bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic structure of Ni2MnIn for use in spin injection
Authors: Kilian K. A. , Victora R. H.
Published: July 2002

Data and Das [S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)] proposed an electronic analog to the optoelectronic modulator which would employ spin injection from a ferromagnet into a semiconductor. We investigate the ferromagnet–semiconductor system consisting of the Heusler alloy Ni2MnIn and the semiconductor InAs. Using a full-potential electronic structure code with a basis set of Slater-type orbitals, we have calculated the band structure of Ni2MnIn. We calculate the spin polarization for each atom. It resides primarily on the Mn atom (3.51), with a small moment (0.31) on the Ni. Interestingly, In has a very small moment (−0.04), which is antiferromagnetically coupled to the other atoms. Using a simple model, we estimate the transmittance of minority spins in three high-symmetry directions.
Calculation of shallow donor levels in GaN
Authors: Wang H. , Chen A.-B.
Published: July 2002

The energy levels of shallow substitutional donors in GaN are calculated in an effective-mass theory that includes the effects of mass anisotropy, central-cell potential correction, and the host conduction band edge wave function. The impurities studied include C, Si, and Ge on the Ga sites and O, S, and Se on the N sites of GaN in both zincblende and wurtzite structures. We present the results for the ground state energies and the transition energies from the ground state to the first few excited states. While a number of measured donor levels can be identified with the isolated substitutional donors treated here, there are also a number of experimental results that are substantially different from the present calculations. We discuss these cases and suggest possible explanations.
Ammonia dimer studied by photoelectron spectroscopy
Authors: Carnovale Frank , Peel J. Barrie , Rothwell Richard G.
Published: July 2002

The He i photoelectron spectrum of (NH3)2 is shown to have only a single broad unstructured ionization band near 10 eV. This demonstrates the equivalence or near-equivalence of the nitrogen lone-pair orbitals of each NH3 unit in the dimer structure. While theory has generally predicted that (NH3)2 has a classical structure with a near-linear N--H–N linkage, such a structure would be expected to show two ionization bands separated by about 1.5 eV. However analysis of the observed low ionization energy band suggests that a maximum possible band separation would be only 0.5 eV for ionization from the nitrogen lone-pair orbitals. While this result concurs with the nonclassical structure for (NH3)2 proposed by Klemperer and co-workers, it does not exclude the symmetric C2h structure as a likely equilibrium geometry for the dimer. On the contrary, it is argued that the photoelectron spectrum is supportive of the latter structure. Split-valence basis Hartree–Fock calculations on (NH3)2 at various nonclassical geometries indicate that the dimer could have the symmetric C2h geometry but that a relatively shallow potential energy surface for asymmetric van der Waals vibrations would be responsible for the nonzero dipole moment observed by rotational spectroscopy. This conclusion corroborates the finding of the most recent ab initio calculations on (NH3)2 by Latajka and Scheiner.
The limiting ionic conductivity of Na+ and Cl− ions in aqueous solutions: Molecular dynamics simulation
Authors: Berkowitz Max , Wan W.
Published: July 2002

The molecular dynamics computer simulation technique was used to calculate the limiting ionic mobilities for Na+ and Cl− in TIP4P water. The velocity autocorrelation function and the memory kernel for the corresponding generalized Langevin equation were calculated for the mobile ion. The autocorrelation function of the force exerted on the stationary ion was also calculated and compared with the memory kernel. The two are in good agreement, especially at short times (≈0.1 ps). The diffusion coefficients obtained from Brownian motion theory (the stationary ion case) were also in good agreement with the ones obtained from mobile ion calculations. The major assumptions of the molecular theory of the limiting ionic mobility were tested and not confirmed by the simulation.
Thermal spike model for heavy ion induced desorption from surfaces
Authors: Lucchese Robert R.
Published: July 2002

Yields of molecular ions desorbed from surfaces by energetic heavy ions have been studied using a thermal spike model. A temperature profile was assumed which is appropriate for high linear energy transfer (LET) ion-solid interactions. The desorption kinetics were assumed to follow a simple Arrhenius rate law which has been found to be useful in understanding desorption due to rapid temperature jumps. With these assumptions, we have fit recent desorption experiments where the exciting ions had a LET in the range of 10–100 MeV cm2 mg−1 and where the desorbed molecules ranged from valine to insulin. The thermal spike model predicted nonlinear dependence of the yield on LET at lower LET and a nearly linear dependence of the yield on LET at high LET. These features are in good agreement with the experimental data. We also considered modified models which included a loss channel due to fragmentation. These models gave somewhat better agreement with the experimental data. The parameters obtained in these fits were analyzed using sensitivity analysis to determine their uncertainties and to determine the interdependencies between parameters. The best fit model has been used to predict desorption yields at higher and lower LET and for different initial energy densities in the ionized track.
Volume-phase transitions of ionized N-isopropylacrylamide gels
Authors: Hirotsu Shunsuke , Hirokawa Yoshitsugu , Tanaka Toyoichi
Published: July 2002

Equilibrium swelling curves of N-isopropylacrylamide (NIPA) gel and a series of its ionized counterparts were measured as a function of temperature. Nonionic NIPA gels underwent a sharp yet continuous volume change, whereas incorporation of a small amount of ionizable groups into the gel network drives the transition toward a discontinuous one. The critical ionic concentration and polymer network density at which the get undergoes a critical phase transition were determined. The results were analyzed using a mean field theory. Discrepancy between experimental data and theory is nontrivial, and may require the formulation of a more elaborate theory.
Intramolecular electron transfer and solvation
Authors: Simon John D. , Su Shyh-Gang
Published: July 2002

The intramolecular charge transfer reaction and solvation dynamics of 4,4′-dimethylaminophenyl sulphone in alcohols are measured using picosecond time-resolved emission spectroscopy. The results are compared to theoretical models that treat the solvent as a dielectric continuum. In alcohol solvents, the value for τ−1L (τL =τDε∞/εs) using either n2 or εmw for ε∞ do not provide a good gauge for the intramolecular electron transfer rate. These results are discussed in terms of the importance of vibrational contributions to the reaction rates and the applicability of the parameter τL in gauging the solvent dynamics. In addition, the time-resolved emission data clearly show that the charge transfer state is formed out of equilibrium with the surrounding solvent structure. The measured time scales for the ensuing solvent relaxation are significantly longer than the hydrodynamic continuum model prediction of τL. The discrepancy between the observed rates and those predicted by models based on a dielectric continuum description of the solvent reveal the importance of the underlying molecular aspects of the role of the solvent in both intramolecular charge transfer and solvation.
Temperature dependence of conductance of the Li+, Cs+, and Cl− ions in water: Molecular dynamics simulation
Authors: Reddy M. Rami , Berkowitz Max
Published: July 2002

The continuum theory of Hubbard–Onsager predicts for the friction coefficient Δζ (defined as a difference between friction coefficient and the slip Stokes friction coefficient) the following behavior: Δζ>0 and ∂Δζ/∂T<0. In case of the Li+ ion in water these predictions are confirmed by experiment. In contrast, experimental observations on Cs+ and Cl− ions in water show that at low temperatures Δζ<0 and ∂Δζ/∂T>0. To explain the observed behavior of Δζ(Cs+), the passage through cavities (PTC) mechanism was proposed by Nakahara et al. The molecular dynamics computer simulation technique was used to find out if the PTC mechanism is responsible for the nonclassical behavior of Δζ. No passage through cavities was observed in the computer simulations. Computer simulations were performed on the Li+ ion in water, the Cs+ ion in water, and the Cl− ion in water at temperatures T=268 K and T=298 K. Our results indicate that the observed behavior of Δζ for Li+, Cl−, and Cs+ ions is related to the difference in the reorientation times of water molecules in the ion solvation shell and in the bulk.
Computer modeling of liquid propane using three-site potential models
Authors: Gupta Sumnesh , Yang Jia-an , Kestner Neil R.
Published: July 2002

Molecular dynamics simulations and ab initio calculations, at the Hartree–Fock level and Mo/ller–Plesset perturbation to the second order energy, have been performed to investigate the modeling of propane using nonlinear three-site pair potentials. Simulation results for the structure and thermodynamic properties of saturated liquid propane compare well with the experimental data when potential models are used in which the interaction sites are located at or close to the geometric centers of CH3 and CH2 groups. One of the empirical potentials has also been compared with the ab initio results for different molecular orientations. The three-site model favors stacked orientations whereas the ab initio propane dimer favors T orientation. This difference is due to the planar geometry of the model molecules. The effect of attractive forces on the structure and thermodynamic properties in the simulated liquid has also been studied and found to be small.
Effect of the molecule shape on the diffusion in molecular fluids. Molecular dynamics calculation results for SF6–C6H6 and a related model solution
Authors: Hoheisel C.
Published: July 2002

Solutions of molecules of planar and globular shape have been investigated by molecular dynamics (MD) calculations. Six-center Lennard-Jones (LJ) potential functions have been used to describe the molecular interactions for both types of molecules. An extended density range between liquid and dense gas was considered, and the concentration of the flat molecules was kept low. Two sets of potential parameters and ‘‘bond length’’ parameters have been chosen for the calculations. The first set reproduced, approximately, the thermodynamic and diffusive properties of SF6–C6H6 at high dilution of C6H6 as recently determined experimentally. The second one ensured that the planar molecule had the same ‘‘molecular diameter’’ as the globular one. For both mixture kinds, we compared the diffusion coefficient of the highly diluted component with the self-diffusion coefficient of the solvent. In the case of the SF6–C6H6 model, the self-diffusion coefficient of SF6 appeared to be always larger than that of highly diluted benzene. This agrees well with recent measurements. For the ‘‘model mixture,’’ the self-diffusion coefficient of the solute was however found to be larger than that of the solvent, but with a tendency to decrease more rapidly with decreasing density. This change of the relative mobility of the flat molecules depending on density is explained by the rotational properties of these molecules. Determination of the first two reorientational correlation functions confirmed these conclusions.
Classical mechanics of intramolecular vibrational energy flow in benzene. IV. Models with reduced dimensionality
Authors: Lu Da-hong , Hase William L.
Published: July 2002

The classical mechanics of intramolecular relaxation of benzene CH(D) local mode overtone states is studied with the molecular models HC3, DC3, and H3C3. These reduced dimensionality models provide one means to correct for the improper classical mechanical treatment of zero-point motion in complete benzene models. They give significantly smaller homogeneous linewidths for the low energy CH(D) overtones than found from previous classical trajectory calculations for C6H6/C6D6 models. The n=3 and 5 linewidths for the DC3 model are less than 1 cm−1, while for the HC3 and H3C3 models these linewidths are approximately 5–10 cm−1. The energy transfer pathways for the deuterated and nondeuterated models are substantially different. A gradation of couplings are observed from the trajectories. For the low energy HC3/H3C3 overtones a CCH bend is initially the mode most strongly coupled to the excited CH bond, while for the higher overtones it is the B1 CC stretch. In the relaxation of the H3C3 overtones, five modes are essentially inactive on a 0.75 ps time scale.
Fluctuating pseudoatoms in metallic fluids
Authors: Goldstein Raymond E. , Parola Alberto , Smith Arthur P.
Published: July 2002

The nature of long-range many-body interactions in metallic fluids is examined with emphasis on their possible role in the unique features of these systems observed near the liquid–vapor critical point. A reexamination of recent theoretical results demonstrating the existence of van der Waals forces between ‘‘pseudoatoms’’ (ion cores and associated screening electrons) reveals a direct correspondence with dispersion forces in insulating systems. In the limit of high conduction electron number densities ρ, pseudoatoms have an effective frequency-dependent polarizability α(ω)=α(0)ω2p/(ω2p−ω2), where α(0)=z/4πρ, with z the ion valence, and ωp is the classical electron gas plasma frequency (4πρe2/m)1/2. It is the dynamic nature of the interactions (arising from fluctuations of the pseudoatoms) that permits such a long-range interaction to exist. The dimensionless parameter α(0)ρ which in insulating fluids characterizes the relative importance of triplet (Axilrod–Teller) to pair dispersion interactions is thus system independent and significantly larger than in nonmetallic fluids. The nature of this dynamic polarizability is further examined in the context of a transport theory for a classical plasma based on the Boltzmann equation. The statistical mechanics of fluctuating pseudoatoms at finite temperature is studied both for the metallic fluid and in the Wigner crystal. These various approaches suggest that the pseudoatom interaction may be viewed as a potential mediated by the exchange of plasmons, just as conventional van der Waals forces arise from the exchange of virtual excitations of atomic levels. A description of the critical point in terms of pseudoatom interactions appears to explain qualitatively the extreme liquid–vapor asymmetry of the coexistence curves of cesium and rubidium as arising from the magnitude of three-body interactions. Additionally, it suggests that the thermal energy at the critical point scales with the plasmon energy, consistent with experiment.
A comparison between integral equation theory and molecular dynamics simulations of dense, flexible polymer liquids
Authors: Curro John G. , Schweizer Kenneth S. , Grest Gary S. , Kremer Kurt
Published: July 2002

Recently we (J.G.C. and K.S.S.) formulated a tractable ‘‘reference interaction site model’’ (RISM) integral equation theory of flexible polymer liquids. The purpose of this paper is to compare the results of the theory with recent molecular dynamics simulations (G.S.G. and K.K.) on dense chain liquids of degree of polymerization N=50 and 200. Specific comparisons were made between theory and simulation for the intramolecular structure factor ω̂(k) and the intermolecular radial distribution function g(r) in the liquid. In particular it was possible to independently test the assumptions inherent in the RISM theory and the additional ideality approximation that was made in the initial application of the theory. This comparison was accomplished by calculating the intermolecular g(r) using the simulated intramolecular structure factor, as well as, ω̂(k) derived from a freely jointed chain model.The RISM theory results, using the simulated ω̂(k), were found to be in excellent agreement, over all length scales, with the g(r) from molecular dynamics simulations. The theoretical predictions using the ‘‘ideal’’ intramolecular structure factor tended to underestimate g(r) near contact, indicating local intramolecular expansion of the chains. This local expansion can be incorporated into the theory self consistently by including the effects of the ‘‘medium induced’’ potential on the intramolecular structure.
Counterion dependence of water of hydration in DNA gel
Authors: Urabe Hisako , Kato Midori , Tominaga Yasunori , Kajiwara Kazuhito
Published: July 2002

The cooling process of DNA gel (20 wt./wt.%–40 wt./wt.%) with various counterions has been investigated by low frequency Raman spectroscopy and differential scanning calorimetry (DSC). We have found that the dynamics of water of hydration is different among counterion species below 0 °C. For K-, Rb-, Sr-, and Ba-DNA gels, a part of nonfreezing water slowly changes its dynamical structure, when the gel is kept at ∼−30 °C after cooling below −50 °C. This relaxation process can be monitored both by the frequency increment of the lowest Raman band from ∼17 to ∼20 cm−1 and by the intensity increment of an extra endothermic peak in DSC heating curves. We propose that this relaxation process corresponds to the crystallization of a part of nonfreezing water and is caused by the rearrangement of the secondary hydrated water including counterions when the temperature is held at ∼−30 °C. This relaxation process cannot be observed for NH4- and Li-DNA gels. For Na-, Mg-, and Ca-DNA gels, this phenomenon is more complicated. The dynamical behavior of water of hydration in DNA gels is thus concluded to be different among counterion species.
High-resolution low-frequency Raman spectra of liquid H2O and D2O
Authors: Mazzacurati V. , Nucara A. , Ricci M. A. , Ruocco G. , Signorelli G.
Published: July 2002

New high-resolution experimental data on the low-frequency Raman spectrum of water and heavy water are presented. Both polarized and depolarized spectra are studied over a wide temperature range, including the supercooled liquid region. For the first time the absolute Raman cross section of the depolarized spectrum is reported to be ≊2×10−3 Å6/molecule, for both liquids within the experimental error at room temperature, and compared with the integrated intensity of the Brillouin doublet, 0.134 Å6/molecule. The analysis of the temperature behavior of both integrated intensity and line shapes, and the comparison with the results of molecular dynamics simulations indicate that the Raman spectra in the 0–50 cm−1 region are dominated by the dipole–induced dipole contribution. No direct evidence for H-bond polarizability modulations is found in this frequency region.
Ultrafast investigation of condensed phase chemical reaction dynamics using transient vibrational spectroscopy: Geminate recombination, vibrational energy relaxation, and electronic decay of the iodine A′ excited state
Authors: Lingle Robert , Xu Xiaobing , Yu Soo-Chang , Zhu Huiping , Hopkins J. B.
Published: July 2002

The reaction dynamics of iodine geminate recombination on the excited A′ 3Π2u state are investigated using picosecond transient Raman spectroscopy. Vibrational energy relaxation and the dynamics of geminate recombination are directly observed in the transient Raman spectrum. Geminate recombination rates are significantly slower for the excited electronic state compared to the ground electronic state. This observation implies that a fundamentally different geminate recombination mechanism is responsible for the formation of the A′ 3Π2u state. Evidence is reported which suggests the possible role of iodine atom–solvent complexes in the reaction step leading from photodissociated atoms to formation of the excited state. The solvent dependence of vibrational energy relaxation suggests that the rate is dominated by V–T (and possibly V–R) energy relaxation which is also found to be significantly slower in the excited electronic state compared to the ground state. In n-hexane the vibrational relaxation rate appears to be enhanced by V–V energy transfer. Electronic relaxation is investigated and is found to be largely the result of the formation and decay of I2⋅solvent exciplexes.
Critical exponent for the viscosity of carbon dioxide and xenon
Authors: Berg R. F. , Moldover M. R.
Published: July 2002

The viscosities η of carbon dioxide and xenon have been measured near their critical points and the critical exponent y characterizing the asymptotic divergence, η∼‖T−Tc‖−y, has been determined. Both fluids yielded exponents in the range y=0.041±0.001 and thus also fell in the range y=0.042±0.002 from our earlier study of four binary liquids. This agreement between experiments is the first evidence that pure fluids and binary liquids are in the same dynamic universality class. A recent theoretical value for y is 0.032. The 30% discrepancy is much greater than the combined errors from experiment and theory. The torsion oscillator viscometer operated at low frequency and low shear rate to avoid systematic errors caused by critical slowing down. Far from Tc the analysis accounted for the crossover from critical to noncritical temperature dependence, where the latter was obtained from previously published correlations. Corrections for gravitational stratification were included close to Tc.
Coherent laser control of the resonance-enhanced multiphoton ionization of HCl
Authors: Park Seung Min , Lu Shao-Ping , Gordon Robert J.
Published: July 2002

Coherent control of the ionization of HCl was achieved by simulataneous 3+1 and 1+1 multiphoton excitation in a molecular beam machine, using either three 336 nm photons or one 112 nm photon to resonantly excite the intermediate j 3Σ−(Ω=0+) state. The phase difference between the two laser beams was continuously varied by passing the radiation through a cell containing either Ar or H2 gas.
An examination of a
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o results for the helium potential energy curve
Authors: Aziz Ronald A. , Slaman Martin J.
Published: July 2002

Obtaining a ground state potential energy curve for helium has been the subject of much research involving empirical, semiempirical, and ab initio methods. In this work, we examine critically recent ab initio potentials proposed for this interaction with respect to their ability to predict certain accurate experimental data. To accomplish this analysis, potentials with a modified HFD-B form were fit to the recent theoretical work of van Duijneveldt and co-workers [Vos, van Lenthe, and van Duijneveldt, J. Chem. Phys. 93, 643 (1990) and Vos, van Mourik, van Lenthe, and van Duijneveldt (to be published)] and Liu and McLean (LM-2) [J. Chem. Phys. 91, 2348 (1989)]. A well depth (ε/k=10.92 K) and a separation at the minimum (rm=2.9702 A) consistent with both determinations were chosen and the properties of helium were calculated based on these potentials. These ‘‘mimic’’ potentials fail to predict the very low temperature 4He and 3He virials and one of them [Vos, van Maurik, van Lenthe, and van Duijneveldt (to be published)] also fails to predict the very accurate room temperature viscosity of Vogel [Ber. Bunsenges. Phys. Chem. 88, 997 (1984)]. For a potential which was fit as closely as possible to the LM-2 potential at its maximum suggested depth (ε/k=10.97 K), the virials are satisfactorily predicted. This compromise potential appears to be the best characterization of the He–He interaction in its ability to predict a variety of experimental data as well as being consistent with ab initio results.
Raman noncoincidence effect of the carbonyl stretching mode in compressed liquid cyclic carbonates
Authors: Sun T. F. , Chan J. B. , Wallen S. L. , Jonas J.
Published: July 2002

The Raman noncoincidence effect and line width of the symmetric C■O stretching band have been measured in liquid propylene carbonate (PC), chloroethylene carbonate (CC), and dichloroethylene carbonate (DC) as a function of pressure up to 3 kbar and over the temperature range from −20 °C to 40 °C. The transition dipole moments of the C■O mode for these liquids have also been determined by means of infrared spectroscopy at ambient conditions. The temperature, density, and transition dipole moment dependences of the experimental noncoincidence effect for these liquids are quantitatively interpreted in terms of Logan theory. An excellent agreement between the experimental results and theoretical predictions indicates that the observed noncoincidence effect is due to the transition dipole moment coupling and permanent dipole moment coupling. For the study of isotropic bandwidths, the band narrowing with increasing density is found for liquid CC and DC and quantitatively explained by means of intermolecular interactions, whereas band broadening is observed for PC. The latter broadening is unexpected since PC possesses the largest permanent dipole moment of these three liquids. A probable reason for difficulty in the interpretation of this result is given.
Higher-order derivatives in density-functional theory, especially the hardness derivative ∂η/∂N
Authors: Fuentealba Patricio , Parr Robert G.
Published: July 2002

Several third derivatives of the ground-state electronic energy E with respect to the electron number N and/or the external potential v, as defined in density-functional theory, are studied. These include the first derivatives of the hardness η. Legendre transforms of η[N,v] are constructed and corresponding Maxwell relations derived. Various new functions and relations are found. The derivative of the hardness with respect to the electron number at constant potential γ is studied numerically for atoms and positive ions, and suggested values are displayed and discussed. Recommended values of γ are positive for most spherical atoms, negative otherwise. Power series expansions of E(N) are not recommended for reasons given. Simple analytical representations are recommended instead.
The microstructure of polydisperse, charged colloidal suspensions by light and neutron scattering
Authors: Wagner Norman J. , Krause Rainer , Rennie Adrian R. , D’Aguanno Bruno , Goodwin Jim
Published: July 2002

Structure measurements of a suspension of charged polystyrene spheres with significant polydispersity (16%) are reported for a wide range of particle and added salt concentrations and scattering wave vector. Small angle neutron scattering yielded the structure factors of concentrated samples that have had extensive rheological characterization. A Schulz distribution was used as a model of the particle size distribution to reduce the intensity data and as a basis for the theoretical fits of the structure factors. Static light scattering measurements yielded the structure factors of diluted, deionized samples. Dynamic light scattering was also performed to study the relaxation of these structures. These measured structures were fit with a recent theory that accounts for both size and charge polydispersity in the suspension. These results and the previous extensive, rheological investigations and physical characterization yield a complete data set on a well-characterized, model system that can be used to test theories of the statics and dynamics of polydisperse colloidal suspensions.
Reactions in clusters
Authors: Kaukonen Harri-Pekka , Landman Uzi , Cleveland C. L.
Published: July 2002

A new class of cluster-catalyzed reactions is proposed and investigated using extensive molecular-dynamics simulations. These reactions involve the collision between reactants which are embedded in inert gas clusters, whose role is that of a local heat bath (i.e., extended third body). The concept is demonstrated for the reactions [Na4Cl3]+Arq+Cl− (for q=12 and 32) and [Na14Cl12]+2Ar30+Cl− for several initial temperatures, relative translational energies between the reactants, and impact parameters. For associative reactions, i.e., Cl− attachment accompanied by the release of Ar atoms, the dynamics of the reactions, the dependencies of the isomeric structures of the product alkali-halide clusters, and of reaction pathways and branching ratios on reactant size and reaction conditions are systematically studied.
Investigation of the end-to-end vector distribution function for linear polymers in different regimes
Authors: Bishop Marvin , Clarke Julian H. R. , Rey Antonio , Freire Juan J.
Published: July 2002

Monte Carlo simulations employing the pivot algorithm are used to generate off-lattice three-dimensional linear polymers in three regimes: nonexcluded volume, theta, and excluded volume. The end-to-end vector distribution function is calculated from the resulting configurations. It is found that the shape of the distribution function is Gaussian for nonexcluded volume chains, nearly Gaussian for theta chains, and that the scaling form derived by des Cloizeaux fits the data for excluded volume chains well.
Reference interaction site model polaron theory of the hydrated electron
Authors: Laria Daniel , Wu David , Chandler David
Published: July 2002

We have extended the reference interaction site model (RISM)-polaron theory of Chandler et al. [J. Chem. Phys. 81, 1975 (1984)] to treat self-trapping and localized states of excess electrons in polar fluids. The extension is based on a new closure of the RISM equation presented herein. The theory is applied to the hydrated electron employing a simple class of electron-water pseudopotentials. Included in this class are models coinciding with those already examined by others using computer simulations. In those cases, the results for both structural and energetic properties compare well with those of simulation. The work function, or equivalently, the excess chemical potential of the hydrated electron are also computed; the theoretical result agrees with experiment to about 1%. Most interesting, however, is that as the parameter characterizing the pseudopotentials is varied, a critical parameter is found where the electron behavior changes essentially discontinuously from a trapped state to a ‘‘super’’-trapped state. This transition may have a direct bearing on theoretical efforts to explain the properties of solvated electrons.
The role of attractive intermolecular forces in the density functional theory of inhomogeneous fluids
Authors: Sokolowski S. , Fischer J.
Published: July 2002

The density functional theory of Meister and Kroll for nonuniform simple fluids [Phys. Rev. A 31, 4055, (1985)] is modified in order to take into account pair correlations in the attractive energy term. The theory is applied for the calculation of density profiles of argon in the case of the free liquid surface and in the case of adsorption on ‘‘solid carbon dioxide.’’ For the gas–liquid interface the theory yields dew and bubble densities, which are in good agreement with computer simulations. For the adsorption of argon on solid CO2, the first-order thin-film to thick-film transition is confirmed.
The hyperfine structure of (DCCD)2
Authors: Bhattacharjee R. L. , Muenter J. S. , Coudert L. H.
Published: July 2002

An experimental and theoretical study of the quadrupole-coupling hyperfine structure of the nonrigid (C2D2)2 dimer is carried out. This dimer exhibits a large amplitude interconversion motion which splits rotational levels into three sublevels. The quadrupole-coupling hyperfine pattern arising from the four deuterium atoms depends on the symmetry species of the tunneling sublevel. For nondegenerate sublevels, the hyperfine structure is especially interesting since the dimer behaves as if the quadrupole coupling were identical for all four deuterium atoms and the effective hyperfine Hamiltonian is completely symmetrical. The symmetry group used to classify the hyperfine levels is, therefore, the permutation group of four objects S4. For the other tunneling sublevels, which are doubly degenerate, the dimer behaves as if the two monomer units were inequivalent. Prior to the diagonalization of the hyperfine Hamiltonian, symmetry-adapted nuclear spin wave functions in S4 are set up and allow us to select hyperfine levels whose symmetry is compatible with the tunneling symmetry species. This formalism is used to analyze the hyperfine patterns of three rovibrational transitions in (C2D2)2, which were recorded under high resolution. The components of effective quadrupole-coupling tensors are thereby determined. These tensors are related to the eQq of an isolated DCCD monomer to obtain vibrationally averaged angles for large amplitude bending motions within the dimer.
Computer-simulation studies of β-quinol clathrate with various gases. Molecular interactions and crystal structure
Authors: Zubkus V. E. , Shamovsky I. L. , Tornau E. E.
Published: July 2002

The crystal structure of β-quinol clathrate was investigated by empirical force-field calculations using two sets of potential functions—AMBER and CVFF. The crystal was approximated by the fragments containing 3402 and 15 750 quinol atoms. It was shown that the AMBER potentials are more precise when describing the experimental data on structure of β-quinol clathrate. The bond stretching, valence angle and out-of-plane bendings, dihedral torsion, van der Waals and electrostatic interactions, and hydrogen bonding were taken into account in the potential energy U. The contribution of each of these energies to the formation of structure was estimated. The energy U was minimized with respect to the independent coordinates of the lattice, unit-cell parameters, and both translation and orientation parameters of included molecules. The equilibrium states of encaged guest molecules, β-quinol lattice structure, and energy of clathrate formation were determined for 27 encaged guest molecules. It was shown that the β-quinol lattice can contract as well as expand depending on the type of an encaged molecule. The distribution of charges around the cage favors the positively charged atoms of the molecule to be located in the center of a cage, in contrast with those negatively charged which occupy the sites in the vicinity of peripheral hydroxyl hexagons. The electrostatic component of guest–guest interaction strongly affects the equilibrium position of guest molecules with large dipole moment. Quantitative estimates of various structural and energetic characteristics for β-quinol clathrate prove to be in good agreement with experimental data.
Two-dimensional exchange nuclear magnetic resonance of powder samples. IV. Distribution of correlation times and line shapes in the intermediate dynamic range
Authors: Schaefer D. , Spiess H. W.
Published: July 2002

The two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster chain motions in amorphous polymers in the glass transition range. Acquisition of the time domain data with a four-pulse sequence leads to new characteristics in the corresponding 2H 2D line shapes if the correlation times of the motion are in the intermediate dynamic range. From the asymmetric 2D line shapes, the width of the correlation time distribution can be determined with higher accuracy than through conventional 1D NMR methods. Experimental data are presented on two amorphous polymers—atactic polypropylene and cis-1,4-polyisoprene—and are analyzed in terms of isotropic rotational diffusion. Deviations from this simple model due to the presence of conformational transitions within the polymer backbone are detected. Close to Tg, the mean correlation times extracted from 2D exchange NMR exhibit strongly nonArrhenius behavior usually described by the Williams–Landel–Ferry (WLF) equation. In addition, the width of the correlation time distribution is found to decrease with increasing temperature.
Coexistence curve of polystyrene in methylcyclohexane. VII. Coexistence surface and critical double point of binary system in T–p–φ space
Authors: Hosokawa Hiroshi , Nakata Mitsuo , Dobashi Toshiaki
Published: July 2002

Cloud-point measurements were made for binary systems of polystyrene (PS) +methylcyclohexane (MCH) with three PS samples in the pressure range up to 90 MPa. On a temperature (T)–pressure (p) plane, observed cloud-point curves, which were determined at various compositions for the molecular weight Mw=1.61×104 and at the critical composition for Mw=1.02×104 and 3.49×104, decrease first, reach a minimum and increase with increasing pressure. For Mw=1.61×104 the coexistence surface was investigated in a T–p–φ space, where φ denotes the volume fraction of PS. On a T–φ plane the coexistence curves give a critical composition independent of pressure. On a p–φ plane one phase region appears between two coexistence curves. The top and the bottom of the coexistence curves give an upper critical solution pressure (UCSP) and a lower critical solution pressure (LCSP), respectively. The two critical points coincide forming a critical double point (CDP) at T=19.28 °C, p=48.6 MPa, and φ=0.1719. The shape of the coexistence curves near the CDP is very sensitive to temperature and is characterized by the critical exponent 2β. The Gibbs free energy of the present system was evaluated as a function of T, p, and φ from the molecular weight dependence of the observed critical lines.
Cross correlations between hard and soft forces: A molecular dynamics study
Authors: Sridhar G. , Vijayakumar P. , Tembe B. L.
Published: July 2002

A molecular dynamics simulation is performed on a system containing a ferrous ion and a ferric ion in water to understand the importance of the cross correlations between different terms of the forces on the ions and their contribution to the total time dependent friction (TDF). The ion–solvent interaction potential consists of five terms namely, the exponential, r−12, r−8, r−6, and r−1 (Coulombic). Our results show that the contributions from the cross correlation terms to the TDF are significant.
Density functionals and dimensional renormalization for an exactly solvable model
Authors: Kais S. , Herschbach D. R. , Handy N. C. , Murray C. W. , Laming G. J.
Published: July 2002

We treat an analytically solvable version of the ‘‘Hooke’s Law’’ model for a two-electron atom, in which the electron–electron repulsion is Coulombic but the electron-nucleus attraction is replaced by a harmonic oscillator potential. Exact expressions are obtained for the ground-state wave function and electron density, the Hartree–Fock solution, the correlation energy, the Kohn–Sham orbital, and, by inversion, the exchange and correlation functionals. These functionals pertain to the ‘‘intermediate’’ density regime (rs≥1.4) for an electron gas. As a test of customary approximations employed in density functional theory, we compare our exact density, exchange, and correlation potentials and energies with results from two approximations. These use Becke’s exchange functional and either the Lee–Yang–Parr or the Perdew correlation functional. Both approximations yield rather good results for the density and the exchange and correlation energies, but both deviate markedly from the exact exchange and correlation potentials. We also compare properties of the Hooke’s Law model with those of two-electron atoms, including the large dimension limit. A renormalization procedure applied to this very simple limit yields correlation energies as good as those obtained from the approximate functionals, for both the model and actual atoms.
A finite-time variational method for determining optimal paths and obtaining bounds on free energy changes from computer simulations
Authors: Hunter John E. , Reinhardt William P. , Davis Thomas F.
Published: July 2002

This paper elaborates on the method introduced by Reinhardt and Hunter [J. Chem. Phys. 97, 1599 (1992)] for determining upper and lower bounds on free energy changes and systematically improving these bounds by variational optimization of the Hamiltonian switching path connecting the two states of interest. The advantages, conditions of applicability, and potential pitfalls of this finite-time variational method are illustrated in detail in a Monte Carlo calculation of the free energy change accompanying the reversal of magnetic field in a two-dimensional Ising model. The power of its flexibility in an inhomogeneous system is demonstrated by computing the boundary tension between domains of positive and negative magnetization in the Ising model. Its success in a problem involving molecular dynamics and continuous potentials is illustrated by computing the free energy difference between a Lennard-Jones fluid and a soft-sphere fluid. Finally, its connection with the method of dynamically modified slow growth is discussed.
Classical Hamiltonian structures in wave packet dynamics
Authors: Gray Stephen K. , Verosky John M.
Published: July 2002

The general, N state matrix representation of the time-dependent Schrödinger equation is equivalent to an N degree of freedom classical Hamiltonian system. We describe how classical mechanical methods and ideas can be applied towards understanding and modeling exact quantum dynamics. Two applications are presented. First, we illustrate how qualitative insights may be gained by treating the two state problem with a time-dependent coupling. In the case of periodic coupling, Poincaré surfaces of section are used to view the quantum dynamics, and features such as the Floquet modes take on interesting interpretations. The second application illustrates computational implications by showing how Liouville’s theorem, or more generally the symplectic nature of classical Hamiltonian dynamics, provides a new perspective for carrying out numerical wave packet propagation. We show how certain simple and explicit symplectic integrators can be used to numerically propagate wave packets. The approach is illustrated with an application to the problem of a diatomic molecule interacting with a laser, although it and related approaches may be useful for describing a variety of problems.
Magneto-infrared spectra of the Si2, Ge2, and Sn2 molecules in rare-gas matrices
Authors: Li S. , Zee R. J. Van , Weltner W.
Published: July 2002

Absorption spectra of Si2, Ge2, and Sn2, observed between 50 and 5000 cm−1 in neon and argon matrices at 4 K, were assigned to the lowest 3Πu←X 3Σg− electronic transition. In all three cases, the Franck–Condon envelope of vibrational bands allowed T00, ωe′, ωe′xe′ to be determined. In the Si2 molecule [near Case (a)] T00=313 cm−1 and each vibrational band was split into Ω triplets (0,1,2) in the upper state. A Zeeman effect on these lines was detected in magnetic fields up to 4 T. As the spin–orbit coupling becomes larger in Ge2 and Sn2, the 1u←X0+g transition [case (c)] was observed, and the corresponding parameters in the upper state determined. For Sn2 the magnetic-dipole 1g←X0+g transition was also detected and identified by the splitting and broadening of the band in magnetic fields up to 4 T. The results are discussed in the context of the ab initio calculations and other experimental results on these molecules.
Dodecahedral and smaller arsenic clusters: Asn, n=2, 4, 12, 20
Authors: Shen Mingzuo , Schaefer Henry F.
Published: July 2002

Ab initio all-electron quantum mechanical methods, including the Hartree–Fock (HF), second-order perturbation theory, configuration interaction with single and double excitations (CISD), and coupled cluster with single and double excitations (CCSD) methods, have been applied to four arsenic clusters, diatomic As2, tetrahedral As4, cagelike As12 belonging to the D3d point group, and dodecahedral As20. Several basis sets were used. The double-zeta plus polarization (DZP) includes both d and f polarization functions, while the triple-zeta plus double polarization basis includes two sets of f functions on each atom. From the most reliable theoretical results, As12 is energetically lowest among the clusters considered, and As20 is energetically comparable to As4.
The diffusion of H2 in hexagonal ice at low temperatures
Authors: Strauss Herbert L. , Chen Zhan , Loong C.-K.
Published: July 2002

The diffusion of hydrogen molecules in hexagonal D2O ice (Ih) in the temperature range of 25–60 K was investigated by quasielastic neutron scattering. The observed spectra consist of a narrow elastic peak and a broad quasielastic component. The single-particle diffusion coefficient for H2 in D2O ice was obtained using a simple model of diffusion and is remarkably high—comparable to the coefficient of H2 in a liquid.
Rotational spectrum and structure of an (H2O–HCN)–Ar trimer
Authors: Gutowsky H. S. , Hoey Aline C. , Tschopp S. L. , Keen J. D. , Dykstra C. E.
Published: July 2002

Rotational transitions of the (H2O–HCN)–Ar trimer have been measured at 3–17 GHz with the Balle/Flygare Mark II pulsed nozzle FT microwave spectrometer for the parent, 18O, 13C, 15N, D2O, and HDO isotopic species. The isotopomers exhibit both a- and b-dipole transitions with 14N hyperfine structure and all but the HDO have two sets of transitions assigned to 000 and 101 internal rotational states of the H2O or D2O. Rotational constants were determined by fitting the line centers separately for each isotopic set to the Watson Hamiltonian for an asymmetric top. A molecular mechanics for clusters (MMC) calculation of the potential energy surface and an approximate substitution analysis of the rotational constants give a nearly planar, Δ-shaped structure which is a somewhat distorted superposition of the H2O–HCN, H2O–Ar, and Ar–HCN dimers. MMC also gives a barrier of ≲25 cm−1 to internal rotation of the H2O. Factors governing the formation of trimers are discussed. The effects on trimer structure of differences in the pair interactions are found to be appreciable while the role of three-body effects is small.
Ultrafast solvation dynamics in water: Isotope effects and comparison with experimental results
Authors: Nandi Nilashis , Roy Srabani , Bagchi Biman
Published: July 2002

A detailed theoretical study of solvation dynamics in water is presented. The motivation of the present study comes from the recent experimental observation that the dynamics of solvation of an ion in water is ultrafast and the solvation time correlation function decays with a time constant of about 55 fs. The slower decay in the long time can be described by a sum of two exponentials with time constants equal to 126 and 880 fs. The molecular theory (developed earlier) predicts a time constant equal to 52 fs for the initial Gaussian decay and time constants equal to 134 and 886 fs for the two exponential components at the long time. This nearly perfect agreement is obtained by using the most detailed dynamical information available in the literature. The present study emphasizes the importance of the intermolecular vibrational band originating from the O...O stretching mode of the O–H...O units in the initial dynamics and raises several interesting questions regarding the nature of the decay of this mode. We have also studied the effects of isotope substitution on solvation dynamics. It is predicted that a significant isotope effect may be observed in the long time. The experimental results have also been compared with the prediction of the dynamic mean spherical approximation (DMSA); the agreement is not satisfactory at the long time. It is further found that the molecular theory and the DMSA lead to virtually identical results if the translational modes of the solvent molecules are neglected in the former. DMSA has also been used to investigate the dynamics of solvation of a dipolar solute in water. It is found that the dynamics of dipolar solvation exhibit features rather different from those of ion solvation.
Collective protein dynamics and nuclear spin relaxation
Authors: Brüschweiler Rafael
Published: July 2002

Theoretical methods are developed and applied to the protein crambin as a model system to characterize collective normal mode dynamics and their effects on correlations between torsion angle fluctuations and heteronuclear NMR relaxation parameters. Backbone N–H NMR S2 order parameters are found to be predominantly determined by local φ and ψ torsion angle fluctuations induced by collective protein modes. The ratio between Cβ–Hβ and Cα–Hα order parameters directly yields fluctuation amplitudes of the sidechain χ1 torsion angles. The results allow a more direct interpretation of motional effects monitored by nuclear spin relaxation.