bluuebunny/crossref_metadata_2025 huggingface dataset:
Unified derivation of the dipolar field and relaxation terms in the Bloch-Redfield equations of liquid NMR
Authors: Jeener J. , Vlassenbroek A. , Broekaert P.
Published: July 2002

The standard theory of NMR relaxation in liquids (with molecular motion described as a classical Brownian motion, and including intermolecular spin–spin couplings) is re-examined, taking great care not to drop significant contributions from the dipolar coupling between distant molecules. This results in ‘‘modified Bloch–Redfield equations’’ for the spins in a single molecule, valid at all spin temperatures, which contain both the usual relaxation terms and a coupling of each spin with a classical average dipolar field. Delicate issues raised in this derivation, like the neglect of quantum correlations between spins on different molecules at (repeated) initial times, are discussed with the help of exact calculations (for all spin temperatures) performed on a simplified model which includes equal couplings between all N spins of a system. The same model is used to compare the merits of different forms of ‘‘high temperature’’ approximation. We also propose an iterative scheme for solving the ‘‘modified Bloch–Redfield equations,’’ in which the starting point is the well understood solution of the problem without dipolar field. Finally, a short discussion is given of the relation between ‘‘quantum correlations’’ and ‘‘quantum coherences’’ in the perspective of multiple-pulse and multiple-quantum experiments. These two notions are very simply related in the strict first approximation of weak order, and have often not been clearly distinguished. However, in the second order approximation, which is required whenever dipolar field effects manifest themselves, unrelated spins may display observable coherences although they are not coupled (hence not correlated).
Spontaneous curvature and thermodynamic stability of mixed amphiphilic layers
Authors: May Sylvio , Ben-Shaul Avinoam
Published: July 2002

The elastic behavior of mixed bilayers composed of two amphiphilic components with different chain length (and identical head groups) is studied using two molecular level models. In both, the bilayer free energy is expressed as a sum of chain, head group and interfacial contributions as well as a mixing entropy term. The head group and interfacial terms are modeled using simple phenomenological but general expressions. The models differ in their treatment of the chain conformational free energy. In one it is calculated using a detailed mean-field molecular theory. The other is based on a simple ‘‘compression’’ model. Both models lead to similar conclusions. Expressing the bilayer free energy as a sum of its two monolayer contributions, a thermodynamic stability analysis is performed to examine the possibility of spontaneous vesicle formation. To this end, we expand the bilayer free energy as a power series (up to second order) in terms of the monolayer curvatures, their amphiphilic compositions and the average cross sectional areas per molecule; all variables are coupled, with the molecular composition and areas treated as degrees of freedom which are allowed to relax during bending. Using reasonable molecular interaction parameters we find that a second order transition from a planar to a curved (vesicle) geometry in a randomly mixed bilayer is unlikely. Most of our analysis is devoted to calculating the spontaneous curvature and the bending rigidity of the bilayer as a function of its amphiphile chain composition.

We find that adding short chain amphiphiles to a layer of long chain molecules reduces considerably its bending rigidity, as already known from calculations involving only the chain contributions. However, we find that inclusion of head group and interfacial interactions moderates the effect of the added short chains. We also find that the bending rigidity of pure monolayers is approximately linear in chain length, as compared to the nearly cubic dependence implied by the chain free energy alone (at constant head group area). Our main result involves the calculation of the spontaneous curvature as a function of composition. We find, for different chain mixtures, that upon adding short chains to long chain monolayers, the spontaneous curvature first increases nearly linearly with composition and then (beyond mole fraction of about 0.5) begins to saturate towards the spontaneous curvature of a pure short chain layer. Qualitative arguments are provided to explain this behavior.
Carbon monoxide oxidation on Rh(111): Velocity and angular distributions of the CO2 product
Authors: Colonell J. I. , Gibson K. D. , Sibener S. J.
Published: July 2002

The velocity and angular distributions of CO2 produced by CO oxidation on Rh(111) have been measured as a function of surface temperature and oxygen coverage. Both the velocity and angular distributions are bimodal. The velocities of one component are well fit by a Maxwell–Boltzmann distribution at the surface temperature, and the angular distribution of its intensity is cosine. The second component is non-Boltzmann, and the angular distribution is sharply peaked toward normal. The average energy of this feature is a very strong function of the surface temperature, increasing with a slope of 8.7kb, where kb is the Boltzmann constant, between 475 K and 700 K. Surprisal analysis proves useful in condensing and interpreting these data.
Molecular dynamics simulation of an excess charge in water using mobile Gaussian orbitals
Authors: Staib Arnulf , Borgis Daniel
Published: July 2002

For mixed quantum-classical molecular dynamics simulations of solvated excess charges a novel and efficient method to expand the solute electronic wave function in a distributed Gaussian basis with a shell structure is presented. The aggregate of Gaussian orbitals is capable of mimicking the shape fluctuation of the excess charge distribution and its diffusion through the solvent. This approach also offers an easy pathway to treat the solvent electronic polarization in an explicit and self-consistent fashion. As applications, the results of adiabatic molecular dynamics simulations for the hydrated electron and the aqueous chloride are reported. For e−/H2O the computed ground state absorption spectrum is discussed. Adiabatic relaxation as well as nonadiabatic transition rates are evaluated—the latter by means of an original Golden Rule formula—and compared to experimental results. In the case of Cl−/H2O the charge transfer to solvent spectra are analyzed. The ability of the mobile basis set method to describe the photodetachment dynamics of an electron from aqueous chloride is also demonstrated.
A dual-level Shepard interpolation method for generating potential energy surfaces for dynamics calculations
Authors: Nguyen Kiet A. , Rossi Ivan , Truhlar Donald G.
Published: July 2002

We present a new dual-level approach to representing potential energy surfaces in which a very small number of high-level electronic structure calculations are combined with a lower-level global surface, e.g., one defined implicitly by neglect-of-diatomic-differential-overlap calculations with specific reaction parameters, to generate the potential at any geometry where it may be needed. We interpolate the potential energy surface with a small number of accurate data points (the higher level) that are placed along the reaction path by using information on the global shape of the potential from less accurate calculations (the lower level). We confirm the findings of Ischtwan and Collins on the usefulness of single-level schemes including Hessians, and we delineate the regime of usefulness of single-level schemes based on gradients or even single-point energies. Furthermore we find that dual-level interpolation can offer cost savings over single-level schemes, and dual-level methods employing Hessians, gradients, or even only simple energy evaluations can yield reasonable potential energy surfaces with relatively low cost, with the potentials being more accurate along the reaction path. For all methods considered in this paper the accuracy of the interpolation for our test cases is lower when the potentials at points significantly removed from the reaction path are predicted from data that lie entirely on the reaction path.
On the energy invariance of open-shell perturbation theory with respect to unitary transformations of molecular orbitals
Authors: Crawford T. Daniel , Schaefer Henry F. , Lee Timothy J.
Published: July 2002

A number of recently proposed single-reference open-shell perturbation theories based on a spin-restricted open-shell Hartree-Fock reference function are examined, with an emphasis on a consistent formalism within which the theories may be compared. In particular, the effect of unitary transformations among the molecular orbitals on the energy is discussed. Of the seven different perturbation theories examined here, the restricted Mo/ller–Plesset theory, open-shell perturbation theory method 1, the method of Hubač and Čársky, Z-averaged perturbation theory, and invariant open-shell perturbation theory methods are found to be invariant to all types of rotations for which the reference wave function is unaffected, though all are invariant to transformations of a more limited nature. Explicit equations for the generalized invariant forms of each perturbation theory are presented, in order to provide working equations for extension of the theories to local correlation schemes or coupled-cluster perturbational corrections, among others.
Helium atom scattering from isolated CO molecules on copper(001)
Authors: Graham A. P. , Hofmann F. , Toennies J. P. , Manson J. R.
Published: July 2002

Angular distributions have been measured for helium atoms scattering from isolated CO molecules chemisorbed on a Cu(001) surface as a function of incident beam energy between 9.4 and 100 meV and surface coverage from 1.3% ML to 9.3% ML. Up to five oscillations are clearly observed in the angular distributions. The parallel wave vector transfers of the peaks vary only slightly with incident energy and are independent of coverage up to 20% of a c(2×2) layer. New hard wall scattering calculations show that all of the distinct peaks observed can be explained by interference structures involving both Fraunhofer diffraction and illuminated face scattering from CO molecules with an approximate hard wall radius of 2.4 Å with no evidence of the classical rainbows predicted in several recent theoretical studies.
A kinetic model of partially reversible protein adsorption
Authors: Van Tassel Paul R. , Viot Pascal , Tarjus Gilles
Published: July 2002

We present a kinetic adsorption model for proteins that accounts for the experimentally observed properties of partial reversibility and surface induced conformational change. Particles (proteins) are modeled as disks that adsorb sequentially and without overlap at random positions onto a surface. Following adsorption, a particle can either desorb or spread symmetrically to a larger size. If the latter occurs, it remains adsorbed irreversibly. Both of these events obey first order kinetic rate laws. We derive analytical results in the asymptotic regime and report Monte Carlo results for shorter times. This model yields adsorbed phases that are more dense than those predicted by models of purely irreversible adsorption. We attribute this densification to a fluid structure that is quite liquidlike. We show that a number of experimentally observed kinetic behaviors can be reproduced with this model and that it is in good quantitative agreement with recent experiments.
Interfacial curvature free energy, the Kelvin relation, and vapor–liquid nucleation rate
Authors: McGraw Robert , Laaksonen Ari
Published: July 2002

The interfacial curvature free energy is shown to cause a significant barrier height correction to the classical nucleation rate. This correction is found to be temperature dependent, but independent of nucleus size. Density functional (DF) calculations are presented for a nonuniform spherical droplet model of the nucleus. Calculations for the surface tension, as a function of nucleus size, and for the interfacial curvature free energy support theoretical predictions and provide an explanation for systematic discrepancies between classical and DF nucleation theories and between the classical theory and experiment.
Coupled cluster spectroscopic properties and isomerization pathway for the cyanate/fulminate isomer pair, NCO−/CNO−
Authors: Pak Youngshang , Woods R. Claude , Peterson Kirk A.
Published: July 2002

Three-dimensional near-equilibrium potential energy surfaces and dipole moment functions have been calculated for the ground states of NCO− and CNO−, using the coupled cluster method with single and double substitutions augmented by a perturbative estimate of triple excitations [CCSD(T)] with a quadruple zeta basis set consisting of 150 contracted Gaussian type orbitals. The corresponding equilibrium bond distances at their linear geometries are re(NC)=1.1934 Å and re(CO)=1.2306 Å for NCO−, and re(CN)=1.1866 Å and re(NO)=1.2741 Å for CNO−. Full three-dimensional variational calculations have also been carried out using the CCSD(T) potential energy and dipole moment functions to determine the rotation–vibrational energy levels and dipole moment matrix elements for both NCO− and CNO−. The predicted band origin of the ν3 band in the NCO− isomer (2114.4 cm−1) agrees well with the gas phase diode laser infrared result (2124.4 cm−1). The variational analysis suggests possible revisions in the assignment of the two experimentally observed hot bands which are affected by Fermi resonance. The calculated dipole moments of NCO− and CNO− in their ground vibrational states are 1.504 and 1.482 D, respectively. The CCSD(T) method with a triple zeta basis set was employed to more broadly explore the isomerization path between the two isomers. In agreement with previous lower level calculations a broad and shallow minimum corresponding to a cyclic oxazirine form was found. The immediate vicinity of this local minimum in the potential energy surface and those of the two saddle points separating it from the linear isomers were further refined using the same quadruple zeta basis used for the two near-equilibrium regions. The equilibrium structures and energies of the two neutral isomers, NCO, and CNO, were also calculated at the same level of theory. For NCO, whose photoelectron spectrum has recently been studied in detail, the predicted electron affinity and neutral-ion bond distance changes agree well with the photoelectron results. The reported spectroscopic structure of NCO, however, is not supported by the present CCSD(T) calculations.
Identifying patterns in multicomponent signals by extended cross correlation
Authors: Coy Stephen L. , Jacobson Matthew P. , Field Robert W.
Published: July 2002

The analysis of current problems in physical chemistry often requires the identification of patterns that encode the composition, structure, and dynamics of a system. Overlapped patterns, unexpected patterns, and patterns whose forms are initially unknown are especially difficult to identify and to extract. We have developed two new techniques for pattern recognition and extraction designed for these situations. These techniques, extended cross correlation (XCC) and extended auto correlation (XAC), identify and extract multiple patterns from experimental data even when the number of derived patterns exceeds the number of experiments. The XCC, which is the focus of this paper, allows the rapid identification and extraction of patterns that are repeated in multiple experimental records. The related XAC technique permits the identification of complex patterns that are parameterized in a multidimensional way, even when the patterns are obscured by the presence of interfering data. The XCC and XAC provide straightforward methods for extracting the features which comprise a pattern, and can be applied in a model-free way. This paper provides a formal description of multidimensional forms of the XCC technique, and illustrates use of the XCC on large data sets with multiple patterns.
Semiclassical theory of multi-channel curve crossing problems: Nonadiabatic tunneling case
Authors: Zhu Chaoyuan , Nakamura Hiroki
Published: July 2002

Based on the new two-state theory of curve crossing recently completed by the authors, a compact and powerful theory is formulated for a general resonant multi-channel scattering with nonadiabatic tunneling (NT) type curve crossings. This theory is demonstrated to work remarkably well by comparing with the numerical solutions of close-coupling equations. Even detailed structures of overlapping resonances are nicely reproduced by the theory. Furthermore, this theory is very simple, not requiring any nonunique diabatization procedure, any complex calculus and any information on the couplings, neither diabatic nor nonadiabatic. The theory is based only on the adiabatic potentials on the real axis. Together with the previously proposed theory for the Landau-Zener (LZ) type curve crossings, the present semiclassical theory provides a complete picture of and a very powerful tool for multi-channel curve crossing problems.
Anomalous size-dependence of interfacial profiles between coexisting phases of polymer mixtures in thin-film geometry: A Monte Carlo simulation
Authors: Werner Andreas , Schmid Friederike , Müller Marcus , Binder Kurt
Published: July 2002

The interfacial profile between coexisting phases of a binary mixture (A,B) in a thin film of thickness D and lateral linear dimensions L depends sensitively on both the linear dimensions and on the nature of the boundary conditions and statistical ensembles applied. These phenomena, generic for systems in confined geometry, are demonstrated by Monte Carlo simulations of the bond fluctuation model of symmetric polymer mixtures, using chains containing NA=NB=N=32 effective monomers connected by effective bonds with an attractive interaction between monomers of the same type and a repulsive interaction between different types. We use short-range potentials at the walls, the right wall favoring A monomers and the left wall B monomers. Periodic boundary conditions are applied in the directions parallel to the walls. Both the canonical and semi-grand-canonical ensemble are studied. We argue that the latter case is appropriate for experiments with a lateral resolution L much less than the actual lateral sample size, in thermal equilibrium. In the canonical ensemble, the interfacial width w increases (from small values that are of the same order as the “intrinsic profile”) like w∝D, before a crossover to a saturation value wmax (wmax2∝ ln L) sets in. In the semi-grand-canonical ensemble, however, one finds the same widths w∝D as in the canonical ensemble for not too large L, while for large L the interfacial profile is smeared out over a finite fraction of the film thickness (w∝D for D→∞). We discuss the implications of these findings for the interpretation of both simulations and experiments.
Oscillating chemical reactions and phase separation simulated by molecular dynamics
Authors: Geisshirt Kenneth , Praestgaard Eigil , Toxvaerd So/ren
Published: July 2002

Molecular dynamics (MD) of stationary chemical kinetics is used to simulate oscillating chemical reactions in a system of N classical mechanical particles with Lotka–Volterra kinetics. The MD includes oscillations in a (closed) system with conserved energy and time reversible dynamics as well as oscillating chemical reactions in an open and driven non-equilibrium system, and with and without a competing phase separation of the different components in the reactions. The approach allows a detailed investigation of the kinetics and demonstrates on a molecular level, the phenomenon oscillating reactions for various chemical and reaction kinetics details. When phase separation takes place during the oscillations the kinetics is no longer simple diffusion driven.
Optimized density functionals from the extended G2 test set
Authors: Schmider Hartmut L. , Becke Axel D.
Published: July 2002

A recently suggested procedure for the systematic optimization of gradient-corrected exchange-correlation functionals [A. D. Becke, J. Chem. Phys. 107, 8554 (1997)] has been applied to the extended G2 test set [L. A. Curtiss et al., J. Chem. Phys. 106, 1063 (1997)], which consists of the standard heats of formation of 148 molecules. The limit of reproduction of the experimental data in this test set is found to be 1.78 kcal/mol mean absolute error, with a maximum of 8.89 kcal/mol error for the ozone molecule. This compares rather well with previous results for G2 theory itself (1.58 and 8.2 kcal/mol, respectively). We show that fair stability can be obtained by our optimization procedure.
A theoretical study of the near-edge x-ray absorption spectra of some larger amino acids
Authors: Carravetta Vincenzo , Plashkevych Oleksandr , Ågren Hans
Published: July 2002

The near-edge x-ray absorption (NEXAFS) spectra at the carbon K edge of the five amino acids, glycine, phenylalanine, histidine, tyrosine, and tryptophan, have been computed by the ab initio static-exchange method and compared to recently collected experimental data. The analysis of the spectra is carried out on the basis of a building-block decomposition taking advantage of the separate spectra for each carbon center. It is shown that different subunits can be clearly identified by such an analysis and that their spectral features remain largely unperturbed in the different molecules. The results support the suggestion by Boese et al. [J. Electron. Spectrosc. Relat. Phenom. 85, 9 (1997)] that differences in the NEXAFS spectra might be used for mapping proteins which differ in their amino acid content.
A Voronoi analysis of lipid area fluctuation in a bilayer
Authors: Shinoda Wataru , Okazaki Susumu
Published: July 2002

Membrane area fluctuation of the lipid bilayer has been investigated based upon two-dimensional Voronoi tessellation analysis for the centers of mass of the lipid molecules projected on the bilayer plane. Long-time trajectories of the molecules used in the analysis have been generated by molecular dynamics calculations. The single-molecular area defined by Voronoi polygon showed a broad Gaussian distribution, from which area distribution of the membrane composed of N lipid molecules may satisfactorily be predicted. The fluctuation was found to be caused mainly by thermal motions of the alkyl chains. The number of gauche conformation and alkyl chain length was strongly correlated to the molecular area. Head group motions, however, showed little contribution to the fluctuation. Geometry of Voronoi polygons and the number of nearest neighbor molecules showed rather broad distribution due to the thermal fluctuation. This is in contrast to the structure found in the ripple, gel, and crystal phases. Formation of large pores in the membrane was also investigated.
Fluctuations in a small hard-disk system: Implicit finite size effects
Authors: Román F. L. , White J. A. , González A. , Velasco S.
Published: July 2002

The influence of implicit finite size effects on the fluctuation in the number of particles in a subvolume is studied for a small system of hard disks with a fixed number of particles. The implicit (or anomalous) finite size effects—that arise from the use of periodic boundary conditions—are taken into account by including the periodicity of the total system into a model pair correlation function. Two pair correlation functions are considered; the accurate Percus–Yevick result and an approximation proposed by Baus and Colot that yields an excellent isothermal compressibility. Although very good agreement with canonical ensemble Monte Carlo results is obtained in both cases, it appears that the theoretical expression obtained for the fluctuation in the number of particles is rather sensitive to the thermodynamic and structural information conveyed by the pair correlation function.
Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet detection of the products
Authors: Ni Chi-Kung , Huang J. D. , Chen Yit Tsong , Kung A. H. , Jackson W. M.
Published: July 2002

Vacuum ultraviolet (VUV) laser photoionization is combined with time-of-flight (TOF) mass spectrometry to determine the photofragments produced from the laser photodissociation of allene and propyne in a molecular beam. Detection of C3H3+ confirms that atomic hydrogen elimination is the primary process for both of these molecules. A hydrogen molecule elimination channel and a low mass carbon fragmentation channel of allene to produce C3H2+H2 and CH2+C2H2, respectively, have also been identified. Different ratios of various dissociation channels from these two molecules suggest that the dissociation mechanisms of these two isomers are different. Dissociation must occur before complete isomerization. These results are discussed in terms of recent theoretical calculations on the ground and excited states of these molecules. Secondary photodissociation of the products has been observed, even though the laser energies that have been used are less than 8 mJ/cm2 and the photolysis laser is not focused. Therefore, the present results show how important it is to determine product distributions as a function of the laser energy.
Stochastic Liouvillian algorithm to simulate dissipative quantum dynamics with arbitrary precision
Authors: Stockburger Jürgen T. , Mak C. H.
Published: July 2002

An exact and efficient new method to simulate dynamics in dissipative quantum systems is presented. A stochastic Liouville equation, deduced from Feynman and Vernon’s path-integral expression of the reduced density matrix, is used to describe the exact dynamics at any dissipative strength and for arbitrarily low temperatures. The utility of the method is demonstrated by applications to a damped harmonic oscillator and a double-well system immersed in an Ohmic bath at low temperatures.
Colloidal aggregation in energy minima of restricted depth
Authors: Molina-Bolı́var J. A. , Galisteo-González F. , Hidalgo-Álvarez R.
Published: July 2002

Coagulation rates of bare and protein-covered colloidal particles show a different dependence on experimental conditions. While the rapid coagulation rate for the bare particles obeys the modified Smoluchowski theory and is independent of pH and the nature of the cation and the anion, the value for the coated particles is lower and depends on pH and ions’ nature. The variation in the Hamaker constant and the existence of a shallow primary minimum of the interparticle potential for the latex–protein complex, both attributed to the layer of water molecules and ions adsorbed on protein, may explain these results. Coagulation rates were measured with a low angle light scattering apparatus, and the experimental curves of stability fitted using Fuchs’ equation and the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory. In the case of covered particles, a modified expression of the van der Waals attraction was used. This attraction depends on the Hamaker constant for the protein in the vacuum, whose value was estimated from contact angle measurements.
Pressure-induced phase transition in mixed-valence gold complexes Cs2Au2X6 (X=Cl and Br)
Authors: Liu X. J. , Moritomo Y. , Nakamura A. , Kojima N.
Published: July 2002

Effects of hydrostatic pressure on Raman spectra have been investigated in mixed-valence (MV) gold complexes Cs2Au2X6 (X=Cl and Br). We have observed pressure-induced deactivation of the Raman-active Au-X stretching modes at various temperatures, indicating a pressure-induced phase transition from the MV state to a single-valence (SV) state. The critical pressure at 300 K decreases from 10.6 GPa for X=Cl to 6.8 GPa for X=Br, perhaps due to reduced electron–phonon interaction in Cs2Au2Br6. We further derived comprehensive electronic phase diagrams of Cs2Au2X6 (X=Cl and Br) as a function of pressure and temperature.
Characterization of the Ã(1A″) state of HCF by laser induced fluorescence spectroscopy
Authors: Schmidt Timothy W. , Bacskay George B. , Kable Scott H.
Published: July 2002

An extensive experimental exploration of the Ã(1A″)←X̃(1A′) transition of supersonically cooled fluoromethylene has been performed using laser induced fluorescence spectroscopy. Several new bending vibrational bands are reported, which provide the harmonic frequency and anharmonicity constant for this vibration and lead to an estimate of the height of the barrier to linearity as 6400±500 cm−1. Assignment of the vibrational hot-band structure leads to the first measurement of the Ã state CF stretching frequency as ν3′=1260 cm−1 and tentatively the CH stretching frequency as ν1′=2852 cm−1. The A′ rotational constant increases strongly with increasing quanta of bending vibration, which indicates that the molecular structure is becoming more linear. Consideration of only the average bond angle, calculated from ab initio data for this state, is insufficient to account for the change in A′. The coupling of a-axis rotation with bending vibration must be included. A number of other dynamical effects were observed in the spectra, including lifetime shortening and disappearance of rotational transitions with K′⩾1. These were explained in terms of the Renner–Teller interaction between the X̃ and Ã states.
Quantum Monte Carlo study of the dipole moment of CO
Authors: Schautz F. , Flad H.-J.
Published: July 2002

We have calculated the dipole moment of CO along the potential energy curve using the pure diffusion quantum Monte Carlo method. Good agreement with coupled cluster singles–doubles calculations has been obtained throughout. An accurate treatment at the equilibrium distance requires a multi-reference trial wave function for which we obtained a dipole moment in accordance with experiment. We discuss the Hellmann–Feynman theorem within the fixed-node approximation in the case of field-dependent smoothly varying nodes. It is shown that the Hellmann–Feynman theorem is applicable to this case due to vanishing boundary contributions. A comparison has been made between the finite field correlated sampling approach and a direct calculation of the expectation value for the dipole moment operator. In the present application both methods perform equally well.
Isosbestic points in liquid water: Further strong evidence for the two-state mixture model
Authors: Robinson G. Wilse , Cho Chul Hee , Urquidi Jacob
Published: July 2002

Three different types of isosbestic points observed for liquid water are dealt with in this paper—temperature induced Raman, temperature induced structural, and pressure induced structural isosbestic points. These isosbestic points leave no doubt as to the precise two-state outer-neighbor mixture model description of this important substance from supercooled temperatures up to at least 40 °C. New pressure-dependent Raman experiments are suggested that will help confirm further this most simple idea that has already been shown to be quantitatively consistent with the temperature- and pressure-dependent properties of liquid water, including all its anomalies.
High resolution molecular beam study of the origin band of the B̃ 2Σ+–X̃ 2Σ+ system of yttrium imide
Authors: Simard Benoit , Jakubek Zygmunt , Niki Hideaki , Balfour Walter J.
Published: July 2002

The (0,0,0)–(0,0,0) band of the B̃ 2Σ+–X̃ 2Σ+ system of three isotopomers of yttrium imide (Y14NH, Y15NH, and Y14ND) has been studied by laser-induced fluorescence in a molecular beam apparatus. Rotational, fine, and nuclear magnetic hyperfine structures have been resolved and analyzed. The B̃ 2Σ+(0,0,0) state of Y14NH, Y14ND, and Y15NH is severely perturbed below J=30.5 by eight, three, and two vibronic states, respectively. Although, the nature of these perturbing states can only be speculated upon, their symmetries are either Σ2 or Π2, and this has made it possible to deperturb the B̃ 2Σ+ state successfully. The spectra can be reproduced within 140 MHz (0.0047 cm−1). The analyses confirm that the molecule is linear in both states with the nuclear arrangement Y–N–H. The bond lengths in the ground X̃ 2Σ+ state and the B̃ 2Σ+ state have been derived to be rY–N=1.877 57(13) Å, rN–H=1.0067(10) Å, and rY–N=1.8839(43) Å, rN–H=1.242(30) Å, respectively. The results are compared with the values of ab initio calculations on YNH and YN, and the experimental data on YN and YO. The atomic character of the unpaired electron in the ground state is 58% Y + 5s and 42% Y + 5p. The electron configurations for the ground X̃ 2Σ+ state and the B̃ 2Σ+ state are discussed and compared with ab initio calculations whenever possible.
Fully coupled 6D calculations of the ammonia vibration-inversion-tunneling states with a split Hamiltonian pseudospectral approach
Authors: Gatti Fabien , Iung Christophe , Leforestier Claude , Chapuisat Xavier
Published: July 2002

An efficient pseudospectral method for performing fully-coupled six-dimensional bound state dynamics calculations is presented. A Lanczos-based iterative diagonalization scheme produces the energy levels in increasing energies. This scheme, which requires repetitively acting the Hamiltonian operator on a vector, circumvents the problem of constructing the full matrix. This permits the use of ultralarge molecular basis sets in order to fully converge the calculations. The Lanczos scheme was conducted in a symmetry adapted six-dimensional spectral representation. The Hamiltonian operator has been split into only four different terms, each being Hermitian and symmetry-adapted. The potential term is evaluated by a pseudospectral scheme of Gaussian accuracy, which guarantees the variational principle. Spectroscopic levels are computed with this method for one ammonia potential, and compared to experimental results. The results presented below are a direct application of our vector formulation. The latter has shown to be particularly well adapted to the split pseudospectral approach for it yields a compact and symmetry-adapted Hamiltonian.
The effect of hydrophobic and hydrophilic channel walls on the structure and diffusion of water and ions
Authors: Allen T. W. , Kuyucak S. , Chung S.-H.
Published: July 2002

Molecular dynamics simulations are carried out to determine the effects of channel wall structure on water and ion properties. We compare hydrophobic (Lennard-Jones 5-3 and atomic) and molecular-hydrophilic cylindrical pores of 2–6 Å in effective radius, relevant to the study of most significant biological ion channels including gramicidin A, ACh, and potassium channels, and to the study of many microporous materials. Large variations in levels of self-diffusion and rotational correlation within hydrophobic channels are explained in terms of water geometry, hydrogen bonding, and dipole correlation. The differing levels of water structure and self-diffusion in hydrophobic and hydrophilic pores arise because of marked differences in the preferred orientation of water dipole moments, and due to hydrogen bonding with molecules on the pore lining. Axial sodium ion diffusion does not experience large variations with pore size, despite anomalous stability in moderate-sized hydrophobic pores. We attribute this to the ability of ions to diffuse along troughs of water density. Ion diffusion along the pore axis exhibits a general increase with channel radius in hydrophobic channels but remains fairly low in hydrophilic channels.
Adsorption of CO2 and Ar on glass surfaces. Computer simulation and experimental study
Authors: Bakaev V. A. , Steele W. A. , Bakaeva T. I. , Pantano C. G.
Published: July 2002

Isotherms of adsorption of CO2 and Ar are simulated by the grand canonical Monte Carlo on four model surfaces of amorphous silica. The surfaces designated A through D differ progressively in their degree of annealing, A being an unannealed, nonequilibrium surface and D being the most extensively annealed. The gas–gas interaction potentials for both gases were taken from the literature and the gas–solid interactions were modeled by applying Lorentz–Berthelot combining rules to the gas–gas potentials plus the TTAM representation of the atom–atom interactions in the solid. The simulated isotherms of Ar on surfaces A and D are close to each other and to the experimental isotherm for nonporous silica. In contrast, the simulated isotherms and isosteric heats of adsorption of CO2 on these surfaces differ considerably from each other. This leads to the conclusion that argon adsorption is not sensitive to the changes in surface structure that occur during annealing, but CO2 is. Since these gases differ considerably in their polarity, these results indicate that the observed differences in adsorption behavior are due in large part to the annealing-induced changes in the electrostatic part of the CO2–SiO2 interaction. The isotherm of CO2 on D was made to be very close to an experimental isotherm on dehydroxylated nonporous silica by diminishing the electrostatic part of the CO2–SiO2 interaction by 30% from that in the original model of the potential. Isotherms of adsorption of CO2 on multicomponent glass fibers measured at 194.5 K are reported here. The sub-monolayer experimental isotherms on glass lie higher than an isotherm on hydroxylated silica which in turn lies higher than that for dehydroxylated silica. The sensitivity of the physical adsorption of CO2 to the chemical nature and the structure of the SiO2 surface indicates that comparisons of experiment with simulations of the isotherms of polar or quadrupolar molecules like CO2 on such model surfaces can be a useful probe of surface structure of silica and silicate glasses.
Lennard-Jones binary mixture: A thermodynamical approach to glass transition
Authors: Coluzzi Barbara , Parisi Giorgio , Verrocchio Paolo
Published: July 2002

We study the liquid-glass transition of the Lennard-Jones binary mixture introduced by Kob and Andersen from a thermodynamic point of view. By means of the replica approach, translating the problem in the study of a molecular liquid, we study the phase transition due to the entropy crisis and we find that the Kauzmann’s temperature TK is ∼0.32. At the end we compare analytical predictions with numerical results.
Global renormalization calculations compared with simulations for Lennard-Jones fluid
Authors: White John A.
Published: July 2002

A recently developed renormalization theory for fluids, that treats nonuniversal as well as universal thermal properties both near and to far from the critical point, is applied to a Lennard-Jones potential. Predictions of volumetric properties by the theory for Lennard-Jones particles for temperatures in the range 0.53⩽T/Tc⩽4.6 and densities 0.016⩽ρ/ρc<4.0 are compared with results of molecular dynamics simulations. The comparison suggests that the theory may be capable of making volumetric predictions for the fluid accurate to ∼1%–2% (for perpendicular distance between calculated isotherms and data points obtained by simulation) for temperatures and densities throughout much of this extended neighborhood of the critical point.
Finding transition structures in extended systems: A strategy based on a combined quantum mechanics–empirical valence bond approach
Authors: Sierka Marek , Sauer Joachim
Published: July 2002

A method for efficient localization and description of stationary points on the potential energy surface of extended systems is presented. It is based on Warshel’s empirical valence bond approach, for which we propose a modification, and combines the potential function description of the total system with a quantum mechanical description of the reaction site (QM-Pot). We describe the implementation of the method in the QMPOT program, which is basically an optimizer for minima and saddle points and has interfaces to existing quantum mechanical (e.g., TURBOMOLE, GAUSSIAN94) and interatomic potential function codes (e.g., GULP, DISCOVER). The power of the method is demonstrated for proton transfer reactions in zeolite catalysts, which may have as many as 289 atoms in the unit cell. As a test case the zeolite chabazite is considered in this study. Its limited unit cell size (37 atoms) makes comparison with the full periodic ab initio limit possible. The inclusion of long-range effects due to the periodic crystal structure by the QM-Pot method proves crucial in obtaining reliable results. The combined quantum mechanics–interatomic potential function calculations yield reaction barriers within 6 kJ/mol and reaction energies within 3.5 kJ/mol of the periodic ab initio limit. The zero-point vibrational energy corrected reaction barriers are between 58 and 97 kJ/mol for the six different proton jump paths. These are density functional results employing the B3LYP functional.
Continuous surface switching: An improved time-dependent self-consistent-field method for nonadiabatic dynamics
Authors: Volobuev Yuri L. , Hack Michael D. , Topaler Maria S. , Truhlar Donald G.
Published: July 2002

We present a new semiclassical method for electronically nonadiabatic collisions. The method is a variant of the time-dependent self-consistent-field method and is called continuous surface switching. The algorithm involves a self-consistent potential trajectory surface switching approach that is designed to combine the advantages of the trajectory surface hopping approach and the Ehrenfest classical path self-consistent potential approach without their relative disadvantages. Viewed from the self-consistent perspective, it corresponds to “on-the-fly histogramming” of the Ehrenfest method by a natural decay of mixing; viewed from the surface hopping perspective, it corresponds to replacing discontinuous surface hops by continuous surface switching. In this article we present the method and illustrate it for three multidimensional cases. Accurate quantum mechanical scattering calculations are carried out for these three cases by a linear algebraic variational method, and the accurate values of reactive probabilities, quenching probabilities, and moments of final vibrational and rotational distributions are compared to the results of continuous surface switching, the trajectory surface hopping method in two representations, the time-dependent self-consistent-field method, and the Miller–Meyer classical electron method to place the results of the semiclassical methods in perspective.
Theoretical study of XONO2 (X=Br, OBr, O2Br): Implications for stratospheric bromine chemistry
Authors: Parthiban Srinivasan , Lee Timothy J.
Published: July 2002

The equilibrium structure, dipole moment, harmonic vibrational frequencies, and infrared intensities of XONO2 (X=Br, OBr, O2Br) are determined using density functional theory in conjunction with a TZ2P (triple zeta double polarized) basis set. The B3LYP functional was used since this has previously been shown to perform well for similar bromine compounds. The equilibrium geometry and vibrational spectra of BrONO2 are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations. The vibrational spectrum of O2BrONO2 has been compared with that of the chlorine analog, O2ClONO2, for which some experimental data exist. The bonding in OBrONO2 is shown to be more similar to that in BrONO2. Using large atomic natural orbital basis sets, the singles and doubles coupled-cluster method that includes a perturbative correction for triple excitations, denoted CCSD(T), was employed to compute energies for three isodesmic reactions in order to determine heats of formation for OBrONO2 and O2BrONO2. Our best estimates are 36.7 and 38.7 kcal/mol, respectively. Finally, the possible formation of O2BrONO2 in the stratosphere by adduct formation and oxidation of OBrONO2 and the implications for stratospheric bromine chemistry are discussed.
Integrable equations in (2+1) dimensions associated with symmetric and homogeneous spaces
Authors: Athorne Chris , Fordy Allan
Published: July 2002

Generalizations of the N-wave, Davey–Stewartson, and Kadomtsev–Petviashvili equations associated with homogeneous and symmetric spaces are presented. These equations are (2+1)-dimensional generalizations of those presented by Fordy and Kulish [Commun. Math. Phys. 89, 427 (1983)] and Athorne and Fordy [J. Phys. A, 20, 1377 (1987)]. Examples are explicitly presented that are associated with the simplest spaces. In particular, a new single component, (2+1)-dimensional generalization of the KdV equation is presented.
Heat equation asymptotics of ‘‘nonminimal’’ operators on differential forms
Authors: Gilkey Peter B. , Branson Thomas P. , Fulling Stephen A.
Published: July 2002

Let D=αdδ+βδd−E be a natural operator on differential forms. For example, (Dv)j=−■k∇kvj−C1■j∇kvk +C2Rjkvk has this form. It will be shown that when the lower-order term E is absent, the integrated heat kernel coefficients an(D) can be simply expressed in terms of the (familiar) coefficients an(Δj) for the Laplacian operators on forms. The first two coefficients when a zeroth-order term is present are evaluated.
Polynomial relations in the Heisenberg algebra
Authors: Fleury Norbert , Turbiner Alexander
Published: July 2002

Polynomial relations between the generators of the classical and quantum Heisenberg algebras are presented. Some of those relations can have the meaning of the formulas of the normal ordering for the creation/annihilation operators which occur in the method of the second quantization. Polynomial relations of a special type invariant under quantization and q deformation are discovered.
Boundary integral equations for the scattering of electromagnetic waves by a homogeneous chiral obstacle
Authors: Ola Petri
Published: July 2002

The time-harmonic scattering of electromagnetic waves by a homogeneous chiral media are considered herein. The scattering problem is reduced using Bohren’s decomposition to an equivalent boundary integral equation which is shown to be uniquely solvable except for a discrete set of values of electromagnetic parameters of the scatterer. It should be noted that the boundary integral operators appearing in this equation are the same as for a dielectric scatterer, and hence their mapping properties are well known.
The Levi–Civita space–time
Authors: da Silva M. F. A. , Herrera L. , Paiva F. M. , Santos N. O.
Published: July 2002

Two exact solutions of Einstein’s field equations corresponding to a cylinder of dust with net zero angular momentum are considered. In one of the cases, the dust distribution is homogeneous, whereas in the other, the angular velocity of dust particles is constant [Nuovo Cimento B 21, 64 (1974)]. For both solutions the junction conditions to the exterior static vacuum Levi–Civita space–time were studied. From this study we find an upper limit for the energy density per unit length σ of the source equals 1/4 for both cases. Thus the homogeneous cluster provides another example [Class. Quant. Grav. 8, 727 (1991); J. Math. Phys. 27, 152 (1980)] where the limit of σ is 1/4. It was also found that the cluster of homogeneous dust has a superior limit for its radius, depending on the constant volumetric energy density ρ0.