bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of linear and nonlinear optical response of model Langmuir–Blodgett films
Authors: Munn R. W. , Shabat M. M.
Published: July 2002

Linear and nonlinear optical response are calculated for lattice models of Langmuir–Blodgett films. Molecules are treated as a set of five point submolecules, when interactions outside a layer are negligible. Molecular polarizabilities and hyperpolarizabilities are treated as axially symmetric with an anisotropy of 5:1. Local fields, linear susceptibilities, refractive indices, and quadratic and cubic nonlinear susceptibilities are calculated. All the properties depend markedly on molecular tilt away from the normal to the film, but very little on distortions from axial packing in the film. One-dimensional approximations to the hyperpolarizabilities tend to underestimate significantly the magnitude of the nonlinear susceptibility coefficients. Cascading terms quadratic in the first hyperpolarizability augment the cubic susceptibility substantially from the direct terms linear in the second hyperpolarizability. Extensions to more realistic models are outlined.
Wetting properties of simple binary mixtures and systems with one self-associating component
Authors: Pérez C. , Roquero P. , Talanquer V.
Published: July 2002

The presence of additional chemical equilibria in an otherwise simple system can induce unexpected phase behavior. We analyze the effect of this phenomenon on the wetting properties of binary mixtures with one self-associating component. As a first step we characterize the global wetting phase diagram of a mean-field lattice model for a simple binary system. We evaluate its reliability in generating an adequate topological description of bulk critical and wetting transition manifolds. These results serve as a basis to study the influence of self-association. We find that under appropriate conditions the appearance of new species in solution can lead to irregular wetting behavior such as ‘‘reentrant’’ wetting and ‘‘dewetting.’’
Binary hard sphere, hard ellipsoid liquid crystal mixtures
Authors: Samborski Adam , Evans Glenn T.
Published: July 2002

A phase diagram for a fluid comprised of hard ellipsoids and hard spheres has been calculated on the basis of a density functional theory. Needed in the theory was the direct correlation function and this was abstracted from previous equations of state of fluid mixtures of hard convex bodies. Orientational correlations in the direct correlation function were implemented only to the extent that the pair excluded volume was calculated exactly. The hard sphere, hard ellipsoid fluid mixture did not undergo phase separation in the isotropic phase. However, after the isotropic to nematic phase transition occurred, the nematic phase was enriched in hard ellipsoids. The isotropic to nematic phase transition, with its entrained phase separation, was found to be a strongly first order process.
Comparison of the open-shell state-universal and state-selective coupled-cluster theories: H4 and H8 models
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

The spin nonadapted or partially spin adapted spin orbital multireference (MR) state-universal (SU) or Hilbert space coupled cluster (CC) method is analyzed from the viewpoint of possible spin symmetry breaking or spin contamination and contrasted with the recently introduced state specific (SS), automatically spin adapted, spin-free unitary group based CC method. It is shown that the partial spin-adaptation of the two-determinantal MR SUCC approach can only be effective if the complementary singlet and triplet states have a similar structure in the sense that their weights at corresponding singlet and triplet configurations, having the same orbital occupancies, are identical or nearly identical. The performance of both SS and SU CC methods, and of corresponding CI method (all truncated at the double excitation level) for the lowest lying open-shell singlet and triplet states of the H4 and H8 model systems was examined, showing the superiority of the unitary group based CC approach in all studied aspects.
The Hessian biased singular value decomposition method for optimization and analysis of force fields
Authors: Dasgupta Siddharth , Yamasaki Terumasa , Goddard William A.
Published: July 2002

We present methodology (HBFF/SVD) for optimizing the form and parameters of force fields (FF) for molecular dynamics simulations through utilizing information about properties such as the geometry, Hessian, polarizability, stress (crystals), and elastic constants (crystals). This method is based on singular value decomposition (SVD) of the Jacobian describing the partial derivatives in various properties with respect to FF parameters. HBFF/SVD is effective for optimizing the parameters for accurate FFs of organic, inorganic, and transition metal compounds. In addition it provides information on the validity of the functional form of the FF for describing the properties of interest. This method is illustrated by application to organic molecules (CH2O, C2H4, C4H6, C6H8, C6H6, and naphthalene) and inorganic molecules (Cl2CrO2 and Cl2MoO2).
Condensation of supersaturated vapors. X. Pressure and nonideal gas effects
Authors: Fisk Jeffery A. , Katz Joseph L.
Published: July 2002

Calculations of temperatures and supersaturations in nucleation experiments often include the assumption that all gases are ideal and they ignore the increase in vapor pressure due to the presence of noncondensible gases. Regardless of experimental technique, such assumptions can lead to substantial errors when comparing experiment to theory, especially when included in only one or the other. To demonstrate these effects, the procedures for calculating the temperatures and supersaturations in thermal diffusion cloud chambers are examined in detail. Nonideal gas effects do alter them; however, these effects also alter the rates calculated by nucleation theory and the effect is larger on nucleation theory than on the analysis of the experimental data. Due to the presence of noncondensible gases there also is a Poynting type effect, but it very nearly cancels in thermal diffusion cloud chambers when included consistently in both theory and experiment. An additional effect which arises only in nucleation theory is the nonzero compressibility of the critical nuclei. The net result (for n-nonane), when these effects are included consistently in both experiment and theory, is to reduce the temperature dependence of the multiplicative correction to theory necessary for agreement with experiment. None of these effects account for the previously observed apparent decrease in nucleation rate with increasing total pressure. Also critically examined are the uncertainties in the transport properties used to calculate the chamber profiles. Of particular interest is the uncertainty in the thermal diffusion factor which changes systematically with composition and thus with total pressure.
Electronic excitation by electron impact of the O2 molecule physisorbed on a metal
Authors: Bahrim B. , Teillet-Billy D. , Gauyacq J. P.
Published: July 2002

The electronic excitation process by low energy electron impact is studied theoretically for the case of O2 molecules physisorbed on a model jellium metal (Al). The spin forbidden excitations to the a 1Δg and b 1Σ+g states are considered. Only the resonant contribution corresponding to the 2Πg O−2 resonance, which dominates at low energy in the free molecule is included in the present work. The characteristics of this resonant process involving a resonant state hidden below the excitation threshold are analyzed; in contrast with the free molecule case, a very important excitation process occurs below the energy threshold. The dependence of the excitation process on the symmetries of the problem is also discussed.
Orientational correlations and entropy in liquid water
Authors: Lazaridis Themis , Karplus Martin
Published: July 2002

The molecular pair correlation function in water is a function of a distance and five angles. It is here separated into the radial distribution function (RDF), which is only a function of distance, and an orientational distribution function (ODF), which is a function of the five angles for each distance between the molecules. While the RDF can be obtained from computer simulations, this is not practical for the ODF due to its high dimensionality. Two approaches for obtaining an approximation to the ODF are introduced. The first uses a product of one- and two-dimensional marginal distributions from computer simulations. The second uses the gas-phase low-density limit as a reference and applies corrections based on (a) the orientationally averaged interactions in the liquid calculated by simulations, and (b) the observed differences in the one- and two-dimensional marginal distributions in the gas and in the liquid. The site superposition approximation was also tested and found to be inadequate for reproducing the orientationally averaged interaction energy and the angular distributions obtained from the simulations. The two approximations to the pair correlation function are employed to estimate the contribution of two-particle correlations to the excess entropy of TIP4P water. The calculated value is comparable to the excess entropy of TIP4P water estimated by other methods and to the experimental excess entropy of liquid water. More than 90% of the orientational part of the excess entropy is due to correlations between first neighbors. The change in excess entropy with temperature gives a value for the heat capacity that agrees within statistical uncertainty with that obtained from the change in energy with temperature and is reasonably close to the experimental value for water. The effect of pressure on the entropy was examined and it was found that increase in the pressure (density) causes a reduction of orientational correlations, in agreement with the idea of pressure as a ‘‘structure breaker’’ in water. The approach described here provides insight concerning the nature of the contributions to the excess entropy of water and should be applicable to other simple molecular fluids.
Critical evolution of hot van der Waals droplets
Authors: Kondratyev V. N. , Lutz H. O. , Ayik S.
Published: July 2002

The dynamical evolution and fragmentation of a hot finite van der Waals system is investigated in a classical molecular dynamics approach. It is demonstrated that, under certain conditions determined mainly by the initial deposited energy, the fragmentation of rare-atom clusters exhibit a critical behavior. This behavior leaves its signatures in the mass distributions and the conditional moments, as well as the scaled factorial moments of the produced fragments. The connection between the observed critical behavior and the second-order liquid-gas phase transition is investigated by employing Fisher’s droplet model for the average fragment mass distributions, and performing Campi and intermittency analysis for the correlation and fluctuation properties of the fragment mass distributions. The size dependence of the signatures of critical behavior is discussed.
Uniaxial deformation of bridging polymer systems: A Monte Carlo study
Authors: Hölzl Th. , Wittkop M. , Kreitmeier S. , Göritz D.
Published: July 2002

A new approach for the equilibrium deformation of three-dimensional chains, that are bigrafted to parallel planes is presented. The underlying lattice Monte Carlo algorithm is the bond fluctuation model. In addition to the excluded-volume interaction of this a priori athermal algorithm, we incorporated external potentials in order to enable direct detection of forces. The whole deformation process is split up into a series of separate steps. Each step consists of a generation process and subsequent relaxation procedures. Stress and strain are simultaneously calculated as time-averaged quantities of sufficiently equilibrated systems. Stress–strain relations ranging from compression to the highly stretched regime were simulated by variation of both chain length, N, and grafting density, σ. In the high-density limit the simulation data agree perfectly with a simple one-dimensional theory. The N and σ dependency of the distance, h0(N,σ), of grafting planes at vanishing force is in qualitative agreement with theoretical predictions for an intermediate regime of σ. The simulated force–length relations are in satisfactory agreement with current scaling predictions.
Dynamic light scattering from binary mixtures of polyelectrolytes. I. Influence of mixing on the fast and slow polyelectrolyte mode behavior
Authors: Sedlák Marián
Published: July 2002

Dynamic light scattering experiments were performed on salt-free aqueous binary mixtures of sodium poly(styrene sulfonate) with different molecular weights. Three diffusive modes were observed and investigated as a function of the mixture composition, total polymer concentration, and molecular weights of the components of the mixture. The fast and slow polyelectrolyte modes, which are typical for binary polyelectrolyte solutions, are observed also in binary mixtures and maintain their basic characteristics. The fast diffusive mode is independent of the total polymer concentration, mixture composition, and molecular weights of the two components, in agreement with results obtained on binary solutions. Only one slow diffusive mode is observed in binary mixtures, in spite of the fact that polyions with appreciably different molecular weights in binary solutions give slow modes on appreciably different time scales. Therefore, the slow polyelectrolyte mode cannot be identified with any single-chain property. Oppositely, our results confirm “multichain character” of the processes behind the slow mode mechanism. In addition to the fast and slow polyelectrolyte modes, a new mode, named the “medium mode,” was observed. The investigation of the medium mode is presented in the subsequent paper in this issue of the journal.
Photon emission from gas phase fullerenes excited by 193 nm laser radiation
Authors: Heszler P. , Carlsson J. O. , Demirev P.
Published: July 2002

Continuous, black-body-type light emission is observed upon irradiation of gas phase C60 and C70 by 193 nm ArF excimer laser at fluences from 3 to 80 mJ/cm2 in Ar and He ambient. Cluster temperatures are estimated by calibrating the detection system against a tungsten filament and applying Wien’s displacement law. Time-resolved spectroscopic measurements show that the initial internal temperature of the irradiated fullerenes (around 2800 K) decreases linearly, while the emitted light intensity decreases exponentially with time, respectively. Excited C60 and C70 molecules are predominantly cooled via inelastic collisions with noble gas atoms above ∼0.5 mbar ambient pressure and below ∼2800 K temperature. The quenching rates are ∼7.1 bar−1 s−1 for C60, and ∼6.4 bar−1 s−1 for C70 in Ar, and 81 bar−1 s−1 for C60 in He ambient, respectively, determined from Stern–Vollmer type relations. The inelastic quenching cross section for He gas (∼4.4×10−23 cm2) is ∼3.7 times higher than for Ar. This observation may provide further insight on the mechanisms of fullerene synthesis by coalescence of hot carbon vapor in a noble gas atmosphere. At laser fluence above 30–40 mJ/cm2 the fullerene temperature saturates at ∼2800 K, indicating a kind of phase-transition—“boiling” of the excited fullerenes. The cluster temperature stabilizes by other cooling mechanisms like electron and/or C2 “evaporation” (i.e., ionization and/or fragmentation). The unperturbed (i.e., extrapolated to zero ambient pressure) lifetime of the temperature-stabilized species is 100±25 μs for C60 and 44±4 μs for C70, respectively, at high (80 mJ/cm2) laser fluences. The measured two- and three-photon multiplicities of the excitation at low laser fluences (<15 mJ/cm2) are in good agreement with the observed cluster temperatures.
Theoretical study of the molecular complexes of Fe+ with small alkanes
Authors: Hendrickx M. , Gong K. , Vanquickenborne L.
Published: July 2002

Ab initio calculations have been performed in order to study the binding of a singly charged iron cation with methane, ethane, and propane. All three adducts are predicted to possess a quartet ground state with the sextet equilibrium structures lying just slightly higher in energy. The calculated binding energies are found to increase with increasing size of the alkane and amount to 13.1, 16.7, and 21.9 kcal/mol for the methane, ethane, and propane adduct, respectively. These values confirm the experimental data available for these molecular complexes. For all three adducts the quartet and sextet potential energy surfaces are found to intersect at low energies with respect to the quartet ground state and the sextet equilibrium structures.
Gyroid versus double-diamond in ABC triblock copolymer melts
Authors: Matsen M. W.
Published: July 2002

Monodisperse melts of ABC linear triblock copolymer are examined using self-consistent field theory (SCFT). Our study is restricted to symmetric triblocks, where the A and C blocks are equal in size and the A/B and B/C interactions are identical. Furthermore, we focus on the regime where B forms the majority domain. This system has been studied earlier using density functional theory (DFT), strong-segregation theory (SST), and Monte Carlo (MC), and it corresponds closely to a series of isoprene–styrene–vinylpyridine triblocks examined by Mogi and co-workers. In agreement with these previous studies, we find stable lamellar, complex, cylindrical, and spherical phases. In the spherical phase, the minority A and C domains alternate on a body-centered cubic lattice. In order to produce alternating A and C domains in the cylinder phase, the melt chooses a tetragonal packing rather than the usual hexagonal one. This amplifies the packing frustration in the cylinder phase, which results in a large complex phase region. Contrary to the previous evidence that the complex phase is double-diamond, we predict the gyroid morphology. The earlier theoretical results are easily rationalized and the experimental data are, in fact, more consistent with gyroid.
Phase-modulated nutation nuclear quadrupole resonance spectroscopy
Authors: Vaca Chávez F. , Casanova F. , Robert H. , Pusiol D.
Published: July 2002

Nutation nuclear quadrupole resonance spectroscopy enables one to determine the asymmetry parameter on powder samples. The method relies on two-dimensional methodology: The first period is the duration of the radio frequency exciting pulse and the second dimension is the free-evolution period of the quadrupolar nucleus. Varying lengths of the radio frequency pulse induce an amplitude modulation of the free-induction decay signals which is characteristic of the anisotropic nutation frequency distribution. Fourier analysis along the first dimension provides a powder nutation line shape that allows determination of the electric field gradient tensor’s asymmetry parameter. We describe the first application of two-dimensional phase-modulated spectroscopy to a quadrupolar nucleus at zero field. It is shown that a phase-modulated variant of the nutation spectroscopy is feasible in the pure quadrupole regime and it provides a gain in the signal-to-noise ratio compared to the amplitude-modulated method.
Electron–ion coincidence measurements: The neutral dissociation plus excitation cross section for N2
Authors: Mi L. , Bonham R. A.
Published: July 2002

Pulsed electron beam time-of-flight measurements of the electron energy loss spectrum of molecular nitrogen at scattering angles of 28°, 45°, 71°, 112°, and 135° were made at impact energies of 24.5, 33.1, and 33.6 eV in coincidence with positive ions. The angular-dependent elastic, total inelastic, ionization, and neutral dissociation plus excitation cross sections were measured. The data were integrated over electron energy loss for each scattering angle. These angular-dependent data were then fitted with smooth analytic functions and integrated over angle to obtain individual total cross sections. All cross-section data were placed on an absolute scale by matching to total cross-section measurements reported in the literature. The angular-dependent elastic cross section and the integrated cross sections for elastic and total inelastic scattering were found to be in good agreement with literature values. Values obtained for the neutral dissociation plus excitation cross section agree with the sum of previous separate measurements of neutral dissociation and excitation. The total ionization cross sections are in fair agreement with accurate values reported in the literature.
Intensities of infrared transitions in the two-dimensional algebraic model
Authors: Oss S. , Temsamani M. Abbouti
Published: July 2002

In a previous paper,Reference 1 the Lie algebraic method was applied to two-dimensional problems, with special attention to the vibrational analysis of degenerate bending modes of linear molecules. This paper completes the two-dimensional algebraic model with the study of infrared transition intensities, whose theoretical aspects were only hinted at in the previous work.
Structures, binding energies, and spectra of isoenergetic water hexamer clusters: Extensive ab initio studies
Authors: Kim Jongseob , Kim Kwang S.
Published: July 2002

We investigated five lowest energy structures of the water hexamer (ring, book, bag, cage, and prism) using extensive ab initio calculations. High levels of theory using various basis sets were employed. On the basis of Mo/ller–Plesset second order perturbation (MP2) calculations using a large basis set [9s6p4d2 f1g/6s4p2d]+diffuse(2sp/s), the lowest energy structure with zero point energy (ZPE) correction is the cage conformer, followed by the book (within 0.1 kcal/mol) and the prism (within 0.2 kcal/mol). The spectra of the five conformers have been investigated. The predicted rotational constants and dipole moments of the cage conformer are in good agreement with the experiment [Liu et al., Nature 381, 501 (1996)] as compared to other structures. This proves that the experiment surely found the cage structure, which was first reported by one of the authors [Kim et al., Chem. Phys. Lett. 131, 451 (1986)]. However, the five structures would still be nearly isoenergetic within 0.7 kcal/mol at 0 K. Above ∼40 K, the free energy of the book is slightly lower than the cage, which might imply that the book structure would be detected. Upon deuteration, the cage structure is the lowest energy conformer, followed by two competing structures of the book and prism whose energies are only 0.2 kcal/mol higher at 0 K; above ∼55 K the book would be more populated than the cage.
Closed-form expression relating the second-order component of the density functional theory correlation energy to its functional derivative
Authors: Ivanov Stanislav , Lopez-Boada Roberto , Görling Andreas , Levy Mel
Published: July 2002

For helping to improve approximations to the density-functional exchange-correlation energy, Exc[n], and its functional derivative, the difference between the second-order component of the correlation energy, Ec(2)[n], and the integral ∫dr vc(2)([n];r)n(r), involving its functional derivative, vc(2)([n];r), is given in terms of only the occupied Kohn–Sham orbitals and the exchange potential. The quantity 2Ec(2)[n] is especially significant because it is the initial slope in the adiabatic connection formula for Exc[n]. The analytic expression for 2Ec(2)[n]−∫dr vc(2)([n];r)n(r) is obtained for any spherically symmetric two-electron test density. Numerical examples are presented.
Approximating state-space manifolds which attract solutions of systems of delay-differential equations
Authors: Roussel Marc R.
Published: July 2002

Although the theory of delay-differential equations (DDEs) is generally best set in a function space, some systems of DDEs have solutions which, after the decay of transients, lie on a low-dimensional manifold in their state space. When the delay is small, highly accurate approximations to the state-space manifold which attracts the solutions can be constructed by a simple functional equation treatment. This allows the reduction of the original system of DDEs to a smaller system of ordinary differential equations. The simplified model obtained may be used to facilitate bifurcation analysis. The method is applied to two biochemical models, namely to a delay-differential version of Michaelis–Menten kinetics (the Brown model) and to a simple inducible operon model.
Numerical study of gas–liquid nucleation in partially miscible binary mixtures
Authors: ten Wolde Pieter Rein , Frenkel Daan
Published: July 2002

We report a numerical study of homogeneous gas–liquid nucleation in a binary mixture. We study the size and the composition of the critical nucleus as a function of the composition and supersaturation of the vapor. As we make the (Lennard-Jones) mixture increasingly nonideal, we find that there is a regime where the critical nucleus is still miscible in all proportions, even though the bulk liquid phase is not. When these critical nuclei grow, their composition “bifurcates” to approach the value of one of the two bulk phases. For more strongly nonideal mixtures, the two species in the critical nucleus are no longer completely miscible: we observe droplets that are either rich in one species, or in the other. However, we do not find evidence for phase separation inside the critical nucleus—a scenario suggested by Talanquer and Oxtoby [J. Chem. Phys. 104, 1993 (1996)]. In fact, our simulations show that such demixed clusters have a higher free energy than critical nuclei that have an asymmetric composition.
Calibration and comparison of the Gaussian-2, complete basis set, and density functional methods for computational thermochemistry
Authors: Petersson G. A. , Malick David K. , Wilson William G. , Ochterski Joseph W. , Montgomery J. A. , Frisch M. J.
Published: July 2002

We have reexamined several high-accuracy Gaussian-2, complete basis set and density functional methods for computational thermochemistry (in order of increasing speed): G2, G2(MP2), CBS-Q, G2(MP2,SVP), CBS-q, CBS-4, and B3LYP/6-311+G(3df,2p). We have employed ΔfH2980 for the “extended G2 neutral test set” for this comparison. Several errors in previous studies have been corrected and experimental spin-orbit interactions have been included in all calculated atomic energies. The mean absolute deviations from experiment are 1.43, 1.76, 1.19, 1.64, 2.34, 2.66, and 3.43 kcal/mol, respectively. The maximum deviations from experiment are 10.6, 8.8, 8.1, 9.4, 11.4, 12.9, and 24.1 kcal/mol respectively. The species responsible for these maximum errors are in order: SiF4, SiF4, Cl2C=CCl2, F2C=CF2, ClF3, ClF3, and SiCl4. All seven methods have relatively large errors for bonds to halogens, but these errors are sufficiently systematic to benefit from empirical corrections. After a discussion of ill conditioning in the “bond separation reaction” implementation of isodesmic reactions, we determine “isodesmic bond additivity corrections” (BACs) for several types of bonds by least-squares fits to the heats of formation for 76 organic species with up to ten carbons and a variety of heteroatoms. The mean absolute deviations are reduced from 1.49, 1.93, 1.22, 1.53, 2.28, 3.09, and 3.45 kcal/mol to 0.55, 0.57, 0.77, 0.63, 1.03, 0.98, and 1.16 kcal/mol. The maximum errors are reduced to about 3 kcal/mol for all but the DFT method (4.2 kcal/mol). The BACs are especially useful for larger molecules with many similar bonds. For example, the CBS-Q error for Cl2C=CCl2 is reduced from 8.1 to 3.0 kcal/mol and the CBS-4 errors for benzene and naphthalene are reduced from 10.5 and 17.5 to 2.1 and 1.6 kcal/mol, respectively.
Analysis of the molecular density
Authors: Rico J. Fernández , López R. , Ramı́rez G.
Published: July 2002

The minimal deformation criterion, previously proposed for the partition of the molecular density into atomic contributions, is updated and extended. For any Gaussian basis set, these atomic contributions are expanded in a series of real spherical harmonics by radial factors. The terms with l=0 determine the spherical parts of the atomic clouds and the remaining ones, their deformations. This detailed description is complemented with a simplified representation of the molecular density in terms of atomic charges and multipoles. Moreover, these descriptions give a simple way to calculate the electrostatic potential of the molecule as well as the electrostatic interaction between molecules.
Ultraviolet-laser induced desorption of NO from the Cr2O3(0001) surface: Involvement of a precursor state?
Authors: Wilde M. , Seiferth O. , Al-Shamery K. , Freund H.-J.
Published: July 2002

NO molecules interact with the Cr2O3(0001) surface to form a chemisorption bond of 1.0 eV. At higher coverages an additional more weakly bound species appears in thermal desorption spectra with a binding energy of 0.35 eV. By infrared spectroscopy the weakly adsorbed species is identified to be an unusually strong bound NO-dimer exhibiting a weak feature at 1857 cm−1 beside the chemisorbate absorption band at 1794 cm−1. Laser induced desorption experiments performed at 6.4 eV are presented with main emphasis on the high coverage regime. The desorbing molecules are detected quantum state selectively using resonance enhanced multiphoton ionization. The desorbing molecules are strongly rotationally and vibrationally excited conform with a nonthermal excitation process. The velocity distributions of single rovibronic states of desorbing NO are bimodal and exhibit a strong coupling of rotation and translation. With increasing coverages an additional channel is observed appearing in the time-of-flight spectra of v″=0 as smoothly increasing intensity at long flight times. The numeric values of these unusually long flight times are indicative for long residence times on the surface rather than small kinetic energies. The desorption efficiencies weakly depend on the concentration and vibrational state ranging from (2.0±0.3)×10−17 cm2 at low coverages to (1.0±0.4)×10−17 cm2 at high coverages for v″=0. The intensity of the desorption signal per laser pulse only increases proportional to the chemisorbate coverage. The data are interpreted assuming the dimers to act as extrinsic precursors within the desorption process.
The structure of fluid argon from high-pressure neutron diffraction and ab initio molecular dynamics simulations
Authors: Pfleiderer Till , Waldner Isabella , Bertagnolli Helmut , Tödheide Klaus , Kirchner Barbara , Huber Hanspeter , Fischer Henry E.
Published: July 2002

The structure of supercritical argon at 350 K over a broad range of density (pressures between 5 to 90 MPa) is determined experimentally by high-pressure neutron diffraction, and theoretically from quantum chemically calculated ab initio pair potentials applied in molecular dynamics simulations. The origins of small discrepancies between theory and experiment are discussed. Together with previous results at 85 K in the liquid state, the overall agreement between theory and experiment, over a wide range of state points, is found to be quite good. This is remarkable, as no empirical parameters are used for the calculations.
Specific heat of endohedral and higher fullerene thin films
Authors: Allen K. , Hellman F.
Published: July 2002

Using a novel microcalorimeter, we have performed the first specific heat measurements of C84, Sc2@C84, C82, and La@C82 (10–300 K). We analyze these results using a framework based on the phonon modes in these materials. C84 compares qualitatively to C60 with a clear separation between inter- and intraball modes, although the interball modes are significantly softer in C84. For Sc2@C84 the added optical modes due to the metal atoms are high-energy Einstein modes comparable to the on-ball modes. Thus, the specific heat of Sc2@C84 is very similar to that of C84; and likewise, the specific heat of La@C82 resembles that of C82. Remarkably, however, C82 contrasts sharply with the other empty fullerenes in that it shows no separation of energy levels between inter- and intraball modes. We speculate about possible causes of this anomalous behavior.
Structural evolution of W nano clusters with increasing cluster size
Authors: Oh S. J. , Huh S. H. , Kim H. K. , Park J. W. , Lee G. H.
Published: July 2002

We have recorded the x-ray diffraction (XRD) patterns of nanometer-size W metal clusters prepared at different average cluster sizes. Nanometer-size W metal clusters were produced through a collision induced clustering mechanism of W metal atoms generated by decomposing W(CO)6 vapors. The XRD patterns clearly showed that structure changed from amorphous→face- centered-cubic (fcc)→body-centered-cubic (bcc) with increasing average cluster size. This implies that W metal clusters do not simply approach the bulk bcc structure but pass through an intermediate fcc structure before they reach the bulk structure, as predicted by Tománek, Mukherjee, and Bennemann [Phys. Rev. B 28, 665 (1983)].
Equivalence between the “classical” and the “Warren” approaches for the effects of long range dipolar couplings in liquid nuclear magnetic resonance
Authors: Jeener J.
Published: July 2002

I show, on a simple example, that the two approaches lead to exactly the same density operator for the whole spin system, with the same equations of motion for the matrix elements (ignoring relaxation and diffusion). Hence, the two approaches are equivalent in all respects.
The Smoluchowski diffusion equation for structured macromolecules near structured surfaces
Authors: Peters Michael H.
Published: July 2002

Beginning with the molecular-based Fokker–Planck equation obtained previously [M. H. Peters, J. Chem. Phys. 110, 528 (1999); J. Stat. Phys. 94, 557 (1999)], the Smoluchowski diffusion equation is derived here to describe the spatial and orientational dynamics of molecularly structured macromolecules near molecularly structured surfaces. The formal scaling and perturbation methods employed allow the establishment of definite limits on the use of the Smoluchowski equation when surfaces are present. It is shown that the Smoluchowski equation reduces to that given previously [D. W. Condiff and J. S. Dahler, J. Chem. Phys. 44, 3988 (1966)] in the absence of external surfaces. A specific example application is given involving a spherical macromolecule with electrostatic charge segments near a planar surface with an arbitrary charge distribution. Finally, we show that the short-time solution to the Smoluchowski equation obtained here yields a Brownian dynamics method consistent with that given previously [E. Dickinson, S. A. Allison, and J. A. McCammon, J. Chem. Soc. Faraday Trans. 2 81, 591 (1985)] for orientable, interacting Brownian particles. This study has applications to problems involving site-specific adsorption of orientable, structured Brownian particles, such as association or adsorption of biological macromolecules to cellular surfaces and enzyme–substrate docking kinetics, to name a few.
A Monte Carlo study of the structural properties of end-linked polymer networks
Authors: Gilra Nisha , Cohen Claude , Panagiotopoulos A. Z.
Published: July 2002

The formation and structural properties of end-linked polymer networks were studied in the framework of the bond fluctuation model on a three-dimensional simple cubic lattice. Only excluded volume interactions were considered between repeat units (monomers and cross-links). Networks were created with a wide range of the ratio of cross-link sites to precursor polymer chain ends, r, to test the experimental observation that a nonstoichiometric ratio generates a more perfect network. Systems of 10-, 20-, and 50-mer precursor polymer chains with values of r ranging from 0.9 to 1.6 were studied. An algorithm was developed to determine the soluble fraction and the amount of loops and pendent structures. The network properties were evaluated at the same number of Monte Carlo steps per repeat unit for each chain length independent of r value. In agreement with experimental observations, the simulation results show that the optimum r values are nonstoichiometric, increase with increasing chain length of the precursor polymer chain, and increase with side reaction.
Modeling ion–ion interaction in proteins: A molecular dynamics free energy calculation of the guanidinium-acetate association
Authors: Rozanska Xavier , Chipot Christophe
Published: July 2002

The twin nitrogen–twin oxygen, C2v association of guanidinium and acetate in water is examined by means of molecular dynamics free energy calculations, using three approaches for handling Coulomb electrostatic interactions: (i) an Ewald lattice summation, (ii) a generalized reaction field correction, and, (iii) a smoothed spherical truncation. The potential of mean force obtained from the Ewald simulation exhibits the expected minima characteristic of a contact and a solvent-separated ion pair. In contrast, the repulsive and, therefore, physically unrealistic profile resulting from a spherical truncation of the electrostatic interactions at 12 Å calls into question the validity of such a protocol for simulating charged proteins. The introduction of a generalized reaction field improves the description of the solution significantly, in spite of an artifactual behavior towards the edge of the cutoff sphere. Whereas this approach, compared to a conventional spherical truncation, implies virtually no additional computational effort, it rapidly becomes costly for large ion–water and water–water cutoff radii. At 12 Å, the computational investment is comparable to that of an Ewald simulation with an appropriately chosen number of k-vectors and separation parameter, making the latter more cost-effective.
New approach to the state-specific multireference coupled-cluster formalism
Authors: Adamowicz Ludwik , Malrieu Jean-Paul , Ivanov Vladimir V.
Published: July 2002

A new development is presented in the framework of the state-specific multireference (MR) coupled-cluster (CC) theory (MRCC). The method is based on the CASSCF (complete active space self-consistent field) wave function and it is designed specifically for calculating excited electronic states. In the proposed approach, the cluster structure of the CC wave operator and the method to determine this operator are the key features. Since the general formulation of the CASCC method is uncontracted, i.e., allows the interaction between the nondynamic and dynamic correlation effects to affect both the CAS reference function and the CC correlation wave operator, the method is expected to perform better than contracted perturbative approaches such as the CASPT2 (second-order perturbation theory based on the CAS wave function) method. Also, the CASCC method is not a perturbative approach and is not based on selection of an unperturbed Hamiltonian, which in the case of the CASPT2 method often leads to the “intruder state” problem. CASCC calculations of the lowest totally symmetric excited state of the H8 model system using the internally contracted and uncontracted approaches reveal some interesting features of the methodology.
Lie–Bäcklund symmetries of certain nonlinear evolution equations under perturbation around their solutions
Authors: Lakshmanan M. , Tamizhmani K. M.
Published: July 2002

A generalization of the Lie–Bäcklund (LB) theory for coupled evolution equations is discussed. As a consequence, the direct connection between the LB symmetries and constants of motion of these systems is also established. Furthermore, as an application of this theory we investigate the existence of infinitely many commuting LB symmetries of certain nonlinear evolution equations under perturbation around their solutions. Then the corresponding constants of motion are derived, which are in agreement with the known results.
Characteristic functional structure of infinitesimal symmetry mappings of classical dynamical systems. II. Mappings of first-order differential equations
Authors: Katzin Gerald H. , Levine Jack
Published: July 2002

Infinitesimal velocity-dependent symmetry mappings of second-order dynamical systems (a) Ei(ẍ, ẋ, x, t)≡ẍi −Fi(ẋ, x, t)=0, i=1,..., n, were studied in considerable detail in a previous paper [J. Math. Phys. 2X, xxxx (198X), the first of this series]. Among the results developed in that paper was a procedure for determining the characteristic functional structure of symmetry mappings for such second-order systems. In this present companion paper it is shown that a similar procedure may be used to obtain the characteristic functional structure of infinitesimal symmetry mappings (b) ȳI=yI+δyI, δyI ≡ηI(y, t)δa; (c) t̄=t+δt, δt≡η0(y, t)δa, for systems of first-order differential equations (d) EI(ẏ, y, t)≡ẏI−λI (y, t)=0, I=1,..., N. This characteristic structure is the same for all first-order systems (d) and is explicitly dependent upon constants of motion of the system. For the special case in which (d) is a system of N=2n equations derived from a system of n second-order equations (a) it is shown how the respective symmetry equations based upon these two equivalent dynamical descriptions are related and how their symmetry solutions are correlated. Two examples are given.
A physical interpretation of the Kerr solution
Authors: Punsly Brian
Published: July 2002

The pseudo-Riemannian nature of the Kerr solution is exploited to obtain a special relativistic analysis. Lorentz contraction and time dilation factors are found that represent the metric coefficients. The special relativistic treatment is extended to the study of the hypersurface at r=0. A nontrivial topology is revealed and new extensions of the metric are found.
Global solution of the Boltzmann equation for rigid spheres and initial data close to a local Maxwellian
Authors: Palczewski A. , Toscani G.
Published: July 2002

In this paper, which is an extension of the previous paper of one of the authors [Arch. Rat. Mech. Anal. 102, 231 (1988)], a global existence theorem is presented for the Boltzmann equation for initial data close to a local Maxwellian of a special type. Contrary to the previous result, the global existence is proved for rigid spheres and ‘‘hard’’ potentials with angular cutoff.
Curvature collineations in general relativity. I
Authors: Hall G. S. , da Costa J.
Published: July 2002

Although curvature collineations (curvature preserving transformations) have been studied within the context of general relativity for 20 years, there has been little attempt to study them systematically and there does not appear to have been a detailed mathematical investigation of their properties. This is the first of two papers that are intended as a contribution to this deficiency. This paper presents a discussion of the more general mathematical aspects of curvature collineations and suggests a program for studying them. The implementation of this program and the analysis of specific examples will be carried out in the second paper.
Coherent state formulation of multiconfiguration states
Authors: Deumens E. , Öhrn Y. , Weiner B.
Published: July 2002

The theory of vector coherent states is applied to multiconfiguration states that are given a complex parametrization, based on the underlying special linear group. Using the time-dependent variational principle, equations of motion for such states are formulated as evolution equations in a generalized phase space. The classical Hamiltonian for these equations is given in terms of the reduced first- and second-order density matrices, which can be expressed in terms of partial derivatives with respect to the group parameters of the overlap kernel. The multiconfiguration self-consistent-field (MCSCF) and configuration interaction (CI) equations arise as special cases of these equations. Using the coherent state formulation we obtain very compact and numerically efficient expressions for the time-dependent quantum mechanical description of chemical reactions.
Method of self-similar approximations
Authors: Yukalov V. I.
Published: July 2002

A new method is suggested to approximately find out the limit for a sequence of functions when solely several first terms of a sequence are known. The method seems to be very useful for those complicated problems of mathematical physics, statistical mechanics, and field theory in which, because of technical difficulties, one is able to calculate, using some iteration procedure or perturbation scheme, only several first approximations for quantities of interest. As an illustration, the quartic oscillator is considered, and it is shown that invoking only the terms of a perturbation theory up to the second order, the ground-state energy is defined with an accuracy of 0.1% for all values of the anharmonicity parameter, from zero up to infinity.
Quantum mechanics of the general spatially homogeneous geometry coupled to a scalar field
Authors: Christodoulakis T. , Korfiatis E.
Published: July 2002

The canonical quantization of the general spatially homogeneous geometry minimally coupled to a scalar field is considered. The classical reduction of the full Einstein plus matter action is performed in a way that is independent of both the coordinate system and the dimension of the isometry group acting on each hypersurface. Exact solutions of the Wheeler–DeWitt equation are given for all but one Bianchi types in the case of two-dimensional minisuperspace, and for some Bianchi types in the case of three-dimensional minisuperspace. The results of the quantization suggest that the probability of the classically collapsed configurations to occur as quantum states is zero.
The unified Ito formula has the pseudo-Poisson structure df(x)=[f(x+b)−f(x)]μνdaνμ
Authors: Belavkin V. P.
Published: July 2002

A universal functional Ito formula, recently discovered within the *-Ito noncommutative algebraic approach, is described herein. The formula unifies the classical Ito formulas for Wiener, Poisson, and quantum cases, extends the chain rule df(x1,...,xl) = ∑f′i(x)dxi to the case of noncommuting stochastically differentiable operators Xi(t), and also admits a nonadapted generalization.
Translation normalizers of Euclidean groups. I. Elementary theory
Authors: Kopský Vojtěch
Published: July 2002

Translation normalizers of Euclidean groups are introduced as normalizers with respect to the translation subgroup of the full Euclidean group. It is shown that the concept can be easily generalized to translation normalizers of affine groups; the results for normalizers of Euclidean groups can be at once adopted for those affine groups which have orthogonalizable point groups. Some of elementary and basic properties of translation normalizers are proved. The translation normalizer is identical for all groups of the generalized arithmetic class—a concept borrowed from mathematical crystallography, because such a generalization is quite natural. It is further shown, how to use translation normalizers in consideration of sets of conjugate subgroups or supergroups and hence also of normal subgroups of Euclidean groups. For that purpose is derived the space shift diagram and normality criterion. The exposition is closed by a brief discussion of the connection of translation normalizers with location properties of Euclidean motion groups.