bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal fluctuations in a lamellar phase of a binary amphiphile–solvent mixture: A molecular-dynamics study
Authors: Loison C. , Mareschal M. , Kremer K. , Schmid F.
Published: December 2003

We investigate thermal fluctuations in a smectic A phase of an amphiphile–solvent mixture with molecular-dynamics simulations. We use an idealized model system, where solvent particles are represented by simple beads, and amphiphiles by bead-and-spring tetramers. At a solvent bead fraction of 20% and sufficiently low temperature, the amphiphiles self-assemble into a highly oriented lamellar phase. Our study aims at comparing the structure of this phase with the predictions of the elastic theory of thermally fluctuating fluid membrane stacks [Lei et al., J. Phys. II 5, 1155 (1995)]. We suggest a method which permits to calculate the bending rigidity and compressibility modulus of the lamellar stack from the simulation data. The simulation results are in reasonable agreement with the theory.
Characterization of the three lowest-lying singlet electronic states of AlOH
Authors: Li Se , Sattelmeyer Kurt W. , Yamaguchi Yukio , Schaefer Henry F.
Published: December 2003

Two linear (1Σ+ and Π)1 and three bent (1 1A′, 2 1A′, and 1 1A″) lowest-lying electronic singlet states of AlOH have been systematically investigated employing ab initio self-consistent-field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and CCSD with iterative partial triple excitations (CCSDT-3 and CC3) quantum mechanical methods with basis sets up to augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ). The linear Σ+1 state is found to be a remarkably low-energy transition state between the two equivalent bent 1 1A′ structures, while the linear Π1 state is a second-order saddle point, which leads to the bent 2 1A′ and 1 1A″ states. The bent ground (X̃ 1A′) state of AlOH is predicted to have a bond angle of 157° at the aug-cc-pVQZ CC3 level of theory and is classified as a quasilinear molecule, confirming previous studies. Employing the equation-of-motion coupled cluster method, the first singlet excited state (Ã 1A′) is predicted to have a bond angle of 110° and to lie 114 kcal/mol (39 900 cm−1, 4.94 eV) above the ground state, whereas the second singlet excited state (B̃ 1A″) is predicted to have a bond angle of 116° and to be located 119 kcal/mol (41 700 cm−1, 5.17 eV) above the ground state. These theoretical energy separations are in excellent agreement with the experimental values T0 (Ã 1A′)=114.57 kcal/mol (40 073 cm−1, 4.968 eV) and T0 (B̃ 1A″)=119.36 kcal/mol (41 747 cm−1, 5.176 eV). The barriers to linearity for the two bent singlet excited states are determined to be 11.6 kcal/mol for the Ã 1A′ state and 6.2 kcal/mol for the B̃ 1A″ state.
Phase behavior in thin films of cylinder-forming ABA block copolymers: Experiments
Authors: Knoll Armin , Magerle Robert , Krausch Georg
Published: December 2003

We experimentally establish a phase diagram of thin films of concentrated solutions of a cylinder forming polystyrene-block-polybutadiene-block-polystyrene triblock copolymer in chloroform. During annealing the film forms islands and holes with energetically favored values of film thickness. The thin film structure depends on the local thickness of the film and the polymer concentration. Typically, at a thickness close to a favored film thickness parallel orientation of cylinders is observed, while perpendicular orientation is formed at an intermediate film thickness. At high polymer concentration the cylindrical microdomains reconstruct to a perforated lamella structure. Deviations from the bulk structure, such as the perforated lamella and a wetting layer are stabilized in films thinner than ≈1.5 domain spacings.
Nonadiabatic reaction rates for dissipative quantum-classical systems
Authors: Sergi Alessandro , Kapral Raymond
Published: December 2003

The dynamics of a quantum system which is directly coupled to classical degrees of freedom is investigated. The classical degrees of freedom are in turn coupled to a classical bath whose detailed dynamics is not of interest. The resulting quantum-classical evolution equations are dissipative as a result of coupling to the classical heat bath. The dissipative quantum-classical dynamics is used to study nonadiabatic chemical reactions and compute their rates. The reactive flux correlation formalism for the calculation of nonadiabatic rate constants is generalized to dissipative quantum-classical dynamics and implemented in terms of averages over surface-hopping Langevin trajectory segments. The results are illustrated for a simple quantum-classical two-state model. The techniques developed in this paper can be applied to complex classical environments encountered, for example, in proton and electron transfer processes in the condensed phase where local environmental degrees of freedom must be treated explicitly but the remainder of the environment can be treated simply as a heat bath.
A super-linear minimization scheme for the nudged elastic band method
Authors: Chu Jhih-Wei , Trout Bernhardt L. , Brooks Bernard R.
Published: December 2003

In this article, we present a superlinear minimization scheme for the nudged elastic band (NEB) method, which determines a minimum-energy path (MEP) of a reaction via connecting intermediate “replicas” between the reactant and the product. The minimization scheme is based on a quasi-Newton method: the adopted basis Newton–Raphson (ABNR) minimization scheme. In each step of ABNR minimization, the Newton–Raphson procedure is performed in a subspace of a user-defined dimension. The tangent directions of the path at a new Newton–Raphson step are determined self-consistently in the subspace. The acceleration of the proposed scheme over the quenched molecular-dynamic minimization, the current practice for minimizing a path using NEB, is demonstrated in three nontrivial test cases: isomerization of an alanine dipeptide, α-helix to π-helix transition of an alanine decapeptide, and oxidation of dimethyl sulfide. New features are also added such that the distances between replicas can be defined in the root of mean squared (RMS) best-fit space with flexible weighting options. This offers a way to incorporate the effects of a mobile solvent in the process with a finite number of replicas. MEPs obtained from various initial structures can be used to investigate different proposed reaction mechanisms, and the speedup of minimizing a path enhances the applicability of the NEB method. The combination of NEB force projection procedures, the flexible distance definition in the RMS best fit space with arbitrary weighting options, and the superlinear minimization scheme provides a framework to aid in the study of transition processes of biological molecules as such proteins.
Theoretical calculation of vibronic levels of C2H and C2D to 10 000 cm−1
Authors: Tarroni Riccardo , Carter Stuart
Published: December 2003

The rovibronic levels for the X 2Σ+,A 2Π electronic system of C2H, C2D and all C13 substituted isotopomers are calculated using a recently developed variational method [Carter et al., Mol. Phys. 98, 1967 (2000)] and high level ab initio diabatic potential energy surfaces. Energies, rotational constants and spin–orbit splittings of all levels of Σ, Π, Δ, Φ symmetry are reported up to 6400 cm−1 for C2H and up to 5500 cm−1 for C2D. Computed energies of Σ, Π levels up to 10 000 cm−1 are also reported for all C13 substituted isotopomers. Experimental data are reviewed in the light of the present results.
A discrete solvent reaction field model for calculating frequency-dependent hyperpolarizabilities of molecules in solution
Authors: Jensen Lasse , van Duijnen Piet Th. , Snijders Jaap G.
Published: December 2003

We present a discrete solvent reaction field (DRF) model for the calculation of frequency-dependent hyperpolarizabilities of molecules in solution. In this model the solute is described using density functional theory (DFT) and the discrete solvent molecules are described with a classical polarizable model. The first hyperpolarizability is obtained in an efficient way using time-dependent DFT and the (2n+1) rule. The method was tested for liquid water using a model in which a water molecule is embedded in a cluster of 127 classical water molecules. The frequency-dependent first and second hyperpolarizabilities related to the electric field induced second harmonic generation (EFISH) experiment, were calculated both in the gas phase and in the liquid phase. For water in the gas phase, results are obtained in good agreement with correlated wave function methods and experiments by using the so-called shape-corrected exchange correlation (xc)-potentials. In the liquid phase the effect of using asymptotically correct functionals is discussed. The model reproduced the experimentally observed sign change in the first hyperpolarizaibility when going from the gas phase to the liquid phase. Furthermore, it is shown that the first hyperpolarizability is more sensitive to damping of the solvent–solute interactions at short range than the second hyperpolarizability.
Propyne and allene photolysis at 193.3 nm and at 121.6 nm
Authors: Qadiri Rafay H. , Feltham Emma J. , Hendrik Nahler N. , Pérez Garcı́a Rodrigo , Ashfold Michael N. R.
Published: December 2003

The fragmentation dynamics of allene and propyne molecules following photoexcitation at 193.3 nm and at 121.6 nm have been investigated by H(D) Rydberg atom photofragment translational spectroscopy. The total kinetic energy release (TKER) spectra of the H (and D) atoms resulting from H2CCCH2, H3CCCH, and D3CCCH photolysis at 193.3 nm are found to be essentially identical. The results contradict conclusions reached in several previous studies of propyne photochemistry at this wavelength. The observed energy disposal, and the isomer independence, are most readily rationalized by assuming that the fragmentation of both molecules following excitation at 193.3 nm is preceded by internal conversion to the ground (S0) state potential energy surface, and that the isomerization rate of the resulting highly vibrationally excited S0 molecules is faster than their unimolecular decay rate. The time-of-flight (TOF) and TKER spectra of the H and D atoms resulting from 121.6 nm photolysis of allene, propyne and propyne-d3 show significant differences, however. The differences can be reconciled by assuming two competing pathways for forming H(D) atoms following photoexcitation of propyne. One, involving selective cleavage of the acetylenic H3CCC–H bond, is assumed to occur from the excited electronic state prepared by photon absorption or from a recognizably “propynelike” state to which it couples efficiently. The other, which yields a slower distribution of H(D) atoms, is considered to arise via radiationless transfer to a lower electronic state, isomerization, and subsequent unimolecular decay. The TOF and TKER spectra of the H atoms resulting from 121.6 nm photolysis of allene are indistinguishable from those associated with this second, “statistical” fragmentation channel in propyne.
A rigorous test of the statistical model for atom–diatom insertion reactions
Authors: Rackham Edward J. , Gonzalez-Lezana Tomas , Manolopoulos David E.
Published: December 2003

The statistical model of atom–diatom insertion reactions is combined with coupled-channel capture theory and used to calculate differential cross sections for the reactions of C(1D), N(2D), O(1D) and S(1D) with H2. In the case of C(1D) and S(1D), the resulting statistical differential cross sections are found to be in excellent agreement with the recent quantum reactive scattering calculations of Honvault and Launay. They are therefore also in good agreement with molecular beam experiments for the S(1D)+H2 reaction, in contrast to the results of earlier calculations based on a less rigorous statistical theory. However, because the exact quantum mechanical differential cross sections for N(2D) and O(1D) exhibit a slight forward–backward asymmetry, the agreement with the statistical model for these reactions is not quite so good. The difference between the two cases can be rationalized in terms of the greater exoergicities of the N(2D) and O(1D) reactions, which lead to broader resonances and hence to shorter lifetimes of the H2O and NH2 collision complexes than those of CH2 and H2S.
Transition path sampling with a one-point boundary scheme
Authors: Chen L. Y. , Nash Patrick L.
Published: December 2003

Studying the motion of Lennard-Jones clusters in an external potential having a very narrow channel passage at the saddle point, we present a one-point boundary scheme to numerically sample transition (reaction) paths. This scheme does not require knowledge of the transition states (saddle points) or that of the final states. A transition path within a given time interval (0,tf) consists of an activation path during (0,tM) and a deactivation path during (tM,tf) (0<tM<tf) joined at an intermediate time tM. The activation path is a solution to a Langevin equation with negative friction, while the deactivation path is that to a regular Langevin equation with positive friction. Each transition path so generated carries a determined statistical weight. Typical transition paths are found for two-particle and three-particle clusters. A two-particle cluster adjusts its orientation to the direction of the narrow channel and then slides through it. A three-particle cluster completes a transition by openning one of its three bonds, becoming linear, and sliding through the channel.
The heterogeneity of the hydroxyl groups in chabazite
Authors: Mihaleva Velitchka V. , van Santen Rutger A. , Jansen A. P. J.
Published: December 2003

Two different clusters that have the topology of chabazite but different shapes have been used as a model for the Brønsted sites in chabazite. One of the clusters consists of eight tetrahedral atoms (8T) arranged in a ring and the other represents an intersection of two 8T rings. The adsorption of water and methanol on the two stable proton positions in chabazite has been studied using the B3LYP functional. The coordination of water and methanol with respect to the zeolite fragments were found to be similar, but with methanol situated closer to the acid site than water. The anharmonic zeolite OH stretch frequencies were found to be in the range of 2170–2500 cm−1 and 1457–2074 cm−1 in the presence of water and methanol, respectively. As a measure of the acidity of the bridging hydroxyl groups in chabazite the shift of the zeolite OH stretch frequency upon adsorption has been used. We have found that the proton attached to the oxygen atom O1 to be more acidic than the proton attached to the oxygen atom O3. Also, in the closed ring clusters the zeolite hydroxyl groups are more acidic than in the open clusters. This is not due to a steric effect as the orientation of the adsorbates with respect to the zeolite site is very similar for both clusters. The anharmonicities of the zeolite O–H bond account for about 40% in the redshift upon the adsorption of water or methanol.
Enhancements of third-order nonlinear optical response in the triplet excited state of finite open single-walled carbon nanotubes
Authors: Cheng W.-D. , Wu D.-S. , Zhang H. , Li X.-D. , Lan Y.-Z. , Chen D.-G. , Wang H.-X.
Published: December 2003

Spin multiplicity enhancements of third-order nonlinear polarizabilities have been investigated at the excited state for finite single-walled carbon nanotubes (5,0) and (6,0) in terms of the time-dependent density functional theory combined with the sum-over-states method. The time-dependent density functional theory at the B3LYP/3-21G level is employed to calculate the transition moments and the excited state energies of optimized tubes. The obtained results show that the third-order polarizabilities of γ at the excited triplet state are enhanced about from 2 to 3 orders of magnitude and the estimated third-order optical susceptibility of χ(3) is as large as 10−8 esu magnitude order. The electron transitions among the three triplet states make significant contributions to third-order polarizabilities of the (5,0) nanotube, and the electron transition between the two triplet states makes main the contribution to those of the (6,0) nanotube.
Surfactant adsorption density calculation from Fourier transform infrared external reflection spectroscopy (FTIR/ERS)
Authors: Fa Keqing , Miller Jan D.
Published: December 2003

An equation to calculate surfactant adsorption density from Fourier transform infrared external reflection spectra was established. The derivation and limitation of this equation are discussed in detail. The validation of the FTIR/ERS adsorption density equation was experimentally verified from the analysis of Langmuir–Blodgett films of stearic acid with and without calcium ions in the subphase at the air–water interface and at fluorite surface. In this way the properties of Langmuir–Blodgett films at the air–water interface are further characterized using FTIR/ERS.
Numerical simulation of transmission coefficient using c-number Langevin equation
Authors: Barik Debashis , Bag Bidhan Chandra , Ray Deb Shankar
Published: December 2003

We numerically implement the reactive flux formalism on the basis of a recently proposed c-number Langevin equation [Barik et al., J. Chem. Phys. 119, 680 (2003); Banerjee et al., Phys. Rev. E 65, 021109 (2002)] to calculate transmission coefficient. The Kramers’ turnover, the T2 enhancement of the rate at low temperatures and other related features of temporal behavior of the transmission coefficient over a range of temperature down to absolute zero, noise correlation, and friction are examined for a double well potential and compared with other known results. This simple method is based on canonical quantization and Wigner quasiclassical phase space function and takes care of quantum effects due to the system order by order.
Propagation of a planar flame front studied by molecular dynamics
Authors: Schmidt Hansen Jesper , Toxvaerd Søren , Præstgaard Eigil L.
Published: December 2003

In this paper we study the propagation of a planar traveling wave in a nonisothermal autocatalytic reaction diffusion system. This is done by performing a series of molecular dynamics simulations for different densities and heat of reactions. In the special case where the reaction is isothermal, it is shown that the molecular dynamics simulation qualitatively resembles the approximative macroscopic description that is based on the reaction diffusion equation. Simulations for different densities and heat releases further show that the speed of propagation is not simply the minimum speed of the corresponding macroscopic equation and that the speed is always constant even though the Lewis number is greater than unity.
Orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate
Authors: Cang Hu , Li Jie , Fayer M. D.
Published: December 2003

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments are used to study the orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM+NO3−) over time scales from ∼1 ps to ∼2 ns, and the temperatures range from 410 to 295 K. The temperatures cover the normal liquid state and the weakly supercooled state. The orientational dynamics exhibit characteristics typical of normal organic glass-forming liquids. The longest time scale portion of the data decays as a single exponential and obeys the Debye–Stokes–Einstein relation. The decay of the OHD-OKE signal begins (∼1 ps) with a temperature independent power law, t−z, z=1.02±0.05, the “intermediate power law.” The power law decay is followed by a crossover region, modeled as a second power law, the von Schweidler power law. The longest time scale decay is the exponential α relaxation. The intermediate power law decay has been observed in van der Waals supercooled liquids previously. These are the first such observations on an ionic organic liquid. The observation of the dynamical signatures observed in other liquids demonstrates that the orientational dynamics of ionic organic liquids are fundamentally the same as van der Waals liquids and supports the universality of the intermediate power law decay in the dynamics of complex liquids. Within the mode-coupling theory (MCT) framework, the MCT critical temperature TC is estimated to be TC≅255 K.
Semiflexible polymer solutions. I. Phase behavior and single-chain statistics
Authors: Spakowitz Andrew J. , Wang Zhen-Gang
Published: December 2003

We study the thermodynamics and single-chain statistics of wormlike polymer solutions with Maier–Saupe-type interactions using self-consistent-field (SCF) theory. The SCF equations are derived using a systematic field-theoretical approach which yields the SCF equations as the lowest order approximation, but permits fluctuation corrections to be incorporated. We solve the SCF equations using the spheroidal functions, which provides a nonperturbative description of the thermodynamics and single-chain statistics in the nematic state for arbitrary degrees of nematic order. Several types of phase diagrams are predicted, with an emphasis on the limit of metastability (spinodal) associated with each phase. The shape and location of these spinodals suggest interesting scenarios for the phase transition kinetics. A large but finite persistence length is shown to significantly decrease the isotropic–nematic transition temperature relative to that for rigid rods. In the nematic state, the mean-square end-to-end distance in the parallel and perpendicular directions are governed by two separate correlation lengths. An exact relationship between these correlation lengths and the eigenvalues of the spheroidal functions is provided, which reproduces the analytical expressions predicted from earlier studies in the limit of large nematic strength. The dominant contribution to the single-chain thermodynamics is shown to arise from small amplitude undulations in the directions perpendicular to the nematic direction; the presence of hairpins, though crucial for determining the dimensions of the polymer, has insignificant consequences on the single-chain thermodynamics.
Equilibration of long chain polymer melts in computer simulations
Authors: Auhl Rolf , Everaers Ralf , Grest Gary S. , Kremer Kurt , Plimpton Steven J.
Published: December 2003

Several methods for preparing well equilibrated melts of long chains polymers are studied. We show that the standard method in which one starts with an ensemble of chains with the correct end-to-end distance arranged randomly in the simulation cell and introduces the excluded volume rapidly, leads to deformation on short length scales. This deformation is strongest for long chains and relaxes only after the chains have moved their own size. Two methods are shown to overcome this local deformation of the chains. One method is to first pre-pack the Gaussian chains, which reduces the density fluctuations in the system, followed by a gradual introduction of the excluded volume. The second method is a double-bridging algorithm in which new bonds are formed across a pair of chains, creating two new chains each substantially different from the original. We demonstrate the effectiveness of these methods for a linear bead spring polymer model with both zero and nonzero bending stiffness, however the methods are applicable to more complex architectures such as branched and star polymer.
Reactivity of restatoms and adatoms in Cl adsorption at a Si(111)-7×7 surface
Authors: Shudo K. , Washio H. , Tanaka M.
Published: December 2003

We observed the adsorption process of chlorine at a Si(111)-7×7 surface by means of surface differential reflectance spectroscopy, which can evaluate the surface densities of both the Cl-terminated dangling bonds at the Si adatoms and the broken back bonds beneath. The total coverage of chlorine was obtained with thermal desorption spectroscopy. The adsorption process on the dangling bonds at the restatoms was concluded to proceed as follows: Chlorine has no preference between the native dangling bond of the adatom and that of the restatom for being adsorbed at the initial phase. After all the native dangling bonds have substantially been terminated, back bond breakage occurs. There are two stages in the breaking process. Until 65% of the breakable back bonds are cut, the chlorine atom tends to break a further back bond rather than to terminate the dangling bond at the restatom that emerges after the back bond breakage. On a more Cl-dense surface, this tendency is reversed; the Cl atoms to be adsorbed will break a further back bond at only 23% probability, and will terminate the dangling bonds that have emerged on the restatoms at 77% probability. These results indicate that the emerging dangling bonds should be taken into consideration, as well as the native dangling bonds at the restatom, in the overall mechanism of the surface reaction.
Long-range order of dipolar fluids
Authors: Morozov Konstantin I.
Published: December 2003

The general theory of the isotropic–ferromagnetic transition of dipolar hard sphere fluids is considered on the base of the formalism of the direct correlation function. The equation of the Lovett–Mou–Buff–Gubbins type, described the one-particle orientational distribution, is generalized on a case of long-range interparticle potential and ellipsoidal sample shape. The exact criterion of the isotropic–ferromagnetic transition is found as a result of bifurcation analysis. It is shown that the formalism reduces to all known approximations in partial cases. A new expression for the susceptibility is obtained analytically within the framework of the generalized mean-spherical approximation. It is demonstrated that the susceptibility diverges at appropriate values of short-range correlations. In this way, the decisive role of short-range correlations outside the core in the transition appearance is established whereas the long-range correlations are found to be of secondary importance. The qualitative estimations show that the short-range pair correlations in dipolar fluids are most likely antiferromagnetic in character so as the transition to ferromagnetic liquid becomes questionable.
Long-time tails in the dynamics of Rouse polymers
Authors: Tothova J. , Lisy V. , Zatovsky A. V.
Published: December 2003

The Rouse model of the dynamics of polymers in solution is generalized by taking into account the hydrodynamic memory during the motion of the polymer monomers in an incompressible liquid. This leads to new peculiarities in the behavior of the time correlation functions describing the polymer motion. It is demonstrated by the appearance of long-time tails of these functions. The mean square displacement of the polymer coil as a whole contains, in addition to the Einstein term ∼t, other contributions, the leading of which is ∼t1/2. The behavior of the correlation functions for internal modes differs from the traditional exponential relaxation: as t→∞, they consist of tails decaying like t−n/2, the longest-lived being ∼t−3/2.
Structure of carbon onions and nanotubes formed by arc in liquids
Authors: Alexandrou I. , Wang H. , Sano N. , Amaratunga G. A. J.
Published: December 2003

Since carbon nanotubes and onions were discovered, many methods have been proposed for their production. For applications the main requirements are low capital cost, high purity of the produced material, simplicity of technique, and its potential for scale up. Recently a cathodic arc between two graphitic electrodes immersed in liquids has been demonstrated to be a simple method to produce carbon nanoparticles such as nanotubes and onions. In this paper high-resolution transmission electron microscopy is employed to examine the shape of the nanoparticles and the purity of the final material produced under various conditions. In this study we have used an arc discharge in two different liquids—liquid nitrogen and distilled water—and we have changed the grade of the carbon electrodes. The variety in structure, shape, and size of the produced particles is discussed in line with a model proposed to describe the physical process.
Classical density functional theory of orientational order at interfaces: Application to water
Authors: Jaqaman Khuloud , Tuncay Kagan , Ortoleva Peter J.
Published: December 2003

A classical density functional formalism has been developed to predict the position-orientation number density of structured fluids. It is applied to the liquid–vapor interface of pure water, where it consists of a classical term, a gradient correction, and an anisotropic term that yields order through density gradients. The model is calibrated to predict that water molecules have their dipole moments almost parallel to a planar interface, while the molecular plane is parallel to it on the liquid side and perpendicular to it on the vapor side. For a planar interface, the surface tension obtained is twice its experimental value, while the surface potential is in qualitative agreement with that calculated by others. The model is also used to predict the orientation of water molecules near the surface of droplets, as well as the dependence of equilibrium vapor pressure around them on their size.
Hydrolysis of a two-membered silica ring on the amorphous silica surface
Authors: Du Mao-Hua , Kolchin Andrew , Cheng Hai-Ping
Published: December 2003

We have combined density functional theory (DFT) with classical interatomic potential functions to model hydrolysis of amorphous silica surfaces. The water–silica interaction is described by DFT with incorporation of a long-range elastic field described by classical interatomic potentials. Both physisorption and chemisorption of water on a surface site, known as the two-membered silica ring, are studied in detail. The hybrid quantum-mechanical and classical mechanical method enables more realistic treatment of chemical processes on an extended surface than previous methods. We have studied cooperative events in the hydrolytic reactions and discovered a new reaction pathway that involves a double proton transfer process. In addition, the evaluation of the total energy in a hybrid quantum-mechanical and classical mechanical system is discussed.
Classification of secondary relaxation in glass-formers based on dynamic properties
Authors: Ngai K. L. , Paluch M.
Published: December 2003

Dynamic properties, derived from dielectric relaxation spectra of glass-formers at variable temperature and pressure, are used to characterize and classify any resolved or unresolved secondary relaxation based on their different behaviors. The dynamic properties of the secondary relaxation used include: (1) the pressure and temperature dependences; (2) the separation between its relaxation time τβ and the primary relaxation time τα at any chosen τα; (3) whether τβ is approximately equal to the independent (primitive) relaxation time τ0 of the coupling model; (4) whether both τβ and τ0 have the same pressure and temperature dependences; (5) whether it is responsible for the “excess wing” of the primary relaxation observed in some glass-formers; (6) how the excess wing changes on aging, blending with another miscible glass-former, or increasing the molecular weight of the glass-former; (7) the change of temperature dependence of its dielectric strength Δεβ and τβ across the glass transition temperature Tg; (8) the changes of Δεβ and τβ with aging below Tg; (9) whether it arises in a glass-former composed of totally rigid molecules without any internal degree of freedom; (10) whether only a part of the molecule is involved; and (11) whether it tends to merge with the α-relaxation at temperatures above Tg. After the secondary relaxations in many glass-formers have been characterized and classified, we identify the class of secondary relaxations that bears a strong connection or correlation to the primary relaxation in all the dynamic properties. Secondary relaxations found in rigid molecular glass-formers belong to this class. The secondary relaxations in this class play the important role as a precursor or local step of the primary relaxation, and we propose that only they should be called the Johari–Goldstein β-relaxation.
Negative ion formation in electron-stimulated desorption of CF2Cl2 coadsorbed with polar NH3 on Ru(0001)
Authors: Solovev S. , Kusmierek D. O. , Madey T. E.
Published: December 2003

Photon-induced dissociation of CF2Cl2 (freon-12) in the stratosphere contributes substantially to atmospheric ozone depletion. We report recent results on dissociation and negative ion formation in electron-stimulated desorption (ESD) of CF2Cl2 on Ru(0001), when CF2Cl2 is coadsorbed with a polar molecule (NH3), for electron energies ranging from 50 to 300 eV. Two different time-of-flight methods are used in this investigation: (a) an ESD ion angular distribution detector with wide collection angle and (b) a quadrupole mass spectrometer with narrow collection angle and high mass resolution. Many negative ESD fragments are seen (F−,Cl−,FCl−,CF−,F2−, and Cl2−), whose intensities depend on the surface preparation. Using both detectors we observe a giant enhancement of Cl− and F− yields for ESD of CF2Cl2 coadsorbed with ∼1 ML of NH3; this enhancement (>103 for Cl−) is specific to certain ions, and is attributed to an increased probability of dissociative electron attachment due to “trapped” low-energy secondary electrons, i.e., precursor states of the solvated electron in NH3. In further studies, the influence of polar NH3 spacer layers (1–10 ML) on ESD of top-layer CF2Cl2 is determined, and compared with thick films of condensed CF2Cl2. The magnitudes and energy dependences of the Cl− yields are different in these cases, due to several contributing factors.
The effects of solute–solvent electrostatic interactions on solvatochromic shifts in supercritical CO2
Authors: Nugent Severin , Ladanyi Branka M.
Published: December 2003

Solvent clustering around attractive solutes is an important feature of supercritical solvation. We examine here the effects of the local density enhancement on solvatochromic shifts in electronic absorption and emission spectra in supercritical CO2. We use molecular dynamics (MD) simulation to study the spectral line shifts for model diatomic solutes that become more polar upon electronic excitation. The electronic transition is modeled as either a change from a quadrupolar to a dipolar solute charge distribution or as an increase in the magnitude of the solute dipole. Our main focus is on the density dependence of the line shifts at 320 K, which corresponds to about 1.05 times the solvent critical temperature, Tc, but results for higher temperatures are also obtained in order to determine the behavior of the line shifts in the absence of local density enhancement. We find that the extent of local density enhancement at 1.05Tc is strongly correlated with solute–solvent electrostatic attraction and that the density dependence of the emission line shifts resembles the behavior of the effective local densities, ρeff, obtained from the first-shell coordination numbers. The differences that are seen are shown to be due to solute–solvent orientational correlations which provide an additional source of enhancement for electrostatic solvation energies and spectral line shifts.
Variational transition state theory calculations for the rate constants of the hydrogen scrambling and the dissociation of BH5 using the multiconfiguration molecular mechanics algorithm
Authors: Kim Kyung Hyun , Kim Yongho
Published: December 2003

The BH5 molecule contains a weak two-electron-three-center bond and it requires extremely high level of theories to calculate the energy and structure correctly. The potential energy of the hydrogen scrambling in BH5 has been generated by the multiconfiguration molecular mechanics algorithm with 15 high-level Shepard interpolation points, which would be practically impossible to obtain otherwise. The high-level interpolation points were obtained from the multicoefficient correlated quantum mechanical methods. The more high-level points are used, the better the shape of the potential energy surface. The rate constants are calculated using the variational transition state theory including multidimensional tunneling approximation. The potential energy curve for the BH5 dissociation has also been calculated, and the variational transition state was located to obtain the dissociation rate constants. Tunneling is very important in the scrambling, and there is large variational effect on the dissociation. The rate constants for the scrambling and the dissociation are 2.1×109 and 2.3×1012 s−1 at 300 K, respectively, which suggests that the dissociation is three orders of magnitude faster than the scrambling.
Effects of pulse strength, width, and sample spinning speed on the spectral spin diffusion of multiquantum coherences of spin-32 quadrupolar nuclei
Authors: Liu Zhihong , Deng Feng , Ding Shangwu
Published: December 2003

The effects of radio-frequency pulse strength, width, and sample spinning speed on the spin-diffusion spectrum of half-integer quadrupolar spins in solids have been studied by theoretical, numerical, and experimental investigations. It is revealed that the line shape of the cross peaks changes nonmonotonically with respect to the change of pulse strength, pulse width, or sample spinning speed. It is also found that the sample spinning speed has much more pronounced influence on the spin diffusion spectral line shape. In many cases of practical importance, the effect of sample spinning must be included in spectral analysis, in contrast to the practice of previous studies. Moreover, this effect can be exploited to further improve the precision in the determination of relative orientation of the electric-field gradient tensors of the exchange partners.
From transition paths to transition states and rate coefficients
Authors: Hummer Gerhard
Published: December 2003

Transition states are defined as points in configuration space with the highest probability that trajectories passing through them are reactive (i.e., form transition paths between reactants and products). In the high-friction (diffusive) limit of Langevin dynamics, the resulting ensemble of transition states is shown to coincide with the separatrix formed by points of equal commitment (or splitting) probabilities for reaching the product and reactant regions. Transition states according to the new criterion can be identified directly from equilibrium trajectories, or indirectly by calculating probability densities in the equilibrium and transition-path ensembles using umbrella and transition-path sampling, respectively. An algorithm is proposed to calculate rate coefficients from the transition-path and equilibrium ensembles by estimating the frequency of transitions between reactants and products.
Fluctuation conductivity and critical currents in YBCO films
Authors: Solovjov A. L. , Svetlov V. N. , Stepanov V. B. , Dmitriev V. M.
Published: December 2003

A comparative analysis of the results obtained in measurements of the fluctuation conductivity and of the critical current density jc(T) in YBa2Cu3O7−y films containing various numbers of defects is carried out for the first time. It is found that the value and temperature dependence of the fluctuation conductivity are interrelated with the values and temperature dependences of the critical current and resistivity of the samples. It follows from measurements of the fluctuation conductivity that the variation of these temperature dependences is directly related to the variation of the number of defects and, hence, the number of pinning centers in the films studied. It is shown that in films containing practically no defects the jc(T) curve completely follows a model giving jc(T) in high-Tc superconductors with low-angle grain boundaries between crystallization blocks. On the other hand, if the sample has a specific defect structure that is formed under certain conditions in c-oriented epitaxial high-Tc superconducting films, one observes a sharp increase in jc and in the slope of the jc(T) curve.
Antiferromagnetic correlations in superconducting YBa2Cu3O6+x samples from optical absorption data; comparison with the results of neutron and muon experiments
Authors: Samovarov V. N. , Vakula V. L. , Libin M. Yu.
Published: December 2003

Narrow-band spectral features demonstrating a high sensitivity to the development of the pseudogap state are detected in metallic films of YBa2Cu3O6+x with Tc≈51 and 74 K. Attention is focused on the temperature behavior of the exciton–bimagnon band A+3J (≈2.15 eV) and the exciton–two-magnon band A+4J (≈2.28 eV); these bands arise as a result of phase separation into insulating and metallic regions. By comparing the optical results with published data on the temperature behavior of the muon depolarization rate and the integrated intensity of the (π,π) magnetic resonance it is shown for the first time that all three independent techniques give the same observed temperature dependence f(T/Tc) of measured quantities both in the normal and superconducting states. The established correlation of the optical, neutron, and muon data is analyzed from the standpoint of the formation of stripe ordering and the compatibility of antiferromagnetic order and superconductivity.
Phase diagram of a spin-1 non-Heisenberg antiferromagnet
Authors: Fridman Yu. A. , Klevets Ph. N. , Spirin D. V.
Published: December 2003

The phase transitions in temperature of an anisotropic antiferromagnet with a biquadratic exchange interaction are investigated. The conditions for realization of a quadrupolar phase in the study are determined, and the critical temperatures of the transition to the paramagnetic phase are found. The phase diagram are constructed for different relations among the material constants.
High-temperature quantum size oscillations of the conductivity in thin metal films
Authors: Kozlov I. V.
Published: December 2003

The longitudinal electrical conductivity of a thin conducting film in the presence of electron and hole groups of charge carriers is calculated by the Kubo method under conditions such that the carrier motion is spatially quantized. It is shown that in the case of elastic scattering on impurities there are quantum high-temperature oscillations of the conductivity which are relatively insensitive to the temperature smearing of the Fermi level.
Rate of exciton scattering and exciton absorption in crystals with LO-phonon dispersion
Authors: Grigorchuk N. I.
Published: December 2003

The Ridley–Babiker relation for the LO-phonon energy is used to obtain an exact formula for the exciton scattering rate in the one-phonon approximation. The effect of LO-phonon dispersion on the frequency and temperature dependence of the exciton attenuation is studied theoretically for models of polar crystals with large and small exciton radii. It is shown that the role of LO-phonon dispersion is essentially different in processes with phonon absorption and phonon emission. For some fixed frequency interval, it is established that the effect of dispersion on exciton scattering changes with temperature. The TlBr and ZnS crystals are used as models for illustrations.
Time series analysis of collective motions in proteins
Authors: Alakent Burak , Doruker Pemra , Çamurdan Mehmet C.
Published: December 2003

The dynamics of α-amylase inhibitor tendamistat around its native state is investigated using time series analysis of the principal components of the Cα atomic displacements obtained from molecular dynamics trajectories. Collective motion along a principal component is modeled as a homogeneous nonstationary process, which is the result of the damped oscillations in local minima superimposed on a random walk. The motion in local minima is described by a stationary autoregressive moving average model, consisting of the frequency, damping factor, moving average parameters and random shock terms. Frequencies for the first 50 principal components are found to be in the 3–25 cm−1 range, which are well correlated with the principal component indices and also with atomistic normal mode analysis results. Damping factors, though their correlation is less pronounced, decrease as principal component indices increase, indicating that low frequency motions are less affected by friction. The existence of a positive moving average parameter indicates that the stochastic force term is likely to disturb the mode in opposite directions for two successive sampling times, showing the modes tendency to stay close to minimum. All these four parameters affect the mean square fluctuations of a principal mode within a single minimum. The inter-minima transitions are described by a random walk model, which is driven by a random shock term considerably smaller than that for the intra-minimum motion. The principal modes are classified into three subspaces based on their dynamics: essential, semiconstrained, and constrained, at least in partial consistency with previous studies. The Gaussian-type distributions of the intermediate modes, called “semiconstrained” modes, are explained by asserting that this random walk behavior is not completely free but between energy barriers.
High-level ab initio computations of structures and interaction energies of naphthalene dimers: Origin of attraction and its directionality
Authors: Tsuzuki Seiji , Honda Kazumasa , Uchimaru Tadafumi , Mikami Masuhiro
Published: December 2003

The intermolecular interaction energies of naphthalene dimers have been calculated by using an aromatic intermolecular interaction model (a model chemistry for the evaluation of intermolecular interactions between aromatic molecules). The CCSD(T) (coupled cluster calculations with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the second-order Møller–Plesset perturbation interaction energy near saturation and the CCSD(T) correction term obtained using a medium-size basis set. The estimated interaction energies of the set of geometries explored in this work show that two structures emerge as being the lowest energy, and may effectively be considered as isoenergetic on the basis of the errors inherent in out extrapolation procedure. These structures are the slipped-parallel (Ci) structure (−5.73 kcal/mol) and the cross (D2d) structure (−5.28 kcal/mol). The T-shaped (C2v) and sandwich (D2h) dimers are substantially less stable (−4.34 and −3.78 kcal/mol, respectively). The dispersion interaction is found to be the major source of attraction in the naphthalene dimer. The electrostatic interaction is substantially smaller than the dispersion interaction. The large dispersion interaction is the cause of the large binding energies of the cross and slipped-parallel dimers.
Design and application of a multicoefficient correlation method for dispersion interactions
Authors: Giese Timothy J. , York Darrin M.
Published: December 2003

A new multicoefficient correlation method (MCCM) is presented for the determination of accurate van der Waals interactions. The method utilizes a novel parametrization strategy that simultaneously fits to very high-level binding, Hartree–Fock and correlation energies of homo- and heteronuclear rare gas dimers of He, Ne, and Ar. The decomposition of the energy into Hartree–Fock and correlation components leads to a more transferable model. The method is applied to the krypton dimer system, rare gas–water interactions, and three-body interactions of rare gas trimers He3, Ne3, and Ar3. For the latter, a very high-level method that corrects the rare-gas two-body interactions to the total binding energy is introduced. A comparison with high-level CCSD(T) calculations using large basis sets demonstrates the MCCM method is transferable to a variety of systems not considered in the parametrization. The method allows dispersion interactions of larger systems to be studied reliably at a fraction of the computational cost, and offers a new tool for applications to rare-gas clusters, and the development of dispersion parameters for molecular simulation force fields and new semiempirical quantum models.