bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct perturbation theory of relativistic effects for explicitly correlated wave functions: The He isoelectronic series
Authors: Ottschofski Edgar , Kutzelnigg Werner
Published: July 2002

Direct perturbation theory (DPT) of relativistic effects is formulated for two-electron states described by a Dirac-Coulomb or a Dirac-Gaunt Hamiltonian. The relativistic wave function, a 16-component spinor, is—after a change of the metric—expanded in powers of c−2. An expression for the leading relativistic correction E2 to the energy is derived, that reduces to the Breit-Pauli form if the nonrelativistic problem is solved exactly; otherwise a correction term appears. The method is applied in two ways to the ground state of He-like ions including electron correlation. In the first way, via a conventional configuration interaction (partial wave expansion) in a Slater-type orbital (STO) basis, the nonrelativistic partial wave increments to the energy go as (l+1/2)−4, and those of the leading relativistic correction as (l+1/2)−2. Knowing the exact analytic behaviour of the leading terms in the partial wave expansions, an extrapolation to l→∞ is possible. More accurate results, with a rather rapidly converging partial wave expansion, are obtained in the second way, where the nonrelativistic wave function contains terms linear in the interelectronic coordinate r12 (R12-method). For the He ground state both the nonrelativistic energy and the relativistic correction are obtained with an error of a few nanohartrees (nEh). The importance of various contributions to the energy for different values of the atomic charge Z is discussed.
Third virial coefficients for saturated square well particles
Authors: Speedy Robin J.
Published: July 2002

Saturated square well (SSW) particles provide a model for atoms and molecules with valency in which the interparticle potential is not pairwise additive. The many-body constraints in the potential make it easier to calculate the third virial coefficient for the SSW models than for the simpler, pairwise additive, square well model. Fluid pressures predicted by the truncated virial equation are compared with simulation results to determine the density and temperature ranges over which the truncated series is accurate.
Comparison between molecular geometry and harmonic vibrational frequency predictions from CISD[TQ] and CISDTQ wave functions for hydrogen sulfide
Authors: Hoffman Brian C. , Sherrill C. David , Schaefer Henry F.
Published: July 2002

The selection of which configurations to include in a configuration interaction (CI) wave function is a compromise between accuracy and computational difficulty. A compact and accurate configuration interaction wave function can be constructed by inclusion of all single and double excitations and certain triple and quadruple excitations chosen in an a priori manner according to how many electrons are placed in several subsets of orbitals. Such a wave function, denoted CISD[TQ], has previously been shown to recover a large fraction of the energy of a CI wave function including all single, double, triple, and quadruple excitations (CISDTQ). A comparison of the molecular geometry and harmonic vibrational frequencies of hydrogen sulfide (H2S) predicted by two CISD[TQ] wave functions and the complete CISDTQ wave function are presented. With the largest basis set used, a triple-ζ plus double polarization basis with an additional set of d-type functions added to hydrogen, and an additional set of f-type functions added to sulfur [TZ2P(f,d)], the CISD[TQ] predictions differ from the CISDTQ by 0.0003 Å in the bond length and by 0.02° in the bond angle. The CISD[TQ] harmonic vibrational frequencies differ by less than 2 cm−1 from the full CISDTQ predictions. These results suggest that the CISD[TQ] wave function is an efficient and accurate truncation of the complete CISDTQ and are particularly impressive considering that with a TZ2P(f,d) basis, the larger CISD[TQ] wave functions included roughly 300 000 configurations while the CISDTQ includes almost nine million.
Fast evaluation of the Coulomb energy for electron densities
Authors: Pérez-Jordá José M. , Yang Weitao
Published: July 2002

The evaluation of the Coulomb interaction of the electron density with itself dominates the cost of a density-functional theory calculation, due to its quadratic scaling with the size of the system. A similar problem is found in simulations of systems of particles, where the cost is dominated by the particle–particle interactions. Recently, we have presented a simple method for the particle–particle problem [J. M. Pérez-Jordá and W. Yang, Chem. Phys. Lett. 247, 484 (1995)]. In this paper, our scheme is generalized to densities, in particular for calculations with Gaussian basis functions. Near linear scaling is observed for molecules with about 400 first-row atoms. CPU time savings of up to one order of magnitude are observed for these molecules. The method distinguishes between localized and diffuse distributions in a much simpler way than in other proposed approaches.
Stochastic theories of the activated complex and the activated collision: The RNA example
Authors: Jacob Christine , Breton Nicolas , Daegelen Patrick
Published: July 2002

We propose a common rigorous foundation to the classical collision theory and that of the classical activated complex. Based on the notion of activated complex, this foundation relies on a stochastic approach showing up the different influence factors of a chemical reaction. The thermodynamic formulation is obtained here by assuming the exponential statistical distribution within each stable chemical state. The general model we obtain yields two stochastic formulations called the stochastic transition state theory (denoted STST) and the stochastic activated collision theory (denoted SACT) respectively, depending on whether the rate of the reaction is of the same scale as, either the rate of passing over the potential energy barrier, for the STST, or the rate of reaching the activated complex state (which is a generalization of the collision rate), for the SACT. The modeling is first done in the case of a closed small system undergoing stochastic changes of chemical state and then, by extension, in the frame of classical chemical kinetics. Some properties of these two theories are studied. Furthermore the formulation is applied to the stochastic example of RNA structure changes and to the classical simple example of a reaction between two atoms A and B to form the diatomic molecule AB.
Shear flow simulations of biaxial nematic liquid crystals
Authors: Sarman Sten
Published: July 2002

We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by performing molecular dynamics simulations. The equations of motion have been augmented by a director constraint torque that fixes the orientation of the directors. This makes it possible to fix them at different angles relative to the stream lines in shear flow simulations. In equilibrium simulations the constraints generate a new ensemble. One finds that the Green–Kubo relations for the viscosities become linear combinations of time correlation function integrals in this ensemble whereas they are complicated rational functions in the conventional canonical ensemble. We have evaluated these Green–Kubo relations for all the shear viscosities and all the twist viscosities. We have also calculated the alignment angles, which are functions of the viscosity coefficients. We find that there are three real alignment angles but a linear stability analysis shows that only one of them corresponds to a stable director orientation. The Green–Kubo results have been cross checked by nonequilibrium shear flow simulations. The results from the different methods agree very well. Finally, we have evaluated the Miesowicz viscosities [D. Baalss, Z. Naturforsch. Teil A 45, 7 (1990)]. They vary by more than 2 orders of magnitude. The viscosity is consequently highly orientation dependent.
A fluid in contact with a semipermeable surface: Second-order integral equation approach
Authors: Bryk P. , Henderson D. , Sokołowski S.
Published: July 2002

An integral equation approach for a binary hard-sphere mixture interacting with a planar semipermeable wall (membrane) is formulated by using the second-order nonuniform or pair Ornstein–Zernike equation as well as the usual singlet Ornstein–Zernike equation. The results of the pair theory are compared with those obtained from the singlet theory and with computer simulation data. The pair approach is more accurate than the singlet theory.
A thermodynamic model for polymerized unary liquids
Authors: Comas Carles , Mora Maria Teresa
Published: July 2002

In this paper a thermodynamic model for a polymerized unary liquid is presented. The model assumes there is a distribution of species as a result of a polymerization equilibrium which involves chain formation and scission. According to the model, the distribution of species depends on the Gibbs energy of bond formation and on the temperature of the liquid. Such a polymerization equilibrium is widely believed to occur in the Se-Te melts. The paper discusses the heat capacity, which is compared with the experimental heat capacity of the Se-Te melts. A very close behavior is found and an explanation from the point of view of a polymerization equilibrium is suggested.
Metastable and collision induced Coulomb explosion of doubly charged ammonia cluster ions
Authors: Rexer Eric F. , DeLeon Robert L. , Garvey James F.
Published: July 2002

A continuous neat ammonia cluster beam was ionized by electron impact ionization, then analyzed by a tandem triple quadrupole mass spectrometer. Metastable Coulomb explosion was observed for the {NH3}51Hx=0,1,22+ and the {NH3}53Hx=0,1,22+ cluster ions. Coulomb explosion was observed in larger doubly charged cluster ions activated by low energy collisions with argon gas.
Mass resolved photoionization/fragmentation studies of Cr(CO)6 at photon energies of ∼8–40 eV
Authors: Qi Fei , Yang Xin , Yang Shihe , Gao Hui , Sheng Liusi , Zhang Yunwu , Yu Shuqin
Published: July 2002

We have carried out a comprehensive study on the photoionization/fragmentation of Cr(CO)6 by using the time-of-flight mass spectrometry (TOFMS) technique and a synchrotron radiation source in the photon energy range of ∼8–40 eV. The appearance potentials (AP) of all the observed fragment ions were accurately determined, and those of some fragments, e.g., the doubly charged ions and the carbide fragments, were determined for the first time. The corresponding thermodynamic properties of these fragment ions have been derived.
Double-cone problem revisited: Effect of the geometric phase on the broad semiclassical resonances
Authors: Nikitin E. E.
Published: July 2002

We present a semiclassical analysis of resonance states supported by a conical potential well coupled to a conical peak. The positions of the energy levels are calculated by Wentzel–Kramers–Brillouin (WKB) procedures, which are applied to an adiabatic Hamiltonian with the contribution from the geometric phase taken into account. The probability of escape from the well is calculated by resorting to a comparison equation of the Zener–Dykhne–Chaplik type. The widths of the energy levels are calculated via the escape probability by using a general relation derived recently by Zhu, Nikitin, and Nakamura [J. Chem. Phys. 104, 7059 (1996)]. It is shown that the present calculations are in excellent agreement with accurate numerical data for the positions and widths as recovered from an analysis of the scattering matrix and from a direct calculation of the complex-valued energy levels. The results obtained explain the very fast decay of the low-lying states and the good performance of the surface-hopping approximation.
The effect of a solid wall on polymer chain behavior under shear flow
Authors: Lemak Alexander S. , Balabaev Nikolai K. , Karnet Yuliya N. , Yanovsky Yurii G.
Published: July 2002

Molecular dynamics computer simulations are used to investigate the properties of isolated bead–rod chains with excluded volume effects under a simple shear flow. With the help of the collisional dynamics technique, the simulations are performed with both the chains end grafted to an impenetrable wall and the isolated free-space chains immersed in a solvent. The comparison of these chains shows up the effect of the wall presence. We have calculated both the microscopic properties (such as end-to-end distance of polymer chain, the deformation ratio) and the macroscopic rheological properties of the model polymer solution (such as shear viscosity, first normal stress difference). A comparative analysis of the chains dynamics shows that the response of the grafted chains to simple shear flow is much different from that of the free chains. It was demonstrated that the grafting of polymer chains also gives rise to the distinctive rheological properties of the polymer solutions. For example, the effective viscosity of the grafted chains is an order of magnitude higher then the viscosity of the free chains.
A chemical application of the algebraic concept of internal space
Authors: Granucci G. , Cassam-Chenaı̈ P. , Ellinger Y.
Published: July 2002

We present a general method for extracting the internal space of an arbitrary wave function, exploiting the molecular symmetry. This permits the rewriting of a wave function built with nonorthogonal orbitals as a compact expansion of Slater determinants over orthogonal orbitals. The method is applied to valence bond (VB) and projected unrestricted Hartree–Fock (PUHF) functions along the potential curves of some diatomics, namely He2+, F2−, and F2. These functions are then used as references for further configuration interaction and perturbative Mo/ller–Plesset calculations.
A small system grand ensemble method for the study of hard-particle systems
Authors: Soto-Campos Gerardo , Corti David S. , Reiss Howard
Published: July 2002

This paper introduces an alternative statistical geometrical method for the evaluation of the thermodynamic properties of hard-particle systems. We develop a general theory which allows one to express average quantities in terms of geometrical “measurables.” The theory is tested with the known exact results for the one-dimensional hard rod fluid. We also discuss the potential benefits of the theory in efficiently calculating the chemical potential and pressure (using Monte Carlo methods) of dense hard-particle fluids in two and three dimensions.
A spin-density polarization index
Authors: Worsnop S. Kent , Boyd Russell J. , Sarasola Cecilia , Ugalde Jesus M.
Published: July 2002

By analogy with the Carbo overlaplike similarity measure, a spin-density polarization index (SPI) is defined. This index gives the Euclidean distance between the α and β electron densities for an open-shell molecule. An extensive study of the O2 molecule reveals that the SPI is highly dependent on the theoretical method, but is less sensitive to the basis set provided flexible extended basis sets are used. The radial distribution of the O2 spherically averaged electron density also reveals that near the center of the bond there is an inversion of the α and β electron densities and that there is a minimum in the β electron density near the nuclei within the internuclear region. Finally, the SPI is calculated for a variety of small molecules and the results are compared with those obtained for O2.
Analytic energy gradients for multiconfigurational self-consistent field second-order quasidegenerate perturbation theory (MC-QDPT)
Authors: Nakano Haruyuki , Hirao Kimihiko , Gordon Mark S.
Published: July 2002

An analytic energy gradient method for second-order quasidegenerate perturbation theory with multiconfigurational self-consistent field reference functions (MC-QDPT) is derived along the lines of the response function formalism (RFF). According to the RFF, the gradients are calculated without solving coupled perturbed equations. Instead, it is necessary to solve seven sets of linear equations in order to determine Lagrangian multipliers, corresponding to four sets of parameter constraining conditions and three sets of additional parameter defining conditions in the Lagrangian. Just one of these linear equations is a large scale linear equation; the others are reducible to just partial differentiations or simple equations solvable by straightforward subroutines.
Effects of complex formation on low energy H++O2(X 3Σg−,v=0)→H+O2+(X 2Πg,v″) charge transfer
Authors: Grimbert D. , Sidis V. , Cobut V.
Published: July 2002

The mutual effects of complex formation and nonadiabatic transitions in a low energy ion–molecule collision are investigated. This is achieved by studying the exothermic charge transfer (CT) process H++O2(X 3Σg−,v=0)→H+O2+(X 2Πg,v″) at collision energies below the first vibrational excitation threshold (E<0.2 eV). State-to-state vibronic CT cross sections are thus computed in the quantal infinite order sudden approximation using diabatic electronic states of Grimbert et al. [Chem. Phys. 124, 187 (1988)] and a vibrational basis of 14-reactant and 15-product states. The characteristics of the (H–O2)+ interaction potential in the entrance channel allow the formation of both translation-to-vibration and orbiting complexes which reflect themselves in the quantal scattering as Feschbach and shape resonances, respectively. The charge transfer may occur either directly or by a sort of predissociation of these complexes. The strong anisotropy of the relevant interactions in the (H–O2)+ system enables one to explore these effects in a broad range of coupling conditions. Two cases appear which are considered in detail: one in which CT cross sections are small while the electron transfer interaction is important, and the other in which the CT cross sections are large while the electron transfer interaction is rather small. Structures in the rotationally averaged cross section emphasize privileged orientations of the molecule for the studied CT process.
Some constraints involving the statistical properties of trajectories run in the Monte Carlo computation of a rate constant and their use in improving and testing the quality of sampling
Authors: Makarov Dmitrii E. , Metiu Horia
Published: July 2002

We analyze here the formula for the rate constant of a chemical reaction provided by the flux-flux correlation function theory. To evaluate this equation, classical trajectories are launched from a surface separating reactants and products, with initial conditions sampled according to the Boltzmann distribution. They are followed until they exit the transition region. We show here that these trajectories have statistical properties that satisfy several nontrivial constraints. We examine the nature of these constraints and explain how they can be used to improve rate constant calculations.
Generation of charge carriers and formation of antisymmetric double electric layers in glycerine
Authors: Koval’chuk A. V.
Published: July 2002

The effect of increasing (by more than one order of magnitude) of the conductivity of glycerine under the action of a low-frequency electric field (10−4–10−3 Hz) and heating is discovered and investigated. The transition into the high-conductivity state is accompanied by formation of asymmetric double electric layers at the electrodes. Parameters of the electric barrier at the interface are determined. For example, for the sample with thickness 18 μm at the temperature 325 K, the height of the potential barrier is 0.4 V, the thickness is 2.1 nm, the concentration of ionized centers is 3×1026 m−3. The transition into the more conductive state can be induced by desorption of ions from the electrode surface. The properties of glycerine are investigated for the both low and high-conductive states by low frequency dielectric spectroscopy. The dispersion of components ε′ and ε″ of the complex dielectric permittivity for the both states is described by the Debye equation modified by Cole–Cole. The dielectric relaxation time τ and the thickness of the double electric layer WC–C, where the redistribution of charges takes place, are measured. The transition into the high-conductivity state is characterized by the decreasing (more than one order) of τ, its activation energy and WC–C. Comparison of the obtained data with the data for the liquid crystal mixture 1282 suggests that the dispersion of ε′ and ε″ is induced by redistribution of charges between the bulk and the interface.
Analytic first-order properties from explicitly correlated many-body perturbation theory and Gaussian geminal basis
Authors: Bukowski Robert , Jeziorski Bogumil , Szalewicz Krzysztof
Published: July 2002

Theory of analytic first-order properties is formulated in a basis set independent way using the first-quantized many-body perturbation theory. This formulation allows the correlation effects to be described with explicitly correlated basis sets. The basis of Gaussian geminals is employed to calculate the second- and third-order correlation corrections to the lowest multipole moments of the H2 and LiH molecules. The same formalism is also utilized to compute the intra-atomic correlation contribution to the first-order interaction energy for the helium dimer. The results compare favorably with the literature data obtained using the conventional, orbital basis approach.
Quantum chemical studies of carbon-13 equilibrium fractionation in ion–molecule reactions
Authors: Lohr Lawrence L.
Published: July 2002

Ab initio computational quantum chemical methods are used to calculate reduced partition function ratios for all isotopomers of CO, HCO+, and HOC+ involving the nuclides H1, H2 (D), C12, C13, O16, and O18. The ratios are used to calculate equilibrium constants for the reaction pairs HCO+/CO, HOC+/CO, and C+/CO. Both simple proton transfers and more complex isotopic variants involving the breaking and reforming of CO bonds are considered. The probable pathways for the HCO+/CO and C+/CO exchange reactions are explored in detail using high-accuracy quantum chemical calculations. It appears most likely that the HCO+/CO reaction proceeds through exothermic formation of the linear adduct OCHCO+ with D∞h symmetry. Similarly, the C+/CO reaction proceeds along a spin-allowed pathway with exothermic formation of the linear adduct COC+ with D∞h symmetry. An alternate but higher energy spin-allowed pathway for the C+/CO reaction passes through a transition state with only Cs symmetry and a locally stable intermediate with C2v symmetry. In the ISM these reactions may proceed by these direct pathways or indirectly through coupled exothermic reaction pairs involving other species to achieve C13/12C isotope exchange.
Converging toward a practical solution of the Holstein molecular crystal model
Authors: Romero Aldo H. , Brown David W. , Lindenberg Katja
Published: July 2002

We present selected results for the Holstein molecular crystal model in one space dimension as determined by the Global–Local variational method, including complete polaron energy bands, ground state energies, and effective masses. We juxtapose our results with specific comparable results of numerous other methodologies of current interest, including quantum Monte Carlo, cluster diagonalization, dynamical mean field theory, density matrix renormalization group, semiclassical analysis, weak-coupling perturbation theory, and strong-coupling perturbation theory. Taken as a whole, these methodologies are mutually confirming and provide a comprehensive and quantitatively accurate description of polaron properties in essentially any regime. In particular, this comparison confirms the Global–Local variational method as being highly accurate over a wide range of the polaron parameter space, from the nonadiabatic limit to the extremes of high adiabaticity, from weak coupling through intermediate coupling to strong coupling.
Theoretical study of ion–molecule potentials for He+ and Li+ with N2
Authors: Falcetta M. F. , Siska P. E.
Published: July 2002

High-level ab initio calculations have been carried out on the lowest charge-transfer excited state of [HeN2]+ and the [LiN2]+ ground state, over a range of intermolecular distances R of 3–20a0 at a fixed N2 bond length re=2.074 30a0 for three orientation angles γ=0°, 45°, and 90°. The calculations employed extended atomic basis sets, chosen to represent accurately the electrical properties of the interacting partners; for N2 the key properties α∥, α⊥, and Θ are within 1.7%, 2.7%, and 2.1% of the best experimental values. All interaction energies were corrected for basis-set superposition error by the counterpoise method, and fitted by analytic forms incorporating the proper long-range expansion through R−7. Our value for the tetrahexacontapole (26-pole) moment of N2 is −15.95 a.u. The most stable geometries for both systems occur for linear (γ=0°) complexes, with minima −De of −7.00 (−12.65) kcal/mol located at Re=3.048 (2.610) Å for He+(Li+)+N2; the Li+ values are in good agreement with previous theoretical results. At small R the splitting ΔV(R,γ) between the He+ and Li+ surfaces is found to have a radial dependence close to the prediction of the hydrogenic charge-transfer coupling model, while its anisotropy is similar to that of the surfaces themselves.
Determination of vibrational energy relaxation rates of C–H,D,T stretching modes on hydrogen, deuterium, and tritium-terminated H,D,T/C(111) and H,D,T/C(110) diamond surfaces using molecular dynamics simulation: Thermal effect
Authors: Lu Hsiu-Feng , Ho Ming-Shun , Hong Sho-Ching , Liu Ai-Hsin , Wu Pei-Fang , Sun Ying-Chieh
Published: July 2002

Molecular dynamics simulations were carried out to determine the vibrational energy relaxation rates for C–H,D,T stretches on hydrogen-, deuterium-, and tritium-terminated H,D,T/C(111) and H,D,T/C(110) diamond surfaces at high temperatures based on the Bloch–Redfield theory and the calculated power spectra of fluctuating force along C–H,D,T stretches. The lifetime of C–H stretches on H/(110) surfaces at room temperature was found to be 0.8 ps, which is much shorter than the calculated lifetime of 30 ps on a H/C(111) surface attributed to 1:3 resonance. This is due to the blueshift of the 1:2 resonance domain in the force power spectra for a H/C(110) surface. The lifetimes of C–H stretches on a H/C(110) surface and C–D,T stretches on both D,T/C(111) and D,T/C(110) surfaces, which all undergo 1:2 resonance energy relaxation, are all on the time scale of tenths of a picosecond at room temperature and are approximately inversely proportional to the square of the temperature at high temperatures. For C–H stretches on a H/C(111) surface, the lifetimes at high temperatures are shortened much further not only by the rise in the temperature but also due to the thermal broadening of the resonance peaks in the force power spectra. The characteristics of power spectra and the resulting relaxation rates were analyzed using a simple model of a constrained diatomic bond in a harmonic bending potential field. The present results suggest that, since the resonance frequencies of C–H stretches are located within the border region between the 1:2 and 1:3 resonance domains, the vibrational energy relaxation of C–H stretches may differ by more than an order of one on different monohydrided low index unreconstructed diamond surfaces in contrast to the lifetimes of C–D,T stretches on these diamond surfaces, which are all on the same time scale at a given temperature.
Orientational effects on low-energy modes in amorphous poly(ethylene terephthalate) fiber
Authors: Kanaya T. , Zorn R. , Tsukushi I. , Murakami S. , Kaji K. , Richter D.
Published: July 2002

Inelastic and quasielastic neutron-scattering measurements have been performed on an amorphous poly(ethylene terephthalate) (PET) fiber in parallel and perpendicular scattering geometries, i.e., when the fiber axis f is parallel and perpendicular to the scattering vector Q, respectively. The so-called boson peak is observed at around −1.4 meV in neutron energy transfer at low temperatures below about 150 K in both the geometries; as temperature increases, the fast process of picosecond order appears at a certain temperature below the glass-transition temperature Tg(=348 K). Although no drastic differences in the dynamics can be observed, a closer look at the spectra revealed some interesting features of the low-energy modes. From the mean-square displacements evaluated with two different energy resolutions, it was found that the fast process appears only in the direction perpendicular to the polymer chain (not the fiber axis f) in the energy region between −1 and −0.2 meV. In the inelastic scattering spectra, we found that the boson peak is stronger in intensity for the parallel direction while the intensity of the fast process is larger for the perpendicular one. The quantitative analysis based on the recent vibration–relaxation model revealed that the fast process can mainly be explained by the softening of the vibrational modes; the softening occurs at lower temperatures in the perpendicular geometry than in the parallel one. On the other hand, the conventional single-Lorentzian fit to the fast process, in which the fast process is assumed to be a relaxational process and described by a Lorentzian, showed that the onset temperature of the fast process is lower in the perpendicular geometry than in the parallel one. These have been tentatively attributed to weaker force constants, for motions perpendicular to a polymer chain, such as torsional and librational motions, than those along the chain.
Microphase separation in poly(isoprene-b-ethylene oxide) diblock copolymer melts. I. Phase state and kinetics of the order-to-order transitions
Authors: Floudas G. , Ulrich R. , Wiesner U.
Published: July 2002

The phase state and the kinetics of the order-to-order transitions have been studied in a series of poly(isoprene-b-ethylene oxide) (PI-PEO) diblock copolymers with a PI volume fraction in the range 0.25<fPI<0.92, using small angle x-ray scattering (SAXS), and rheology. The mean-field theory (MFT) structure factor is used to describe the SAXS profiles in the disordered phase and to extract the temperature dependence of the interaction parameter χ(T). In general, an agreement is found with the phase diagram proposed by an extended MFT, except at fPI=0.61 where the following sequence of phases was found: Lc→Hex→Gyroid→Dis (Lc is the crystalline lamellar phase, Hex signifies hexagonally packed cylinders, Gyroid is the bicontinuous cubic network with the Ia3̄d symmetry, and Dis is the disordered phase). We found that crystallization disrupts the amorphous ordered morphologies and imposes a layered structure (Lc). The study of the kinetics of the Hex to Lc and the Hex to Gyroid transitions is facilitated by the different viscoelastic contrast and the distinctly different scattering patterns of the three phases involved (Lc, Hex, Gyroid). Our studies show that it is possible to undercool and overheat ordered phases just as we can undercool the disordered phase. The transformation from the Hex to the Lc phase proceeds via a heterogeneous nucleation and growth process and results in the formation of a spherulitic superstructure composed from stacks of lamellar crystals. The transformation of the Hex to the Gyroid phase involves two steps. The first step—which is too fast to be picked up by rheology—involves fluctuations of the hexagonal phase. The second “slow” step involves a nucleation and growth process of elongated objects. The transformation proceeds nearly epitaxially and has an activation energy of 47 kcal/mol which is typical for a collective process.
Neon and argon matrix ESR and theoretical studies of the 12CH3Cd, 12CD3Cd, 13CH3Cd, 12CH3111Cd, and 12CH3113Cd radicals
Authors: Karakyriakos Emmanuel , Davis Jeffrey R. , Wilson Carl J. , Yates Sally A. , McKinley Allan J. , Knight Lon B. , Babb Robert , Tyler D. J.
Published: July 2002

Electron spin resonance (ESR) studies are reported for the first time on the various isotopomers of the CH3Cd radical isolated in neon and argon matrices. The radicals were generated in neon matrices by the reaction of laser-ablated cadmium metal and various methyl precursors, and in argon matrices by x-irradiation. The neon matrix values measured were g⊥=1.9491(1), A⊥(H)=17.0(1) MHz, A⊥(D)=2.6(1) MHz, A⊥(13C)=163(3) MHz, and A⊥(111Cd)=−3083(3) MHz, and estimates were derived for A∥(13C)=230(50) MHz and A∥(111Cd)=−3486(5) MHz. The argon matrix values measured were g⊥=1.952(1), A⊥(H)=16(1) MHz and A⊥(111Cd)=−3301(3) MHz, and an estimate was derived for A∥(111Cd)=−3704(5) MHz. The ESR experimental Adip(111Cd) values for the neon and the argon matrices agree with the reported gas-phase value [J. Chem. Phys. 101, 6396 (1994)]. The matrix ESR Aiso(111Cd) values show small shifts compared with the gas-phase results (5% greater for the neon matrix and 12% greater for the argon matrix). At 4.3 K in the neon matrices, additional ESR lines assigned to tunneling phenomena were observed. The radical geometry obtained from ab initio calculations was consistent with that reported from the various experimental results. Multireference single and double excitation configuration interaction (MRSDCI) calculations of the hyperfine interactions gave values that were consistently below the experimental values of Aiso and Adip for 111Cd, H, and 13C. MRSDCI calculations for the CdH radical showed an analogous trend.
Linear and third order nonlinear optical properties of substituted oligothiophenes
Authors: Van Keuren Edward , Möhwald Helmut , Rozouvan Stanislaw , Schrof Wolfgang , Belov Vladimir , Matsuda Hiro , Yamada Shinji
Published: July 2002

Low band gap polythiophene derivatives synthesized using the method of Stille coupling have shown good third order nonlinear optical properties. In order to gain more insight into the structure/property relationship in these materials, we measured the optical properties of a number of oligothiophene derivatives. In addition to linear absorption spectra, we investigated the third order nonlinear optical properties using Z-scan at a number of wavelengths. As in other conjugated oligomers, the properties depend strongly on chain length. We also observe significant dependence on the substituent type, number, and position, with strong enhancement of the oscillator strength for asymmetric substitution. The resonance nonlinear absorption shows strong saturation behavior, in general changing to induced absorption off-resonance. The nonlinear refractive indices were negative near resonance, suggesting the possibility of thermal effects. While off-resonant nonlinear refractive indices were small, in the five substances for which they could be measured, the values were found to be negative, suggesting that resonant enhancement is still playing a role.
Faber and Newton polynomial integrators for open-system density matrix propagation
Authors: Huisinga Wilhelm , Pesce Lorenzo , Kosloff Ronnie , Saalfrank Peter
Published: July 2002

Two polynomial expansions of the time-evolution superoperator to directly integrate Markovian Liouville–von Neumann (LvN) equations for quantum open systems, namely the Newton interpolation and the Faber approximation, are presented and critically compared. Details on the numerical implementation including error control, and on the performance of either method are given. In a first physical application, a damped harmonic oscillator is considered. Then, the Faber approximation is applied to compute a condensed phase absorption spectrum, for which a semianalytical expression is derived. Finally, even more general applications are discussed. In all applications considered here it is found that both the Newton and Faber integrators are fast, general, stable, and accurate.
A theoretical study on the ionic states of dichloroethylene with an analysis of the vibrational structure of the photoelectron spectra
Authors: Takeshita Kouichi
Published: July 2002

Ab initio calculations have been performed to study the molecular structures and the vibrational levels of the low-lying four ionic states of dichloroethylene. The equilibrium molecular structures and vibrational modes of these states are presented. The theoretical ionization intensity curves including the vibrational structures of the ionic states are also presented and compared with the photoelectron spectra.
Water-soluble fullerene electrolytes. Electrostatic contribution to thermodynamic functions
Authors: Škerjanc Jože
Published: July 2002

The Poisson–Boltzmann equation has been solved to obtain the electrostatic potential for the spherical cell model of a fullerene molecule at which charged groups are attached. The electrostatic internal and free energies, degree of binding of counterions, osmotic coefficient, and distribution of counterions around the charged fullerene shell are calculated. These properties are presented as functions of the concentration, radii of counterions, and surface charge density of the fullerene macroion. Calculated osmotic coefficients are compared with preliminary experimental data for the sodium salt of fullerenehexamalonic acid, C66(COONa)12.
Photodissociation of HOBr. I. Ab initio potential energy surfaces for the three lowest electronic states and calculation of rotational–vibrational energy levels and wave functions
Authors: Füsti-Molnár László , Szalay Péter G. , Balint-Kurti Gabriel G.
Published: July 2002

Potential energy surfaces are presented for the three lowest lying singlet electronic states of HOBr. The surfaces are computed using the recently developed multireference averaged quadratic coupled clusters method and a TZ2P orbital basis set. They provide the basic data needed to compute the dynamics of the HOBr+hν→OH+Br photodissociation process, which plays a key role in the bromine chemistry of the stratosphere. A pseudopotential is used for the core electrons of the Br atom; this is shown not to introduce any errors in the shape of the surfaces through direct comparison with sample all-electron calculations. Transition dipole moment surfaces for the two excitation processes (1 1A″←X̃ 1A′ and 2 1A′←X̃ 1A′) are also presented. These are computed using a multireference singles and doubles configuration interaction method. A grid based method is developed to compute the vibrational–rotational states of the molecule and spectroscopic constants extracted from the computed molecular energy level spacings are compared with experimentally determined quantities.
Temperature and density dependence of self-diffusion in supercooled liquid CS2
Authors: Li Wu-Xiong , Keyes T.
Published: July 2002

The self-diffusion coefficient (D) of liquid CS2 has been determined by computer simulation for seven densities (ρ) and eight temperatures, spanning more than two decades of D in the supercooled and near-melting normal liquid, and bracketing the P=1 atm isobar. Super-Arrhenius behavior of −log D vs 1/T, an increase in slope with decreasing T, is found at 1 atm, but normal Arrhenius T dependence holds along all seven different isochores, even at the highest density and lowest T. The super-Arrhenius behavior is a consequence of the variation in density ρ(T) at constant pressure. Physically meaningful activation energies, representative of the heights of the barriers to diffusion, depend upon ρ only, are smaller than the isobaric slope, and may be obtained by correcting it or from an Arrhenius plot at constant density. Barriers to diffusion are indeed higher at lower temperatures, but only due to the higher density. The importance of T vs ρ as the “control variable” for diffusion is examined. Temperature and density play comparable roles near the melting states, and the relative importance of T grows with supercooling. However that growth is due to the higher activation energy, itself controlled by density.
Thermodynamics and structural properties of the dipolar Yukawa fluid
Authors: Szalai István , Henderson Douglas , Boda Dezsö , Chan Kwong-Yu
Published: July 2002

We report computer simulations and a theoretical study of the thermodynamics and structure of a dipolar Yukawa system. A comparison between the analytical mean spherical approximation (MSA) solution, perturbation theory and Monte Carlo simulation data of pressure, internal energy and dielectric constant is given. In the perturbation theory, the MSA solution of hard core Yukawa fluid is used as a reference system. It was found that the MSA solution is reasonable only at lower dipole moments, while the perturbation theory gives good results at low and high values of dipole moment. Liquid–vapor coexistence data of dipolar Yukawa fluid are also obtained by Monte Carlo simulation and by both MSA and perturbation theory. It was found that at high dipole moments the liquid–vapor equilibrium disappears while chain-like structures appear in the low density fluid phase. The appearance of chain-like structures of dipolar Yukawa fluid is discussed in comparison with the Stockmayer fluid.
Quantum mechanical study of the vibrational relaxation of O2+ colliding with Kr
Authors: Craimer Mark , Pogrebnya Sergei K. , Clary David C.
Published: July 2002

Coupled states calculations on the vibrational relaxation of O2+(v=1) colliding with Kr are reported. In the first stage, calculations have been done on single potential energy surfaces and different surfaces have been used. Then treating O2+ as a molecule in a Π ground electronic state, we have performed quantum scattering calculations on the vibrational relaxation on two 1 2A″ and 1 2A′ surfaces. A significant effect due to the inclusion of the second potential surface is reported. A comparison of the calculated rate constants with available experimental data is made.
Association of diphilic chains near the solvent critical region
Authors: Velichko Yurii S. , Vasilevskaya Valentina V. , Khalatur Pavel G. , Khokhlov Alexei R.
Published: July 2002

By means of computer simulation the association process of diphilic macromolecules with strongly attractive head units (stickers) in the vicinity of the critical point of the solvent is studied. The calculations were carried out by means of Monte Carlo simulation with the bond-fluctuation model of polymer modified by us to mimic the hexagonal packing of monomer units. It was found that the critical concentration of micelle formation and average aggregation number depend crucially not only on the attraction between stickers but on the quality and state of solvent. The average aggregation number can demonstrate nonmonotonic behavior with change in temperature.
Droplet size dependence upon volume expansion rate
Authors: Ashurst Wm. T. , Holian Brad Lee
Published: July 2002

Molecular dynamics of a Lennard-Jones liquid undergoing a homogeneous adiabatic expansion provides a direct numerical simulation of the atomization process. Results from a symmetric-triaxial expansion appear to be consistent with the mean droplet size found in the Knuth and Henne free-jet experiments. New simulations with axisymmetric expansions, similar to flow through an orifice, show only small departures from the symmetric expansion results.
Vibrational relaxation of NO(υ=1) by oxygen atoms
Authors: Dodd James A. , Lockwood Ronald B. , Hwang Eunsook S. , Miller Steven M. , Lipson Steven J.
Published: July 2002

The rate constant kO(υ=1) for NO(υ=1) vibrational relaxation by O has been measured at room temperature using a laser photolysis-laser probe technique. Vibrationally excited NO and relaxer O atoms were formed using 355 nm laser photolysis of a dilute mixture of NO2 in argon bath gas. The time evolution of both the NO(υ=1) and the O atoms was monitored using laser-induced fluorescence (LIF). The required absolute O-atom densities were obtained through a comparison of O-atom LIF signals from the photolysis source and from a titrated cw microwave source. At early times the O atoms constitute the most important loss mechanism for the nascently produced NO(υ=1). Possible effects from NO(υ=1) vibrational ladder-climbing and from thermal expansion have been shown to be minimal. The rate constant kO(υ=1)=(2.4±0.5)×10−11 cm3 s−1 determined herein is a factor of 2 to 3 lower than the generally accepted value of kO(υ=1) used in thermospheric modeling. The present value for kO(υ=1) is the same, within the error bars, as the kO(υ=2,3) previously measured in this laboratory using an entirely different technique, resonant infrared laser excitation of NO(υ=0). This result suggests that the collisional relaxation rates are independent of υ. A recent quasiclassical trajectory calculation, in which both allowed NO–O surfaces have been explicitly considered, predicts a collisional relaxation rate which is in good agreement with the present result. The kO(υ=1) value, along with previously measured rate constants for NO–O high-pressure recombination (krec∞) and isotope exchange (kiso), can serve as a proxy for the rate coefficient kC describing the formation of a long-lived NO2* intermediate from O+NO collisions. The present value for kO(υ=1) is significantly lower, however, than a recent determination of krec∞ and also the value of kC derived from kiso. In the latter case the comparison is not as straightforward.
van der Waals theory for the spatial distribution of the tension in an interface. II. Numerical results
Authors: Baus M. , Lovett R.
Published: July 2002

A van der Waals-based description of a system that approximates a Lennard-Jones fluid is presented as a density functional theory. Numerical calculations of the density and tension profiles for the planar liquid-vapor interface, the wall-fluid interface, and the wall-liquid-vapor interface are made for this system. The forces associated with capillary rise are determined.
Resonant ion stimulated desorption of O+ and H+: A probe of interactions of oxygen and hydrogen with TiO2(110)
Authors: Souda Ryutaro
Published: July 2002

The mechanism of noncollisional H+ and O+ sputtering from reduced, hydrogenated and oxygenated TiO2(110) surfaces has been investigated by low-energy noble-gas ions and electron bombardment. The noncollisional sputtering is found to be initiated by formation of the O 2s core hole via the quasiresonant charge exchange. Desorption of oxygen and hydrogen occurs from core-excited oxygen and hydroxyl group, respectively, which have an antibonding character with a long lifetime. To survive efficient reneutralization, ionization of oxygen should occur after bond breakage via the intraatomic Auger decay of the O 2s hole while hydrogen can be ionized in the course of the O–H bond breakage at the surface via the interatomic Auger decay of the O 2s hole. On the basis of these findings, the interaction of oxygen and hydrogen with the reduced TiO2(110) surface is investigated using resonant ion stimulated desorption (RISD) by He+. At the reduced surface, the chemisorbed oxygen atom either fills a vacancy site of bridging oxygen atoms or chemisorbs at a fivefold-coordinated Ti4+ site as an adatom. In the RISD experiment, the oxygen adatom is detected much more efficiently than the bridging oxygen. Such oxygen adatoms are found to be unstable on the TiO2(110) surface; they diffuse into the bulk at the oxygenated surface or segregate from the bulk at the reduced surface. Hydrogen, bonded to Ti at the reduced surface, tends to be reorganized to form hydroxyl group upon oxygenation.