bluuebunny/crossref_metadata_2025 huggingface dataset:
Variational perturbation calculations for the phase diagram of systems with short-ranged interactions
Authors: Hasegawa M.
Published: July 2002

A variational perturbation theory based on the Gibbs–Bogoliubov inequality is used to predict the phase behavior of systems with short-ranged interactions. We are primarily concerned with the disappearance of a stable liquid phase and the occurrence of an isostructural solid–solid transition, and consider two model systems interacting via a hard-sphere attractive Yukawa (HSAY) potential and a so-called m−n potential, a natural extension of the 12−6 Lennard-Jones potential to higher powers. In the variational calculations, a consistent treatment of the fluid and solid phases is aimed at and the hard-sphere system is used as the reference system for both phases. The predicted phase diagrams for the HSAY system with not very short-ranged potential are confirmed to be in good agreement with essentially the same calculations by Hagen and Frenkel [J. Chem. Phys. 101, 4093 (1994)]. The predicted isostructural solid–solid transition for this system, which occurs for a very short-ranged potential, are somewhat different from the Monte Carlo (MC) simulations by Bolhuis et al. [Phys. Rev. E 50, 4880 (1994)]. In particular, the predicted critical range of the potential for the occurrence of this transition is much shorter than the MC result. For the m−n potential system, a stable liquid phase is found to disappear when the attractive potential range becomes comparable to that of C60. The critical temperature for the m−n potential system, at which the isostructural solid–solid transition terminates, is consistently higher by about 50% than that for the HSAY system, which is interpreted as an effect of the softness of the repulsive core. The solid–solid transition in the high density limit, which could occur for an infinitesimally short-ranged potential, is discussed in some detail for both systems within the framework of the present method.
NMR excitation of quadrupolar order using adiabatic demagnetization in the rotating frame
Authors: Hughes Colan E. , Kemp-Harper Richard , Wimperis Stephen
Published: July 2002

Adiabatic demagnetization and remagnetization in the rotating frame (ADRF and ARRF) are shown to be practical and efficient techniques for exciting and observing quadrupolar order, T2,0, in NMR of quadrupolar nuclei such as H2 or Na23. A detailed theoretical description of ADRF and ARRF of spin I=1 and 3/2 nuclei, based on the well-known vector model of NMR, is presented and used to derive analytically a variety of pulse shapes for ADRF and ARRF. This theory is also used to calculate the state of the spin I=1 and 3/2 density operators following an ADRF or ARRF pulse and it is shown that the desired coherence transfer processes have the maximum amplitudes allowed by the well-known “universal bound” theorem. In principle, therefore, ADRF is shown to be superior as a method of exciting quadrupolar order to the Jeener-Broekaert experiment since the latter fails to excite the maximum T2,0 amplitude for spin I=3/2 nuclei. The performance of ADRF is investigated using computer calculations and simulations and the conditions under which it yields broadband (i.e., non-oscillatory) excitation of quadrupolar order are derived. Using both H2 (I=1) and Na23 (I=3/2) NMR of liquid crystalline and biological samples, ADRF and ARRF are demonstrated experimentally. The predicted broadband excitation behavior is observed for both ADRF and ARRF, whereas both the Jeener-Broekaert and double-quantum filtration experiments show excitation profiles that oscillate sinusoidally as a function of either the quadrupolar splitting parameter, ωQ, or the duration of the pulse sequence, τ. Finally, a more general discussion of ADRF and ARRF of quadrupolar nuclei is presented and it is shown that the maximum coherence transfer amplitudes are achieved for nuclei of any spin quantum number I.
Brillouin spectra and vibrational–translational energy exchange in liquid acetonitrile
Authors: Sassi Paola , Paliani Giulio , Cataliotti Rosario Sergio
Published: July 2002

Polarized light scattering experiments in the GHz frequency range (Rayleigh–Brillouin spectra) are reported for liquid acetonitrile. Hypersonic velocities and absorption coefficients have been measured as a function of temperature and of transferred wave vector. The plots of these quantities versus frequency show characteristic dispersion which is indicative of relaxation phenomena. The occurring relaxation has been interpreted as due to a thermal process in which the translational energy of molecules is transferred, during the collisions or through dipolar interactions, into internal degrees of freedom of low quanta vibrational motions. The quite strange behavior of the relaxation time with the temperature has been interpreted and compared with the behavior of the relaxation time in liquid chloroform.
Coherence motion of photoinduced nonadiabatic charge transfer reaction in solution: A numerical study of pump–probe spectroscopy
Authors: Sheu Sheh-Yi , Yang Dah-Yen
Published: July 2002

We study the photoinduced charge separation processes in solution through a pump–probe spectroscopy theory [Dah-Yen Yang and Sheh-Yi Sheu, J. Chem. Phys. 106, 9427 (1997)] numerically. We investigate the detailed mechanism of nonadiabatic transition processes via the transition differential flux analysis. For the harmonic potential surfaces, an electronic coherence motion is observed in the overdamped exothermic activationless and inverted regimes.
Adsorption mechanisms and surface heterogeneity in the oxidation reaction of CO
Authors: Cortés Joaquı́n , Valencia Eliana , Araya Paulo
Published: July 2002

A Monte Carlo simulation study is made of the sensitivity of the CO oxidation reaction to changes in the characteristics of the catalyst’s surface on which the type of oxygen adsorption mechanism is dependent. Infinite rate models of the Ziff, Gulari, and Barshad (ZGB) type, and mechanisms having kinetics parameters of actual experiments from the literature are studied. It is shown that, if linear adsorption is assumed, the structural insensitivity becomes apparent in the phase diagram but not in the production of CO2. In the case of structural sensitivity it is seen that surface heterogeneity leads to a change in the character of the phase transition curve, and also allows information about the surface to be obtained from the shape of the transition curve.
Theoretical calculation of line shapes and saturation effects in polarization spectroscopy
Authors: Reichardt Thomas A. , Lucht Robert P.
Published: July 2002

The physics of polarization spectroscopy (PS) is investigated by direct numerical integration of the time-dependent density matrix equations. The Zeeman structure of the upper and lower energy levels is included in a multistate formulation of the density matrix equations. The numerical solution of the time-dependent density matrix equations enables us to investigate the effects of strong saturation on PS signal levels and line shapes. Bath levels not directly coupled by the laser radiation are included in the numerical modeling to investigate the effects of collisional rates and different types of collisions on signal levels and line shapes. The effects of Doppler broadening are included by solving the density matrix equations for numerous velocity groups. At low laser power we find that the homogeneously broadened PS line shape is Lorentzian-cubed, as compared to the Lorentzian predicted in several previous low-power analytical solutions. In the low laser power regime, the line-center PS signal is proportional to (collision rate)−6, obviously greatly complicating the application of unsaturated PS for quantitative concentration measurements in flames and plasmas. As the transition begins to saturate at higher laser intensities, the dependences of the signal strength on the laser intensity and on the collision rate decrease drastically, although the line-center PS signal is still approximately proportional to (collision rate)−2. The dependence of the PS signal intensity on the ratio of the population-transfer collision rate to the dephasing collision rate is minimized for saturating pump beam intensities. For resonances that are both Doppler- and collision-broadened, the low-power PS line shape is Lorentzian with a linewidth equal to the collisional width for the case where the Doppler width is much greater than the collisional width. At low pump laser intensities, the PS signal is very dependent on the ratio of Doppler broadening to collisional broadening when the Doppler width is greater than the collisional width. However, at high intensity, the line-center PS signal intensity becomes nearly independent of collision rate when the collisional linewidth is less than the Doppler linewidth. Quantitative application of polarization spectroscopy for concentration measurements in flames and plasmas will almost certainly require resolution of the PS line shape and/or accurate measurement of the saturation curve.
Effect of laser intensity on the determination of intermolecular electron transfer rate constants—Observation of Marcus inverted region in photoinduced back electron transfer reactions
Authors: Weng Yu-Xiang , Chan Kwok-Chu , Tzeng Biing-Chiau , Che Chi-Ming
Published: July 2002

The light intensity and concentration dependence of the photoproduct yield are investigated in a monophotonic process. The relationship of the photoproduct yield with the laser intensity and the complex concentration for a monophotonic process is derived under laser flash photolysis. The relationship is confirmed experimentally in a monophotonic process, i.e., triplet–triplet transition for a Cu(I) complex Cu6(DMNSN′)6 (DMNSN′=4,6-dimethylpyrimidine-2-thiolate). At low light intensity, the relationship can be approximated by a linear inverse square root dependence on the light intensity. Based on this equation, a method is proposed to determine the intrinsic back electron transfer rate constant kETb in photoinduced intermolecular electron transfer reactions, precluding the effect from the diffusional encounter pairs. The Marcus “inverted region” is observed by using the method in photoinduced back electron transfer reactions of [Au2(dppm)2](ClO4)2 (dppm=bis(diphenylphosphino)methane) with a series of substituted pyridinium acceptors.
Equivalent-core calculation of core-level relaxation energies in photoelectron spectroscopy: A molecular-orbital approach
Authors: Chen Y. , Zhuang G. , Ross P. N. , Van Hove M. A. , Fadley C. S.
Published: July 2002

The equivalent-core approximation is implemented in a novel way so as to calculate core-level relaxation energies in photoelectron spectroscopy. The method is based on self-consistent field (SCF) Hartree–Fock molecular-orbital calculations via linear combinations of atomic orbitals, and involves evaluating the difference of sums of two-electron Coulomb and exchange integrals, for all electrons in an atom and in its equivalent-core ion. By thus avoiding SCF calculations with a core hole present (the true final state of photoemission), this procedure is shown to significantly save computing time in comparison with an exact SCF direct-hole calculation. Application of the method in single atoms and selected molecules shows about a 10% difference with respect to direct-hole calculation results. The approximation introduces about 1–6 eV errors compared to the experimental results of gas phase molecules. This method thus should be a generally useful procedure for estimating relaxation energies in core spectra.
Accurate resonances and effective absorption of flux using smooth exterior scaling
Authors: Karlsson Hans O.
Published: July 2002

A general coordinate transformation is used to derive smooth exterior scaling (SES). Different complex paths are discussed and it is also shown how to derive a complex absorbing potential (CAP) from the SES. Accurate resonance values are computed both for short range and long range potentials. It is shown that the SES absorbs outgoing flux very effectively. The approximation of not scaling the potential and its relation to CAPs is discussed. It is emphasized that the SES can be implemented as easy as CAPs for grid methods.
Quantum dynamics of unimolecular dissociation reaction HFCO→HF+CO
Authors: Yamamoto Takeshi , Kato Shigeki
Published: July 2002

Quantum dynamics calculations of the title reaction were performed within planar 4D and 5D models to investigate how the dissociation rate is affected by the change of total energy or vibrational character of the initial states. In the 4D calculations, the possibility for observing the mode specificity in the in-plane modes was examined by generating the initial states that are locally excited with respect to specific vibrational modes and comparing the dissociation rates for these states. In order to generate the appropriate initial states, we employed the spectral filter method combined with the zero-order vibrational Hamiltonian in the bond coordinates. Comparison was also made between the dissociation rate and the intramolecular vibrational energy redistribution (IVR) rate. In the 5D calculations, we have focused on the CH overtone excited states and examined how the dissociation dynamics is changed by the total energy that is above or below the dissociation threshold. We observed quite large fluctuations in the time-dependent rate coefficient for the fifth CH overtone state, which was found to occur due to the resonant oscillations of the CH stretching and bending energies as well as the closeness of the total energy to the dissociation threshold.
Valence bonds in solid hydrogen under pressure
Authors: Vianna Reinaldo O. , Chacham Hélio
Published: July 2002

We present a description of the valence bonding in solid hydrogen under pressure, through ab initio valence bond calculations on a small cluster. Our results show a huge increase in charge transfer components of the ground state wave function at intermolecular separations around 1.7 Å. This provides an explanation, in terms of chemical bonding, for the origin of the observed modifications in the structural and electronic properties of solid hydrogen at that intermolecular separation range.
Adsorption of random copolymers: A scaling analysis
Authors: Sumithra K. , Baumgaertner A.
Published: July 2002

We report on results from Monte Carlo simulations of a single random copolymer adsorbed on a homogeneous planar surface. Although the critical crossover exponent is unaltered with respect to the case of homogeneous polymers, it is found that the scaling behavior is changed by the fraction of adsorptive monomers of the chain. In particular, we present some explicit expressions for energy and radius of gyration at low temperatures.
Kinetics and thermodynamics across single-file pores: Solute permeability and rectified osmosis
Authors: Chou Tom
Published: July 2002

We study the effects of solute-membrane interactions on osmotic transport through pores. By extending single-file, single-species kinetic models to include entrance of solute into membrane pores, we model the statistical mechanics of competitive transport of two species across membrane pores. The results have direct applications to water transport across biomembrane pores and particle movement in zeolites, and can be extended to study ion channel transport. Reflection coefficients, the reduction of osmotic fluxes measured using different solutes, are computed in terms of the microscopic kinetic parameters. We find that a reduction in solvent flow due to solute–pore interactions can be modeled by a Langmuir adsorption isotherm. Osmosis experiments are discussed and proposed. Special cases and Onsager relations are presented in the Appendixes.
Application of a general classical variational theory to the F+H2→FH+H reaction
Authors: Rutenburg Irina , Koeppl Gerald W.
Published: July 2002

A general classical variational theory of reaction rates [J. Chem. Phys. 87, 5746 (1987)] is applied to the F+H2→FH+H reaction for a series of potential-energy functions (PEFs). The variational theory gives the rate as the equilibrium flux of phase points through a trial surface which divides reactants from products and is varied to obtain a least upper bound for the rate. This dividing surface (DS) is defined by a power-series expansion of the H–H internuclear separation (r) in internal coordinates R and θ where R is the distance between atom F and the center-of-mass of H2 and θ is angle which the H2 internuclear axis makes with a line from the center-of-mass of H2 to atom F. The angle-dependent terms in the DS make it possible to describe the dynamical stereochemistry of atom–diatom reactions in a new and useful manner. The profile of the angle-dependent minimum potential energy for reaction versus orientation angle is varied systematically in the PEF series to define a trend toward a “flatter” angle-dependent barrier. Portraits of the dynamical stereochemistry are obtained for each PEF by plotting contours of the density of variational flux on the DS. These reactivity relief maps show how the accuracy of the variational method depends on the expansion order of the DS and how the field of reactivity which surrounds the diatomic reactant expands with increasing temperature and energy. The accuracy of the variational theory was determined by comparing energy-dependent mean reaction cross sections and incremental (angle-dependent) mean reaction cross sections with results obtained by calculating classical mechanical trajectories. The DS was used to show how the accuracy of the no-recrossing assumption of transition state theory depends on orientation angle. Variational and trajectory results were used to calculate energy-dependent transmission and product coefficients.
Inversion in the relative stabilities of HBO and BOH upon ionization
Authors: Boldyrev Alexander I. , Simons Jack
Published: July 2002

Ab initio calculations of the HBO+→BOH+potential energy surface were performed at the QCISD(T)/6−311++G(2df,2pd) level of theory. A linear 2Σ+ +B = O-H species was found to be the global minimum for the cation although the linear 2Σ+ H-B≡O isomer is the global minimum on the neutral HBO→BOH surface. Which isomer of the cation appears in various experimental circumstances depends on how the cation is produced. Ionization of 1Σ+ HBO should occur near 13.2 eV and will produce the metastable linear H-B=O+(2Π) π-radical cation, which is 36.9 kcal/mol higher in energy than 2Σ+ −B=O-H and is separated from +BB=O-H by a 36.3 kcal/mol classical barrier. Ionization of HBO with less than 37 kcal/mol of excess energy will thus yield 2Π H-B=O+; if the excess energy exceeds 37 kcal/mol, rearrangement to the 2Σ+ +B = O-H σ-radical cation can occur. Production of the cation via exothermic chemical reaction will likely give the more stable 2Σ+ +B=O-H isomer. The stability of the 2Σ+ +B=O-H cation relative to 2Π H-B=O+ is due primarily to the large difference in the energies required to remove an electron from a π bonding orbital in H-B≡O or from a nonbonding boron-localized σ orbital in BOH.
Simulation of surface restructuring and oscillations in CO–NO reaction on Pt(100)
Authors: Zhdanov V. P.
Published: July 2002

Kinetic oscillations occurring in the CO–NO reaction on Pt(100) at UHV conditions and relatively high temperatures are accompanied by adsorbate-induced surface restructuring. Employing the Monte Carlo technique, we show the type of spatio-temporal patterns which might be observed in this system on the nm scale. The model used to describe surface restructuring is based on the lattice-gas approximation and predicts the formation of restructured islands even at low adsorbate coverages. Simulations are executed with qualitatively realistic ratio between the rates of different steps involved in the game.
Hydrogen–hydrogen intermolecular structure of polyethylene in the melt
Authors: Londono J. D. , Annis B. K. , Habenschuss A. , Smith G. D. , Borodin O. , Tso C. , Hsieh E. T. , Soper A. K.
Published: July 2002

Three polyethylene samples, which differed in their degree of deuteration, were studied in neutron diffraction isotopic substitution (NDIS) experiments at 428 K. These results were complemented at small wavevectors by small angle neutron measurements. The intermolecular hydrogen–hydrogen (HH) structure function, hHH(Q), was obtained without recourse to intramolecular structure models, as demonstrated in a prior report. The PE experimental results are compared to computer simulation results for the alkanes C100 at 509 K and C44 at 350, 400, and 450 K. The small temperature dependence of the HH intermolecular radial distribution functions, gHH(r) for C44 indicates that the differences observed between the PE, C100, and C44 (450 K) results are, for the most part, not due to just temperature differences. It is shown that the string model, an analytic result from an integral equation theory of polymers (PRISM), can account approximately for the overall shape of the gHH(r) functions, and that this overall shape is dependent on the radius of gyration of the molecule. Further analysis shows that there are two other contributions to gHH(r), both of which are independent of chain length to first order. The first is due to chain–chain packing, and the second is due to local HH intermolecular correlations. These results are significant because they demonstrate that hHH(Q) is a useful function for studying intermolecular polymer structure, which has been shown to underpin phase behavior in polyolefin blends.
Two-component calculations for the molecules containing superheavy elements: Spin–orbit effects for (117)H, (113)H, and (113)F
Authors: Han Young-Kyu , Bae Cheolbeom , Lee Yoon Sup
Published: July 2002

We have calculated bond lengths, harmonic vibrational frequencies, and dissociation energies for (117)H, (113)H, and (113)F using relativistic effective core potentials (RECPs) with one-electron spin–orbit operators at the two-component coupled-cluster levels of theory. It is shown that any reasonable theoretical descriptions of the electronic structures of molecules containing superheavy elements require consideration of relativistic interactions and electron correlations. Comparisons with available all-electron Dirac–Fock (DF) based results indicate that our two-component approaches are very promising tools in the calculations for the molecules containing superheavy elements. The spin–orbit effects calculated from one- and two-component RECPs are in good agreement with those from all-electron Douglas–Kroll and DF results, implying that the potential average scheme is useful for obtaining one-component RECPs even for superheavy elements. Spin–orbit and electron correlation effects are not additive for molecular properties of (117)H, (113)H, and (113)F, but spin–orbit effects are qualitatively similar at all levels of theory considered. Spin–orbit effects contract Re and increase ωe for (113)H and (113)F, whereas they expand Re and decrease ωe for (117)H. Spin–orbit effects decrease De for all molecules considered, but the amount of decrease for (113)H and (117)H is substantially smaller than that estimated from the atomic splittings. For (117)H, our best calculations yield 1.983 Å (Re), 1403 cm−1(ωe), and 1.60 eV (De).
An investigation of the shape and crossover scaling of flexible tangent hard-sphere polymer chains by Monte Carlo simulation
Authors: Haslam Andrew J. , Jackson George , McLeish Tom C. B.
Published: July 2002

Results are presented from our simulation study of flexible, tangent hard-sphere polymer chains, using a Monte Carlo technique with a reptation algorithm. We examine the crossover region from the swollen (dilute) region to the semidilute regime using chains of degree up to N=2000. The (average) chain end-to-end distance is analyzed as a function of the number of chain links, as is a correlation function for the internal separation of segments, expressed in terms of their connectivity. We study the structure of the polymer chains using scattering functions, which are presented in addition to the segment–segment distribution functions from which they are calculated. Not surprisingly, the Flory exponent ν changes gradually from the dilute value of ν≈0.59 to ≈12 at high density. We find that for chains of 2000 segments, this transition begins at the very low molecular volume fraction of about 0.1%. An analysis of the correlation function for internal segments provides a similar but more detailed picture, in that the role of the screening length ξ becomes explicit. The pair distribution and scattering functions in the swollen, crossover, and melt regions are determined and the scaling behavior is reexamined in the context of these functions. The system considered will form a reference system for subsequent studies in which the model will be broadened to include a network and/or attractive interactions. In this paper we seek first of all to establish that reference, and to relate our work to previous studies in this area of interest. As an important technical point, we also note that the use of an inappropriate random number generator can cause the complete failure of simulations of this type.
Flow-induced swelling of poly(vinyl alcohol) gel
Authors: Takigawa T. , Uchida K. , Takahashi K. , Masuda T.
Published: July 2002

Flow and swelling behavior was investigated for poly(vinyl alcohol) (PVA) gel in the solvent flow field. It was found that the PVA gel swelled under solvent flow and the volume of the gel increased with increasing solvent pressure. The flow rate also increased with increasing solvent pressure. On the other hand, the characteristic times determined by swelling and flow kinetics were independent of the applied pressure, and were almost identical, suggesting that the swelling and flow properties of the gels were closely related to each other. A new type of diffusion equation, which can describe the time evolution of flow and volume change of polymer gels under flow, was proposed in this paper. Comparison between experiment and theory showed that the experimental results were explained fairly well by the theory.
Structure and potential energy surface for Na+⋅N2
Authors: Soldán Pavel , Spirko Vladimı́r , Lee Edmond P. F. , Wright Timothy G.
Published: July 2002

Attention is focused on the Na+⋅N2 complex as part of a study of Na+-containing complexes, which have been implicated in the formation of sporadic sodium layers in the upper atmosphere. The equilibrium structure is found to be linear, in agreement with previous studies. A potential energy hypersurface is calculated at the CCSD(T)/aug-cc-pVTZ level of theory, where the N2 moiety is held fixed, but a wide range of Jacobi bond lengths and bond angles are sampled. This hypersurface is fitted to an analytic form and from this anharmonic vibrational separations are calculated, and compared to harmonic values. Rovibrational energy levels are also calculated from the fitted hypersurface. The best estimate of the interaction energy, ΔEe is 2770 cm−1, and ΔHf298(Na+⋅N2)=(136.5±2.0) kcal mol−1.
A closure for the Ornstein–Zernike relation that gives rise to the thermodynamic consistency
Authors: Eu Byung Chan , Rah Kyunil
Published: July 2002

A closure is proposed for the direct correlation function in the grand canonical ensemble theory that gives rise to the thermodynamic consistency, by which it is meant that identical results are obtained when the equations of state are calculated via the virial and compressibility routes, respectively, and when the excess chemical potentials are calculated by means of the thermodynamic derivative and the statistical mechanical formula, respectively. The integral equation for the pair correlation function under the closure is analytically solved in the case of hard spheres. The equation of state for hard spheres turns out to have the same form as that in the scaled particle theory or the compressibility equation of state in the Percus–Yevick theory although the present closure is quite different from that of the Percus–Yevick theory. The excess chemical potential is also found in the same form as that in the scaled particle theory. It seems to suggest that the present closure produces an integral equation theory equivalent to the scaled particle theory.
H–C
60
and low energy H impact with fullerite
Authors: Smith Roger , Beardmore Keith , Belbruno Joe
Published: July 2002

The interaction of energetic hydrogen (25–50 eV) with fullerite is studied theoretically to determine if endohedral H@C60 is feasible. Ab initio quantum calculations are used to calculate the binding energy of various H–C60 configurations and these are used in the fitting of a classical many-body C–H potential. Molecular-dynamics simulations are carried out of the interaction of individual H atoms with a fullerite crystal at both 25 and 50 eV using this classical potential. It is shown to be feasible to implant H atoms with a good probability within the surface layer fullerene molecules, thus suggesting an experimental procedure for the production of H@C60.
Effect of competition between catalytic particles C on the rate of the reversible diffusion-controlled reaction A+C⇌B+C
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

The effect of competition between catalytic particles C on the rate of the reversible diffusion-controlled reaction A+C⇌B+C is studied in a self-consistent approximation. The C-particles are assumed static with random configuration. The A- and B-particles diffuse in a fluid or solid background medium. Frequency-dependent effective rate coefficients are calculated as a function of the volume fraction of C-particles.
Quantum Monte Carlo calculations of molecular electron affinities: First-row hydrides
Authors: Morosi Gabriele , Mella Massimo , Bressanini Dario
Published: July 2002

Very accurate energies can be computed by the fixed-node diffusion Monte Carlo method. They are affected only by the nodal error due to the approximate description of the nodal surfaces by the trial wave function. We examine the cancellation of nodal errors in molecular electron affinity calculations. Ground state energies of the anions of first-row hydrides AH (A=Li–O) have been computed using the fixed-node diffusion Monte Carlo method with a determinant times a correlation factor as the trial wave function. The energies are among the lowest to date. Using the energy values for the neutral molecules computed by Luchow and Anderson [A. Luchow and J. B. Anderson, J. Chem. Phys. 105, 7573 (1996)] we computed adiabatic electron affinity values and found them in agreement with the experimental data. As a consequence, the values of the anion dissociation energies are also correctly evaluated.
Dynamic multipole polarizabilities, reduced spectra, and interaction coefficients for N2 and CO
Authors: Spelsberg Dirk , Meyer Wilfried
Published: July 2002

Reliable ab initio reduced spectra that serve to calculate dynamic polarizabilities as well as long-range interaction coefficients are presented. The reduced spectra are constructed from perturbed functions that are computed in the single excitation multi-reference averaged coupled pair approach. The static dipole polarizabilities are of the same quality as values obtained in a recent study using elaborated coupled cluster expansions. The dynamic dipole polarizabilities are in excellent agreement with all experimental data available. The higher multipole polarizabilities (l⩽3) are also expected to be the most reliable ones up to date. The interaction coefficients for the N2–N2 interaction perfectly match the coefficients obtained from semiempirical anisotropic oscillator strength distributions. For the CO–CO interaction, however, the same quantities differ by up to 10%. We believe our coefficients to be accurate to 3%.
Rotator phases and herringbone order in Langmuir monolayers and alkanes
Authors: Würger Alois
Published: July 2002

Molecular ordering of a 2D array of hydrocarbon chains in alkane crystals or Langmuir monolayers arises from the anisotropic pair interaction for rotations about the molecular axes. Starting from atom–atom potentials for H and C, we determine the relevant Fourier coefficients of the molecular pair interaction and, in particular, their distance dependence. In terms of a molecular-field approximation (MFA), we find two first-order transitions between a high-temperature hexagonal phase, an intermediate orthorhombic phase, and at lower T a herringbone structure with a nonhomogeneous order parameter, in agreement with the phase diagrams of various systems. It turns out that molecular ordering is closely related to an orthorhombic distortion of the hexagonal lattice. The calculated distortion agrees well, without free parameters, with measured values. In order to account for the weak variation of the critical temperatures with the chain length, we propose a simple mean-field model for conformational defects that provides a good fit for alkanes with 10–35 carbon atoms.
Gaussian model of protein folding
Authors: Erman Burak , Dill Ken
Published: July 2002

The thermodynamics and kinetics of protein folding depend on the sequence of monomer units along the chain. To explore the sequence dependence, current modeling—all-atom simulations and lattice models, for example—require time-consuming computer simulations. There are currently no analytical models by which folding properties can be computed directly from the monomer sequence. Here we introduce a simple analytical model of folding, based on assuming springlike forces for covalent and noncovalent interactions. Thermodynamic and kinetic properties of folding can be obtained directly for specific sequences in Go-like models. Remarkably, although it is a continuum model, some choices of parameters give the same stable conformations as in the corresponding lattice model. The main point of elasticity-based folding models is that their properties can be understood in complete detail, and with little computational investment. This may be useful for understanding how the shapes of energy landscapes, including stable states and kinetic barriers, depend on amino acid sequence.
Dynamics of lithium ions in bismuthate glasses
Authors: Pan A. , Ghosh A.
Published: July 2002

The dynamics of lithium ions in lithium bismuthate glasses have been studied in the frequency range from 10 Hz to 2 MHz and in the temperature range from 323 to 543 K. The composition dependence of the dc (direct current) conductivity has been explained in terms of the structure of bismuthate glasses. The activation energy has been analyzed in the framework of the Anderson–Stuart model. An additional energy term arising from the Madelung constant of glasses and the polarizability of the bismuth ions has been suggested to explain the discrepancy between the calculated and experimentally obtained values. The relaxation mechanism of these glasses has been explored by employing the modulus and conductivity formalisms and the microscopic parameters obtained from the analysis have been compared. Furthermore, the stretched exponential relaxation parameter and the dc conductivity have been correlated with the decoupling index.
Density functional study of carbonic acid clusters
Authors: Ballone P. , Montanari B. , Jones R. O.
Published: July 2002

Density functional calculations on carbonic acid H2CO3 are extended to clusters of up to five such units. The most stable forms are the linear, hydrogen-bonded analogs of the dimer with anti–anti orientation. We calculate structures and vibration frequencies, as well as the energy required to bend and stretch the linear isomers. Linear chains of up to ∼20 units should be favored over ring structures, and they have a tensile strength reminiscent of chains of water molecules. We also discuss planar, nonlinear structures as well as three-dimensional isomers.
Some surprising failures of Brueckner coupled cluster theory
Authors: Crawford T. Daniel , Stanton John F.
Published: July 2002

Brueckner coupled cluster (B–CC) methods have seen a considerable rise in popularity over the last decade thanks, in part, to their apparent propensity for avoiding artifactual symmetry-breaking problems that sometimes plague Hartree–Fock-based approaches. Recent B–CC applications to problematic systems such as the tetraoxygen cation have provided encouraging examples of the success of this theory. In the present work, we examine the performance of the Brueckner technique for a number of other well-known symmetry-breaking problems, including the formyloxyl radical, the first excited state of NO2 and the nitrate radical. In these cases, B–CC methods are found to fail dramatically, predicting broken-symmetry equilibrium geometries in conflict with experimental and/or higher-level theoretical results. A framework is developed which indicates that these errors can be attributed to artificially exaggerated second-order Jahn–Teller interactions with nearby electronic states. Hence, in spite of their initial successes, Brueckner methods cannot be considered a panacea for symmetry-breaking problems.
Theoretical study of collective modes in DNA at ambient temperature
Authors: Cocco Simona , Monasson Rémi
Published: July 2002

The instantaneous normal modes corresponding to radial hydrogen bonds vibrations, torsion, and axial compression fluctuations of a DNA molecule model at ambient temperature are theoretically investigated. Due to thermal disorder, normal modes are not plane waves with a single wave number q but have a finite and frequency dependent damping width. The density of modes ρ(ν), the average dispersion relation ν(q), as well as the coherence length ξ(ν) are analytically calculated. The Gibbs averaged resolvent is computed using a replicated transfer matrix formalism and variational wave functions for the ground and first excited state. Our results for the density of modes are compared to Raman spectroscopy measurements of the collective modes for DNA in solution and show a good agreement with experimental data in the low frequency regime ν<150 cm−1. Radial optical modes extend over frequencies ranging from 50 to 100 cm−1. Torsional and compressional acoustic modes are limited to ν<25 cm−1. Normal modes are highly disordered and coherent over a few base pairs only (ξ<15 Å) in good agreement with neutron scattering experiments.
Global renormalization calculations compared with simulations for square-well fluids: Widths 3.0 and 1.5
Authors: White John A.
Published: July 2002

A recently developed “global” renormalization group theory for calculating thermal properties of fluids both near and to far from the critical point is applied to molecules that interact via a square-well potential. Calculations of gas-liquid coexistence curve densities and vapor pressures are compared with results of recently reported Monte Carlo simulations for hard spheres that interact with square-well potentials of range 3σ (σ=hard core diameter) and with both Monte Carlo and molecular dynamics simulations for square-well potential of range 1.5σ. Additionally, values for the effective critical point exponent βe are calculated for both square wells as a function of temperature distance from the critical point. © 2000 American Institute of Physics.
Polymer network dynamics in shrinking patterns of gels
Authors: Tokita Masayuki , Miyamoto Keiich , Komai Takashi
Published: July 2002

The pattern formation process in the shrinking gel is studied using a confocal laser scanning microscope. The time evolution of the spatial distribution of polymer network within the gel, as well as that of the shape of the gel, is clarified. These results indicate that the pattern formation process of shrinking gel consists of four stages. In the early stages, the diffusion of solvent plays essential roles to form the membrane on the surface portion of the gel. The mechanical properties of the membrane thus formed prevent the uniform shrinking of the gel and finally pattern is formed in the gel.
Absorption, excitation, and resonance Raman spectra of Ce2, Pr2, and Nd2
Authors: Shen Xiaole , Fang Li , Chen Xiaoyu , Lombardi John R.
Published: July 2002

We report the absorption, resonance Raman, and excitation spectra of mass selected cerium, praseodymium, and neodymium dimers in argon matrices. Absorption bands were found for each sample. The excitation profiles give more sensitive detail and resemble absorption spectra. Resonance Raman spectra give single progressions for which Ce2: ωe=245.4±4.2 cm−1, Pr2: ωe=244.9±1.2 cm−1, and Nd2: ωe=148.0±1.9 cm−1, ωexe=0.7±0.4 cm−1. Comparison among lanthanide dimers is discussed.
Application of generating function techniques to Lie superalgebras
Authors: Sharp R. T. , Van der Jeugt J. , Hughes J. W. B.
Published: July 2002

The construction of generating functions for multiplicities of irreducible representations of Lie superalgebras from generating functions for characters is examined, and applied in order to obtain polynomial tensors and branching rules. Problems arising because of the existence of typical and atypical representations are discussed in detail. The techniques are applied to osp(1,2), spl(1,2), osp(3,2), and osp(4,2).
A summation method for the Rayleigh–Schrödinger series for the anharmonic oscillator
Authors: Friedlander Leonid
Published: July 2002

We approximate the energy levels of the anharmonic oscillator with any coupling constant by eigenvalues λj( g,T) of the operator −d2/dx2+x2+gVT(x) with VT(x) =x4 when ‖x‖≤T and VT(x) =T4 when ‖x‖>T. The functions λj( g,T) are holomorphic with respect to g in a neighborhood of the non-negative half-axis. The conformal transformation maps this neighborhood onto the unit circle of the complex plane. It gives the summation method for the Rayleigh–Schrödinger series for every g≥0.
Scattering theory for extended elementary particles in nonrelativistic stochastic quantum mechanics
Authors: Gagnon R.
Published: July 2002

Scattering theory for extended elementary particles in stochastic phase space is studied. It is shown that the interacting Hamiltonian is equivalent to an effective potential in configuration representation. Asymptotic completeness can be studied by investigating the behavior of the effective potential. The sharp-point limit of the extension of these particles is studied. It is also shown that scattering theory can also be studied directly in stochastic phase space in the optimal case.
Gauge theory in relativistic Hamiltonian classical mechanics: Tentative unification of electromagnetic and gravitational fields
Authors: Boisseau Bruno
Published: July 2002

The infinitesimal canonical transformations are considered as gauge transformations. Using a method similar to that of Yang and Mills, one deduces, from a general formalism on phase space, the evolution equations of an infinite sequence of long-range fields sized by order of decreasing force. Special attention is given to the electromagnetic and gravitational fields which can be isolated by means of a restricted theory. In this last case, a comparison is done with the usual equations. An interpretation of the general formalism in terms of a vacuum is suggested in conclusion.
Some static solutions in the general scalar-tensor theory
Authors: Singh T. , Singh Tarkeshwar
Published: July 2002

Exact solutions are found for the static gravitational fields for a matter-free space in the general scalar-tensor theory of Nordtvedt–Barker–Schwinger. The singular behavior of some of the invariants has also been discussed for the solutions in Barker and Schwinger theories.
A new way for solving Laplace’s problem (the predictor jump method)
Authors: Vega-Fernández J. M. , Duque-Carrillo J. F. , Peña-Bernal J. J.
Published: July 2002

This paper presents a new method, that we call ‘‘the predictor jump,’’ for driving to a faster solution of Laplace’s equation. Some results obtained by applying this technique are compared with those that have been obtained by the traditional methods.
A note on Gödel-type space-times
Authors: Rebouças M. J. , Åman J. E. , Teixeira A. F. F.
Published: July 2002

The Gödel-type space-times are examined under different conditions on the algebraic structure of the energy-momentum tensor. A generalization of the Bampi–Zordan theorem on Gödel-type models is given. Chakraborty’s results are recast as a special case of the generalized Bampi–Zordan theorem. The Rebouças–Tiomno solution is shown to be the unique Gödel-type metric with an algebraic structure of the tachyon fluid type.
The Thomas precession and velocity-space curvature
Authors: Ben-Menahem Shahar
Published: July 2002

The motion of a physical system acted upon by external torqueless forces causes the relativistic Thomas precession of the system’s spin vector, relative to an inertial frame. A time-dependent force that returns the system to its initial velocity is considered. The precession accumulates to become a finite rotation of the final spin vector, relative to its initial value. This rotation is commonly explained as the Wigner rotation due to the sequence of pure boosts caused by the force. An alternative interpretation is presented here: The rotation is due to the change of the spin vector as it is parallel-transported around the closed trajectory described by the system in hyperbolic three-velocity space. As an application, the angle of precession for a planar motion is shown to be equal to the area enclosed by the trajectory in velocity space.
Probability density for Bose–Einstein and Fermi–Dirac particles: Slater–Kahn functions
Authors: Kostin M. D.
Published: July 2002

A nonlinear form of the Schrödinger equation is used to derive a partial differential equation for a Slater–Kahn function, which in turn yields the probability density of Bose–Einstein and Fermi–Dirac particles at a given temperature.
The occupation statistics for variously shaped particles on a rectangular 2×N lattice space
Authors: McQuistan R. B. , Hock J. L.
Published: July 2002

Utilizing set theoretic arguments, matrices of shift operators have been constructed, the determinants of which yield recursions that describe exactly the occupational degeneracies for three differently shaped particles (trimers, L-particles, T-particles) distributed on a 2×N rectangular lattice. On the basis of these recursions, the expectation, normalization, dispersion, and continuous representation of the statistics have been calculated and compared for each kind of particle.
On a new hierarchy of nonlinear evolution equations containing the Pohlmeyer–Lund–Regge equation
Authors: Pempinelli F. , Potenza S.
Published: July 2002

A hierarchy of local nonlinear evolution equations associated with a new spectral problem is derived. It is shown that each equation is Hamiltonian and that their fluxes commute and a local infinite set of conserved densities is given. An interesting reduction is considered. In this case a hierarchy of local nonlinear evolution equations is generated by a recursion operator and its explicit inverse. Also this hierarchy satisfies a canonical geometrical scheme. It contains as a special case the Pohlmeyer–Lund–Regge equation.