and correcting for effects of inelasticity, which are ignored in the IOS approximation, are considered, including the energy corrected sudden (ECS) method. Suitably averaged cross sections are compared with experimental Raman Q-branch linewidths for temperatures from 295 to 1500 K and with recently measured room temperature state-to-state rates. Agreement is rather good, especially if ECS corrections are applied.
High precision atomic computations from finite element techniques: Second-order correlation energies of rare gas atoms
Authors: Flores Jesús R.
Published: July 2002

The p-version finite element method for atomic computations [J. Chem. Phys. 91, 7030 (1989)] has been implemented within the frame of second-order Mo/ller–Plesset theory and used to calculate correlation energies of the rare-gas atoms from He to Rn. The calculation proceeds through a sequence of levels of computation that is systematic and hierarchic in nature and helps to estimate the error of the final values. It is possible to include virtual orbitals of very high angular momentum (lmax=12) even for the heaviest elements; so very accurate results can be obtained. Comparison with the most accurate values found in the literature indicates that our FEM-MP2 method competes very well with any other method, even with Kutzelnigg’s MP2-R12 method [J. Chem. Phys. 94, 2002 (1991)], especially in the case of heavy atoms. The results presented here could be the most accurate published so far.
Nonlocal electrodynamic modeling of frequency shifts for molecules at rough surfaces
Authors: Leung P. T. , Hider M. H.
Published: July 2002

The problem of frequency shifts for admolecules at distances very close to a metallic substrate surface protrusion is considered. The molecule is modeled as an oscillating point dipole and the morphology of the substrate is represented by a spherical island. Nonlocal electrodynamic effects are taken into account within the hydrodynamic description of the surface electrons. The results show that the nonlocal effects will tend to suppress the frequency shifts from local theory, and to create blue-shifted higher-order dispersion resonances in the frequency shift spectrum. Possible experimental observation for such effects is discussed.
Polarization-resolved (2+1) resonance-enhanced multiphoton ionization spectroscopy of CF3I (6s) Rydberg states
Authors: Taatjes Craig A. , Mastenbroek Johanna W. G. , van den Hoek Ger , Snijders Jaap G. , Stolte Steven
Published: July 2002

The CF3I(5pπ–6s) Rydberg transitions in the energy range 56 700–64 000 cm−1 are investigated using (2+1) resonance-enhanced multiphoton ionization. The polarization of the two-photon transitions is used to definitely assign the symmetries of the resonant intermediate states. The four allowed electronic transitions in the (5pπ–6s) manifold have been assigned and some vibrational constants in the excited states have been determined. Hot band spectra have been obtained in a supersonic expansion of CF3I through an oven. The upper spin–orbit components (the 2E1/2 ion core states) are perturbed by a dissociative state at approximately 63 000 cm−1, possibly the σ–σ* transition centered on the C–I bond. Density functional calculations have been performed in order to help determine the nature of the perturbing states. Vibronic interactions in the excited states are investigated, and evidence is seen for quadratic Jahn–Teller interactions for ν6 in the lower (2E3/2) spin–orbit state.
The rotational spectrum, structure and dynamics of a benzene dimer
Authors: Arunan E. , Gutowsky H. S.
Published: July 2002

The low J (2 to 7) rotational spectrum of a symmetric-top benzene dimer has been obtained with a Balle/Flygare Fourier transform microwave spectrometer. Each transition is a symmetrical quartet with two J- and K-dependent tunneling splittings of 30 to 400 kHz. Rotational constants B̄, DJ, and DJK were determined to be 427.76(2) MHz, 7.2(3) kHz, and 0.869(5) MHz. The dimer is T-shaped with a benzene c.m. to c.m. distance of 4.96 Å.
Resolution of three optically active components of the transition dipole moment for 249 nm photodissociation of ICN
Authors: Black J. F.
Published: July 2002

Cyanogen iodide (ICN) has been photodissociated at 249 nm. The CN(X 2Σ+) photofragments were probed by laser induced fluorescence (LIF) using multiple distinct excitation–detection geometries, allowing the extraction of bipolar moments describing the distribution and mutual correlations of the fragment velocity, angular momentum, and dipole moment vectors. The results of this analysis are consistent with three optically active transitions at this photolysis wavelength, one with parallel character and two with perpendicular character. One of these perpendicular transitions has the direction of the transition dipole moment perpendicular to the plane of the three atoms, whereas the other has the transition moment in the plane of the three atoms, perpendicular to the I–C bond. This picture can be reconciled with recent ab initio calculations of the level structure of the ICN Ã–X̃ continuum and can also be used as a framework to explain in a consistent manner many of the gross features of the photodissociation dynamics of ICN observed in previous experiments at this wavelength.
Time-resolved kinetic studies on quenching of CH(A 2Δ and B 2Σ−) by (CX3)2CO, CF3COOX, and CXCl3 (X=H or D) and alkane molecules
Authors: Chen Congxiang , Ran Qin , Yu Shuqin , Ma Xingxiao
Published: July 2002

The quenching rate constants kq of CH(A 2Δ and B 2Σ−) radicals by (CX3)2CO, CF3COOX, and CXCl3 (X=H or D), and some alkane molecules have been measured using laser photolysis of CHBr3 at 266 nm to produce CH radicals and time-resolved fluorescence quenching measurements. An isotope effect is found on quenching of both A and B states by deuterated and hydride molecules and the quenching rate constants of both A and B states increase steadily with increase of the number of C–H bonds contained in the alkane molecules. The formation cross sections of complexes of electronically excited CH radicals and alkane molecules were calculated by means of a collision complex model. It is shown that the dependence of the formation cross section of complex on the number of C–H bonds contained in alkane molecules is in agreement with that of the measured quenching cross section.
Dynamic density functional theory for microphase separation kinetics of block copolymer melts
Authors: Fraaije J. G. E. M.
Published: July 2002

In this paper, we describe a numerical method for the calculation of collective diffusion relaxation mechanisms in quenched block copolymer melts. The method entails the repeated calculation of two opposing fields—an external potential field U, conjugate to the density field ρ, and an energetic interaction field E. The external field is calculated by numerical inversion of the density functionals and the energetic interaction field is calculated directly by integration over the density field. When the two fields are balanced U=E, we recover the self-consistent field solutions; when the two fields are off balance, the spatial gradient of E–U is the thermodynamic force which drives the collective diffusion. We introduce a simple local coupling approximation for the Onsager kinetic coefficients of short freely jointed chains in weakly ordered systems. Fluctuations are added by incorporation of a random Langevin force in the diffusion equation. Numerical results of decomposition in symmetric and asymmetric diblock copolymer melts indicate that the method is capable of describing extremely slow defect annihilation relaxation modes. We find that in the nonlinear regime, the density patterns evolve to metastable states, in which isolated defects separate relatively well-ordered crystalline microdomains. These final states are typical for many industrial applications of incompletely relaxed copolymer melts.
Analysis of the Henderson–Abraham–Barker equation in the case of a polar liquid near a neutral hard wall
Authors: Badiali J. P. , Russier V. , Holovko M. E.
Published: July 2002

The physical content of the wall–particle direct correlation function cw(1), defined from the Ornstein–Zernike equation in the framework of the Henderson–Abraham–Barker approach, is analyzed in the case of a dipolar-hard-sphere fluid near a pure and dielectric neutral wall. The exact asymptotic behavior of cw(1) is established and we show that it is not related to simple physical concepts as, e.g., the image potential. We show that the exact Henderson–Abraham–Barker equation introduces some bridge diagrams which are more simple graphs in another approach. Due to this fact, at least for systems with long range interactions, it is misleading to use the usual closures of the theory of homogeneous liquids for cw(1). In the case of a dielectric wall, we emphasize that the diagrammatic structure of cw(1) requires the introduction of a three-body Mayer function. The dipolar-hard-sphere liquid is a good candidate for analyzing cw(1) because some exact results are known and related to simple electrostatic effects; however, the present results are not restricted to this system.
A bond order conservation-Morse potential model of adsorbate–surface interactions: Dissociation of H2, O2, and F2 on the liquid mercury surface
Authors: Sellers Harrell
Published: July 2002

We present a bond order conservation-Morse potential (BOC-MP) model of adsorbate–surface interactions, having only two adjustable parameters, which is based on the BOC-MP model of Shustorovich. We test this model energy expression by determining how well it can represent the dissociation of H2, O2, and F2 on the close-packed mercury surface. This is done by adjusting the two ‘‘free’’ parameters of our BOC-MP energy expression so that it fits the high quality ab initio surface dissociation energy curves that we have computed for these diatomics. Our BOC-MP energy expression is able to reproduce the ab initio surface dissociation energies quite well.
Accurate ab initio quartic force fields for the ions HCO+ and HOC+
Authors: Martin J. M. L. , Taylor Peter R. , Lee Timothy J.
Published: July 2002

The quartic force fields of HCO+ and HOC+ have been computed using augmented coupled cluster methods and basis sets of spdf and spdfg quality. Calculations on HCN, CO, and N2 have been performed to assist in calibrating the computed results. Going from an spdf to an spdfg basis shortens triple bonds by about 0.004 Å, and increases the corresponding harmonic frequency by 10–20 cm−1, leaving bond distances about 0.003 Å too long and triple bond stretching frequencies about 5 cm−1 too low. Accurate estimates for the bond distances, fundamental frequencies, and thermochemical quantities are given. HOC+ lies 37.8±0.5 kcal/mol (0 K) above HCO+; the classical barrier height for proton exchange is 76.7±1.0 kcal/mol.
Electron correlation contributions to one-electron properties from functionals of the Hartree–Fock electron density
Authors: Cioslowski Jerzy , Nanayakkara Asiri
Published: July 2002

Equations for the electron correlation contributions to one-electron properties calculated with correlation energy functionals of the Hartree–Fock density are derived. For any functional that is of an explicitly algebraic form, these contributions are limited to the occupied-virtual block of the one-electron density matrix. This deficiency results in very poor values of one-electron properties, as demonstrated by numerical examples. Implications of these observations to the future of the density functional theory (DFT)-based electronic structure methods are discussed.
Analytical dependence of the viscosity cross sections and viscosity coefficients on parameters of intermolecular potentials
Authors: Kunc Joseph A.
Published: July 2002

General analytical expressions for viscosity cross sections and viscosity coefficients of gases as functions of the parameters of intermolecular potentials are derived from the first principle using the classical scattering theory. Two types (one repulsive–attractive and one purely repulsive) of the central-force potentials, suitable to represent molecule–molecule, atom–molecule and atom–atom interactions, are considered. The expressions for viscosity show good agreement with results of numerical calculations in high-temperature single-component gases (above 2000 K) where viscosity measurements are not available.
Spatially modulated phases in monolayers of mixed surfactants
Authors: Wang Zhen-Gang
Published: July 2002

We present a theoretical study of spatially modulated phases in self-assembled monolayers of mixed surfactants. We propose two models which are appropriate, respectively, for monolayers at a fluid–fluid interface and monolayers on a solid substrate. We show that in both cases, the molecular shape asymmetry, coupled with the local composition variation, can lead to spontaneous formation of periodic structures. In the case of liquid-supported monolayers, the molecular shape asymmetry is manifested as a spontaneous curvature of each component of the film, which induces periodic variations both in the composition of the amphiphiles and in the height profile of the interface (ripples). In the case of solid-supported monolayers, the shape asymmetry is reflected in the spontaneous splay of the orientation of the amphiphiles, and the spatial modulation involves the composition as well as the orientation of the amphiphilic molecules. We analyze these models in some detail near the critical region, where we highlight the roles played by various length scales in determining the critical wavelength. We show that gravity has some very subtle and nontrivial effects for a liquid-supported, tension-free monolayer. We also present some preliminary results for the low temperature cases.
The application of some spherical reference theories to the hard spheroid fluid
Authors: Masters A. J. , Whittle M.
Published: July 2002

We apply three different spherical reference theories to the isotropic phase of the hard spheroid fluid. Of these three, the theory due to Barker and Henderson [J. Chem. Phys. 47, 4714 (1967)] gives the best equation of state. It would not seem, however, to predict an isotropic–nematic phase transition.
Magnetic circular dichroism of Na3Eu(ODA)3⋅2NaClO4⋅6H2O
Authors: Görller-Walrand C. , Verhoeven P. , D’Olieslager J. , Fluyt L. , Binnemans K.
Published: July 2002

Magnetic circular dichroism (MCD), circular dichroism (CD), and axial absorption spectra of Na3Eu(ODA)3⋅2NaClO4⋅6H2O are recorded at 4.2 and 150 K between 4500 and 25 500 cm−1. MCD parameters, rotatory strengths, and intensities are calculated using a parametric model for 4f→ 4f transitions. It is shown that the sign and magnitudes of the MCD parameters are very sensitive to the sign and absolute values of the intensity parameters.
Order–disorder transitions in finite biopolymers: A sequence generating function approach
Authors: Dewey T. Gregory
Published: July 2002

While there are a number of techniques for evaluating partition functions describing helix–coil transitions in infinite biopolymers, few exact techniques exist for the finite polymer problem. In this work, the sequence generating function formalism is extended to allow the treatment of finite length biopolymers. This is achieved by representing the partition function generating function as an expansion that uses a descending series rather than an ascending series. In this formalism the partition function is directly determined by a contour integral that can be evaluated exactly by the calculus of residues. This exact evaluation has advantages over previously used steepest descent methods as it allows finite biopolymers to be treated. With this technique, order–disorder transitions in a nucleic acid double helix model are reexamined. The problem of the perfectly matched double helix is treated exactly. The finite case of this model requires that the roots of a trinomial equation be obtained. This equation is solved and it is shown that each root differs only by a phase factor. Thus, the roots are distributed about a circle in the complex plane. Yang–Lee theory is used to explore conditions under which first-order phase transitions will occur in these systems.
Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr
Authors: Schäfer Ansgar , Huber Christian , Ahlrichs Reinhart
Published: July 2002

Contracted Gaussian basis sets of triple zeta valence (TZV) quality are presented for Li to Kr. The TZV bases are characterized by typically including a single contraction to describe inner shells and three basis functions for valence shells. All parameters—orbital exponents and contraction coefficients—have been determined by minimization of atomic self-consistent field ground state energies. Advantages and necessary modifications of TZV basis sets are discussed for simple test calculations of molecular energies and nuclear magnetic resonance (NMR) chemical shieldings in treatments with and without inclusion of electron correlation.
Bridge function and cavity correlation function for the soft sphere fluid from simulation: Implications on closure relations
Authors: Llano-Restrepo Mario , Chapman Walter G.
Published: July 2002

The soft sphere fluid is of interest as a possible reference fluid since, like the hard sphere fluid, the configurational properties and distribution functions scale with a single parameter. In this paper we present the results of a Monte Carlo simulation study of the cavity correlation function y(r) for the soft sphere fluid. Using the Ornstein–Zernike relationship, the direct correlation function c(r) is determined from simulations of the total correlation function h(r). The bridge function B(r) is calculated by difference. We provide a correlation of the bridge function and demonstrate the usefulness of this reference fluid by calculating some properties of the Lennard-Jones fluid using reference hypernetted chain (HNC) and Rosenfeld and Blum’s prescription for the bridge function state point. The soft sphere bridge function is also compared with the bridge functions for the hard sphere and Lennard-Jones fluids. Finally, it is demonstrated that closures similar to the Percus–Yevick (PY) closure are poor at short range and should only be valid for repulsive fluids; observations are made concerning modifications of the PY closure for repulsive and attractive fluids.
The geometric phase in two electronic level systems
Authors: Koizumi Hiroyasu , Sugano Satoru
Published: July 2002

The effects of the geometric phase on vibronic states associated with the lower potential surface of two electronic level Hamiltonians are examined. We obtain the general formula for the gauge potential arising from the vibronic interaction. It is shown that this gauge potential is split into a topological part and a magnetic part, where the topological part gives rise to the phase factor of +1 or −1 when it is integrated along a closed trajectory in the nuclear coordinate space, and the magnetic part gives rise to a contribution depending on the local character of the trajectory and exists only in systems without time-reversal symmetry. For particular examples, we consider the E⊗e and E⊗(b1+b2) Jahn–Teller systems with strong vibronic interactions. It is demonstrated that the ground states have vibronic standing wave states whose nuclear probability density distributions are localized in one of the equivalent minima on the lower potential surface. We also consider Zeeman splittings of degenerate vibronic states, where the degeneracy arises from time-reversal symmetry.
The chemical potential in atomically inhomogeneous fluids in external force fields by computer simulation
Authors: Powles J. G. , Baker S. E. , Evans W. A. B.
Published: July 2002

The accuracy and practicality of the Widom fictitious-particle insertion method for determining the chemical potential is tested for very inhomogeneous fluids in strong external potentials by a series of molecular dynamics computer simulations. The chemical potential determined in this way is found, as predicted, on an atomic scale to be independent of position in the fluid to a few percent even when the density varies by up to a factor 80. This severe test means that this method of determining chemical potential is established as reliable and accurate in any well-found computer simulation even when the system is very inhomogeneous.
Surface-induced ordering in block copolymer melts
Authors: Brown Gregory , Chakrabarti Amitabha
Published: July 2002

Surface-induced ordering in block copolymer melts is studied numerically. For symmetric copolymers, the thickness of the surface-enrichment layer is found to scale as Req∼Nθ with θ≊0.6, suggesting the system is undergoing a surface-induced strong segregation. The density profile perpendicular to the interacting surface is described quite well by the form predicted by Fredrickson in a mean-field analysis. In asymmetric copolymers, the surface is found to have a profound effect on domain formation. For some off-critical compositions, domains were found to form near the surface with a geometry different from that in the bulk; while for stronger asymmetry in composition, minority domains were nucleated near the wall only, long before any formed in the bulk. These interesting pattern formation processes should be observable in experiments using a depth profiling technique.
Molecular dynamics simulation of crystalline poly(ethylene oxide)
Authors: Neyertz Sylvie , Brown David , Thomas John O.
Published: July 2002

Molecular dynamics (MD) simulation holds great promise as a source of otherwise elusive information concerning ionic conduction mechanisms occurring in the amorphous phases of polymer electrolytes. However, most polymer/salt complexes have a multiphase character at temperatures of interest. Insights into crystalline phases may thus prove meaningful in the subsequent design of strategies to decrease the degree of crystallinity in these systems. We report here the full details of a molecular dynamics model (‘‘md’’ model) for the crystalline phase of the widely used host-polymer poly(ethylene oxide) (PEO). Force-field and computational parameters are optimized to give realistic behavior for crystalline PEO. Analyses of the structure and dynamics obtained from the MD simulations performed at 300 K include mean-square displacements, x-ray powder diffractograms, distributions of bond and torsion angles, and radial distribution functions. These are compared with experimental data, the static x-ray determined PEO structure and that obtained using a ‘‘tethered’’ model, in which the atoms are attached by springs to their initial crystalline positions, and allowed to move according to their experimental mean-square displacements. Agreement is good, but the reliability of the x-ray refined positions is questioned.
Observation of CH A→X, CN B→X, and NH A→X emissions in gas-phase collisions of fast O(3P) atoms with hydrazines
Authors: Orient Otto J. , Chutjian Ara , Murad Edmond
Published: July 2002

Optical emissions in single-collision reactions of fast (20 eV laboratory translational energy) O(3P) atoms with hydrazine, methylhydrazine, and 1,1-dimethylhydrazine have been measured in a crossed-beams geometry. The emissions were observed in the wavelength range 325–440 nm, and were identified as the CH (A 2Δ→X 2Πr) (for methylhydrazine), CN (B 2Σ+→X 2Σ+) (for methylhydrazine), and NH (A 3Π→X 3Σ−) transitions (for all three hydrazines). The experimental vibration-rotation bands were fit to a synthetic spectrum of CH, CN, and NH with given vibrational and rotational temperatures.
The CO (A–X) electric dipole transition moment at large internuclear distances
Authors: Wallaart Hidde L. , Perrin Marie-Yvonne , Martin Jean-Pierre
Published: July 2002

Emissions from the CO(A 1∏–X 1∑+) fourth positive band system have been observed in vibrationally excited CO. The evolution of the (A–X) transition moment as a function of average internuclear distance has been investigated. It was found to have a constant value in the range 1.4 Å≤r̄v′,v″≤1.5 Å.
B KVV and Cl LVV normal and resonance Auger electron spectra in gaseous BCl3 molecules
Authors: Ueda K. , Chiba H. , Sato Y. , Hayaishi T. , Shigemasa E. , Yagishita A.
Published: July 2002

The B KVV and Cl LVV normal and resonance Auger electron spectra in gaseous BCl3 have been observed with use of tunable photon beam from synchrotron radiation. The spectrum excited by photons with energy above the B K and the Cl L2,3 thresholds shows preferentially Cl LVV normal Auger bands. The pure B KVV normal Auger spectrum is obtained using the photons with energy between the B K threshold and the Cl L pre-edge excitation threshold. The spectator and participant Auger processes are observed at the B 1s→2a2″(2pπB) and 1s→4e′(σBCl*) resonances. The spectator Auger processes are also observed in the Cl L2,3 excitation region, while the participant Auger processes are not significant in this region. The observed spectra are discussed with the aid of the one-electron properties of the valence orbitals.