bluuebunny/crossref_metadata_2025 huggingface dataset:
Computational exploration of the six-dimensional vibration–rotation–tunneling dynamics of (NH3)2
Authors: van Bladel J. W. I. , van der Avoird A. , Wormer P. E. S. , Saykally R. J.
Published: July 2002

In order to address the well-known problem that the nearly cyclic structure of (NH3)2 deduced from microwave spectra differs greatly from the hydrogen-bonded equilibrium structure obtained from ab initio calculations, we have calculated the vibration–rotation–tunneling (VRT) states of this complex, and explicitly studied the effects of vibrational averaging. The potential used is a spherical expansion of a site–site potential which was extracted from ab initio data. The six-dimensional VRT wave functions for all the lowest states with J=0 and J=1 were expanded in products of radial (van der Waals stretch) functions and free-rotor states for the internal and overall rotations, which were first adapted to the complete nuclear permutation inversion group G36. Although the (expanded) potential is too approximate to expect quantitative agreement with the observed microwave and far-infrared spectra, we do find several interesting features: The 14N quadrupole splittings and the dipole moment of the complex, which are indicative of the orientational distributions of the NH3 monomers, are substantially affected by vibrational averaging. The interchange tunneling of the two monomers is not quenched. In the ortho–ortho and para–para states, of A and E symmetry, this tunneling manifests itself in a very different manner than in the ortho–para states of G symmetry. In contrast with the interpretation of Nelson et al. [J. Chem. Phys. 87, 6364 (1987)], we believe that the Gα and Gβ states observed by these authors correspond to a single VRT state which is split by (hindered) NH3 monomer inversion.
Coupling of density to concentration fluctuations in concentrated solutions of polystyrene in toluene
Authors: Konak Cestmir , Brown Wyn
Published: July 2002

In concentrated polystyrene (PS) solutions in toluene, close to the glass transition temperature (Tg), both concentration (q2 dependent) and density fluctuations (q independent) are simultaneously observable in the time window of the polarized (VV) dynamic light scattering experiment. At Φp=0.89, both modes (Γc and Γd) are well separated on the time scale. Close to Tg, the diffusion relaxation is ‘‘frozen-in,’’ a phenomenon anticipated from free-volume theory but not previously observed in polymer solutions. At Φp=0.78, a q2-dependent mode is found above about 40 °C, whereas it is q independent at 17 °C. At intermediate temperatures, the two modes are coupled when Γc≊Γd. The coupling of density to concentration fluctuations is analyzed here in terms of a recent theory of Jäckle and Frisch [J. Polym. Sci. Polym. Phys. Ed. 23, 675 (1985)]. The latter is found to provide a good semiquantitative description of the complex dynamic behavior in this system close to the glass transition.
Transport properties of random arrays of freely overlapping cylinders with various orientation distributions
Authors: Tomadakis Manolis M. , Sotirchos Stratis V.
Published: July 2002

We present dimensionless effective transport coefficients (formation factors) for arrays of cylinders of various orientation distributions, namely, cylinders randomly positioned and oriented with their axes parallel to a line, parallel to a plane, or in the three-dimensional space with no preferred orientation. Both cases of conducting cylinders dispersed in a nonconducting matrix and nonconducting cylinders dispersed in a conducting matrix are considered. The transport coefficients are computed by means of a random walk simulation scheme. A comprehensive survey of past studies on transport properties of random arrays of cylinders is also presented, and our results are compared with the predictions of various analytical approximations or bounds and with experimental data of the literature.
Vibrational relaxation, dissociation, and dissociation incubation times in norbornene
Authors: Kiefer J. H. , Kumaran S. S. , Sundaram S.
Published: July 2002

Shock waves in norbornene (bicyclo [2,2,1] hept-2-ene, C7H10)–krypton mixtures have been examined with the laser-schlieren technique over the very wide range of conditions, 542–1480 K, and 34–416 Torr in 0.5%, 2%, and 4% C7H10. The experiments exhibit both vibrational relaxation (542–1480 K) and the retro-Diels–Alder dissociation, norbornene→1,3-cyclopentadiene+ethylene (869–1480 K). Over 869–1304 K, and for pressures below 140 Torr, both relaxation and dissociation are resolved. These experiments provide the first measurements of unimolecular incubation (induction) times for the dissociation of a large polyatomic molecule. The ratio ti/τ decreases from ∼5 to 2 in 900–1300 K. Vibrational relaxation is rapid, log10 Pτ (μs atm)=0.066−6.70/T1/3, with a weak inverse temperature dependence, but is completely consistent with series excitation through the lowest-frequency mode. Dissociation shows very strong unimolecular falloff. A Rice–Ramsperger–Kassel–Marcus (RRKM) model, parametrized to fit the observations (E0=44.2 kcal/mol, 〈ΔE〉down=280 cm−1), finds the reaction closer to the low pressure limit k0 than to k∞ for 1400 K and 100 Torr. Earlier work at low temperatures (<750 K) has Ea too low to extrapolate to the present results. Thus a significant increase in Ea∞ is indicated over 700–1000 K, suggesting a change in mechanism.
Ultralow temperature kinetics of neutral–neutral reactions. The technique and results for the reactions CN+O2 down to 13 K and CN+NH3 down to 25 K
Authors: Sims I. R. , Queffelec J.-L. , Defrance A. , Rebrion-Rowe C. , Travers D. , Bocherel P. , Rowe B. R. , Smith I. W. M.
Published: July 2002

An entirely new experimental method is described which enables the rate constants of neutral–neutral gas-phase reactions to be measured at ultralow temperatures. The measurements are made by applying the pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique of studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The experimental method is described in some detail and its application and limitations are discussed. Results are reported for the reactions of CN radicals with O2 and NH3. For reaction (1) between CN and O2 data are reported for the temperature range T=13–295 K and the rate constants are well-matched by the expression k1(T)=(2.49±0.17)×10−11 (T/298)(−0.63±0.04) cm3 molecule−1 s−1. For reaction (2) between CN and NH3, rate constants in the temperature range T=25–295 K fit the expression k2(T)=(2.77±0.67)×10−11 (T/298)(−1.14±0.15) cm3 molecule−1 s−1. The kinetic data are discussed in terms of the latest quantum chemical and reaction rate theories for these systems.
Reactions of Rydberg states of molecular hydrogen
Authors: Pratt S. T. , Dehmer J. L. , Dehmer P. M. , Chupka W. A.
Published: July 2002

The Rydberg state reaction H*2+H2→H+3+H+e− was studied by exciting the (X 2∑+g)np,v=0,J Rydberg states with n=30–70 by double resonance excitation via the E,F 1∑+g,v=0,J=1 level and by detecting the product ions or electrons. The results are consistent with the reaction occurring between the ion core of the Rydberg molecule and the ground state molecule, with the Rydberg electron acting as a spectator. Thus, these reactions can be used to provide information on the analogous ion–molecule reactions, and the possibility of using the Rydberg state reactions to study the rotational state dependence of the corresponding ion–molecule reactions is discussed.
The dissociation kinetics of NO on Rh(111) as studied by temperature programmed static secondary ion mass spectrometry and desorption
Authors: Borg H. J. , Reijerse J. F. C.-J. M. , van Santen R. A. , Niemantsverdriet J. W.
Published: July 2002

Temperature programmed static secondary ion mass spectrometry (TPSSIMS) and temperature programmed desorption (TPD) have been used to study the kinetics of adsorption, dissociation, and desorption of NO on Rh(111). At 100 K, NO adsorption is molecular and proceeds via mobile precursor state kinetics with a high initial sticking probability. SSIMS indicates the presence of two distinct NO adsorption states, indicative of threefold adsorption at low coverage, and occupation of bridge sites at higher coverages. Three characteristic coverage regimes appear with respect to NO dissociation. At low coverages θNO<0.25 ML, NO dissociates completely at temperatures between 275 and 340 K. If we neglect lateral interactions and assume pure first order dissociation kinetics, we find effective values for the activation barrier and preexponential factor of 40±6 kJ/mol and 106±1 s−1 for the dissociation of 0.15–0.20 ML NO. However, if we assume that a NO molecule needs an ensemble of three to four vacant sites in order to dissociate, the preexponential factor and activation energy are ∼1011 s−1 and 65 kJ/mol, in better agreement with transition state theory expectations. The Nads and Oads dissociation products desorb as N2 and O2, respectively, with desorption parameters Edes=118±10 kJ/mol and νdes=1010.1±1.0 s−1 for N2 in the zero coverage limit.

At higher coverages, the desorption kinetics of N2 is strongly influenced by the presence of coadsorbed oxygen. In the medium coverage range 0.25<θNO<0.50 ML, part of the NO desorbs molecularly, with an estimated desorption barrier of 113±10 kJ/mol and a preexponential of 1013.5±1.0 s−1. Dissociation of NO becomes progressively inhibited due to site blocking, the onset shifting from 275 K at 0.25 ML to 400 K, coinciding with the NO desorption temperature, at a coverage of 0.50 ML. The accumulation of nitrogen and oxygen atoms on the highly covered surface causes a destabilization of the nitrogen atoms, which results in an additional low-temperature desorption state for N2. For high initial NO coverages above 0.50 ML, the dissociation is completely self-inhibited, indicating that all sites required for dissociation are blocked. The desorption of the more weakly bound—presumably bridged—NO does not generate the sites required for dissociation; these become only available after the desorption of—presumably triply coordinated—NO.
Computer simulation studies on β-quinol clathrates with various gases. Interdependence of host and guest molecules
Authors: von Szentpály László , Shamovsky Igor L. , Ghosh Ratna , Zubkus Vladimir E. , Tornau Evaldas E.
Published: July 2002

The equilibrium crystal structure of a β-quinol clathrate occupied by various gases is determined through empirical force field calculations. The guests with van der Waals volume Vm≤27 cm3/mol affect the structure and rigidity of the lattice in a manner independent of their chemical nature: The guest-host attraction leads to a shrinking of the host cages. On the other hand, the inclusion of molecules of the volume 27 cm3/mol≤Vm≤46 cm3/mol expands the cages. Guest molecules render the host lattice more rigid, except for large spherical rotor type molecules, which soften the lattice, eventually destroying it. The inclusion of molecules with Vm≳46 cm3/mol is likely to be impossible. The energy of clathrate formation increases with increasing Vm, with the polar guests forming more stable clathrates than the nonpolar ones with the same Vm. Interestingly, the optimal guest-cage compatibility occurs when the volume of guest molecules differs by less than 50% from the volume of the empty cage. The orientation of a guest molecule in a cage is mainly determined by a competition of three types of interactions. (i) Van der Waals interactions tend to direct the long axis of the guests along the c-axis of the cage. (ii) The electrostatic guest-host interaction tends to place a positively charged guest atom into the center of the cage. (iii) Electrostatic guest–guest interactions tend to align the dipole moments of the included molecules along the c-axis. The different orientations are rationalized in terms of the complementary influences of the guest size, guest polarity and an ‘‘atomic size’’ factor. The calculated barriers hindering the reorientation movement of guest molecules in a cage are found to be in a qualitative agreement with experimental data except for the N2 and CO molecules, which are supposed to be considerably polarized by the cage. The C3-reorientation motion of ethane is predicted to be accompanied by its synchronous rotation around the C–C bond.
Shifts of the 1A−g→1B+uelectronic absorption of carotenoids in nonpolar and polar solvents
Authors: Nagae Hiroyoshi , Kuki Michitaka , Cogdell Richard J. , Koyama Yasushi
Published: July 2002

The 1B+u energy of carotenoids (spheroidene, neurosporene, and β carotene) exhibits a linear dependence on R(n)=(n2−1)/(n2+2) in both nonpolar and polar solvents: The 1B+u energy is more stabilized in polar solvents in the limit of R(n)→0, and the line for polar solvents has a gentler slope. [As a result, the line for polar solvents crosses the line for nonpolar solvents at R(n)≂0.3.] A theory has been developed, which explains the aforementioned observations as follows. In polar solvents, an electric field is generated by fluctuation of the solvent permanent dipoles, and it affects the rodlike, conjugated chain of the carotenoid in a long spheroidal cavity. The electric field stabilizes the 1B+u energy through the polarization effect and it substantially reduces the dispersive interaction. The higher transition multipoles, rather than the transition dipole of carotenoid, play major roles in the aforementioned interactions between the carotenoid and the solvent molecules.
Kinetics of coverage-dependent adsorption
Authors: Meng B. , Weinberg W. H.
Published: July 2002

A method is introduced for evaluating the adsorption probability as a function of surface coverage within the context of a lattice gas model. We delineate the methodology by considering dissociative adsorption for which nearest-neighbor empty surface sites are required. For direct, dissociative adsorption a dynamical Monte Carlo simulation algorithm is used to evaluate the spatial correlation between adsorbates as surface coverage increases over time. The influence on the probability caused by these spatial correlations between adsorbates due to lateral interactions between adsorbates and mobility of the adsorbate are evaluated exactly from Monte Carlo simulations. For precursor-mediated adsorption, Monte Carlo simulations combined with an approximate continuum equation have been used to describe the coverage-dependent adsorption probability. The effects of lateral interactions between adsorbates, lattice geometry, and precursor states on the scaling of the coverage-dependent adsorption probability are quantified using various representative parameters.
Photoelectron spectroscopy of color centers in negatively charged cesium iodide nanocrystals
Authors: Sarkas Harry W. , Kidder Linda H. , Bowen Kit H.
Published: July 2002

We present the photoelectron spectra of negatively charged cesium iodide nanocrystals recorded using 2.540 eV photons. The species examined were produced using an inert gas condensation cluster ion source, and they ranged in size from (CsI)−n=13 to nanocrystal anions comprised of 330 atoms. Nanocrystals showing two distinct types of photoemission behavior were observed. For (CsI)−n=13 and (CsI)−n=36−165, a plot of cluster anion photodetachment threshold energies vs n−1/3 gives a straight line extrapolating (at n−1/3=0, i.e., n=∞) to 2.2 eV, the photoelectric threshold energy for F centers in bulk cesium iodide. The linear extrapolation of the cluster anion data to the corresponding bulk property implies that the electron localization in these gas-phase nanocrystals is qualitatively similar to that of F centers in extended alkali halide crystals. These negatively charged cesium iodide nanocrystals are thus shown to support embryonic forms of F centers, which mature with increasing cluster size toward condensed phase impurity centers. Under an alternative set of source conditions, nanocrystals were produced which showed significantly lower photodetachment thresholds than the aforementioned F-center cluster anions. For these species, containing 83–131 atoms, a plot of their cluster anion photodetachment threshold energies versus n−1/3 gives a straight line which extrapolates to 1.4 eV. This value is in accord with the expected photoelectric threshold energy for F′ centers in bulk cesium iodide, i.e., color centers with two excess electrons in a single defect site. These nanocrystals are interpreted to be the embryonic F′-center containing species, Cs(CsI)−n=41−65.
The performance of density-functional/Hartree–Fock hybrid methods: Cationic transition-metal methyl complexes MCH+3 (M=Sc–Cu,La,Hf–Au)
Authors: Holthausen Max C. , Heinemann Christoph , Cornehl Hans H. , Koch Wolfram , Schwarz Helmut
Published: July 2002

Hybrid methods, including a mixture of Hartree–Fock exchange and density functional exchange-correlation treatment have been applied to the cationic methyl complexes MCH+3 of the first and third-row transition metals (M=Sc–Cu,La,Hf–Au). Bond dissociation energies and optimum geometries obtained with the ‘‘Becke-Half-and-Half-Lee–Yang–Parr’’ and ‘‘Becke-3-Lee–Yang–Parr’’ functionals and from calibration calculations employing quadratic configuration interaction with single and double excitations and with a perturbative estimate of triple excitations are reported. A comparison of the results for the 3d-block species to earlier high-level ab initio calculations and experimental data is carried out in order to assess the reliability of hybrid methods as a practical tool in organometallic chemistry. Furthermore, the bond dissociation energies of the cationic 5d-block transition-metal methyl complexes, many of which have not been investigated so far, are predicted.
Resonant features in the energy dependence of the rate of ketene isomerization
Authors: Gezelter J. Daniel , Miller William H.
Published: July 2002

Calculations of the microcanonical isomerization rates for vibrationally excited ketene are presented. The calculations utilize the quantum reactive scattering methodology of absorbing boundary conditions with a discrete variable representation to obtain the cumulative reaction probability for one form of ketene to isomerize via the oxirene intermediate, and were carried out with model 1-, 2-, and 3-degree-of-freedom potential energy surfaces constructed using ab initio data. Significant differences are seen in the energy dependent features of the microcanonical rate for the single mode and multi-mode potentials; e.g., the single mode potential exhibits tunneling resonances with widths of around 1 cm−1, while the calculations involving more than one degree of freedom have additional resonant features that have widths around 10 cm−1 and also exhibit non-Breit–Wigner resonant line shapes. This suggests that many of the resonance features are best described as Feshbach (energy transfer, or dynamical) resonances that result because of a strongly bent region on the multi-mode potential energy surfaces. The calculated rates show reasonable qualitative agreement with the experimental results of Lovejoy and Moore [J. Chem. Phys. 98, 7846 (1993)].
Breakdown of the Stokes–Einstein relation in supercooled liquids
Authors: Tarjus Gilles , Kivelson Daniel
Published: July 2002

The Stokes–Einstein and Stokes–Einstein–Debye relations hold well in nonsupercooled liquids; however, sizeable deviations from the former appear in supercooled liquids, leading to a ‘‘decoupling’’ of translational diffusion from viscosity and reorientational relaxation. We attribute this breakdown and this ‘‘decoupling’’ to the existence of structured domains in the supercooled liquid.
Time-dependent density functional theory in lattice gas problems
Authors: Reinel D. , Dieterich W.
Published: July 2002

Aiming at a description of the ordering kinetics in stochastic lattice-gases, we propose a time-dependent generalization of classical density functional theory, which is based on a local equilibrium assumption. Our final evolution equations for the single-particle densities are formally similar to time-dependent Ginzburg–Landau theories, but in principle are consistent with the exact thermodynamics of the system. As a first illustration of this theory we briefly discuss some aspects of the interfacial dynamics near an ordering transition in a specific lattice gas model.
Molecular dynamics simulations of CCl4–H2O liquid–liquid interface with polarizable potential models
Authors: Chang Tsun-Mei , Dang Liem X.
Published: July 2002

The results from molecular dynamics simulations of the equilibrium properties of the CCl4–H2O liquid–liquid interface at room temperature are presented. The interactions between H2O–H2O, H2O–CCl4, and CCl4–CCl4 are described using the polarizable potential models developed in our laboratory. To our knowledge, this work is the first molecular dynamics simulations of the liquid–liquid interfacial equilibrium properties that explicitly includes nonadditive polarization effects. Molecular dynamics results of a 300 ps simulation following an extensive equilibration process indicate that the liquid interface is very stable, the density profile of H2O is very smooth, while that of CCl4 exhibits some oscillations. It is found that locally there is a sharp transition from one liquid phase to the other, but the overall interface is broadened by thermal fluctuations as indicated by the liquid density profiles. Calculated radial distribution functions suggest that the local structures of CCl4 and H2O remain unchanged from the bulk liquid to the interface. However, the interface does induce orientational order of H2O and CCl4 molecules. To study the polarization effects on the liquid–liquid interfacial equilibrium properties, we have calculated the total and induced dipole moments of H2O and CCl4 molecules as a function of the distance normal to the interface. The calculated dipole moments of the water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to the bulk values. This behavior can be attributed to the changes of the hydrogen bonding patterns and the orientation of water molecules near the interface. The induced dipole moments of the CCl4 molecules near the interface, on the other hand, are significantly enhanced. This is due in part to the strong local field induced by the water molecules at the interface. The calculated electric potentials using the dipole moment approach help us to analyze the orientations of water and CCl4 molecules at the interface.
Adiabatic nuclear magnetic resonance linewidth contributions for central transitions of I>1/2 nuclei
Authors: Werbelow Lawrence G.
Published: July 2002

The loss of coherence between any pair of spin-inverted states is independent of adiabatic (secular), second-order perturbations associated with the electric quadrupole interaction. Thus, consideration of various dephasing mechanisms, commonly ignored for NMR studies of quadrupolar nuclei (I≳1/2), may be important whenever extreme narrowing approximations fail. In this work, the consequences of various magnetic and higher-order electric effects, as well as mutual interferences, are considered in greater detail.
Vibrational relaxation and energy localization in polyatomics: Effects of high-order resonances on flow rates and the quantum ergodicity transition
Authors: Leitner David M. , Wolynes Peter G.
Published: July 2002

That high-order resonances may be involved in determining rates of vibrational relaxation in highly excited molecules has been recognized for some time. In this paper, high-order resonances are shown to play a central role in establishing the transition to ergodic flow, which we call the quantum ergodicity transition, at low energy in many-dimensional systems. The location of the transition to global vibrational flow in a system of coupled nonlinear oscillators is found to be generally sensitive to high-order resonances. On the other hand, except immediately above the transition, rates of flow once established depend primarily on the familiar low-order terms until the molecule reaches much higher excitation. Both the orders of resonant coupling terms in the vibrational Hamiltonian important in determining the transition to ergodicity, and the orders relevant for determining vibrational relaxation rates in large molecules are found to scale in a simple way with the strength of resonant coupling terms and the size of the molecule.
A scrutiny of the premise of the Rice–Ramsperger–Kassel–Marcus theory in isomerization reaction of an Ar7-type molecule
Authors: Takatsuka Kazuo , Seko Chihiro
Published: July 2002

The validity of the physical premise of the Rice–Ramsperger–Kassel–Marcus (RRKM) theory is investigated in terms of the classical dynamics of isomerization reaction in Ar7-like molecules (clusters). The passage times of classical trajectories through the potential basins of isomers in the structural transitions are examined. In the high energy region corresponding to the so-called liquidlike phase, remarkable uniformity of the average passage times has been found. That is, the average passage time is characterized only by a basin through which a trajectory is currently passing and, hence, does not depend on the next visiting basins. This behavior is out of accord with the ordinary chemical law in that the ‘‘reaction rates’’ do not seem to depend on the height of the individual potential barriers. We ascribe this seemingly strange uniformity to the strong mixing (chaos) lying behind the rate process. That is, as soon as a classical path enters a basin, it gets involved into a chaotic zone in which many paths having different channels are entangled among each other, and effectively (in the statistical sense) loses its memory about which basin it came from and where it should visit next time. This model is verified by confirming that the populations of the lifetime of transition from one basin to others are expressed in exponential functions, which should have very similar exponents to each other in each passing-through basin. The inverse of the exponent is essentially proportional to the average passage time, and consequently brings about the uniformity. These populations set a foundation for the multichannel generalization of the RRKM theory. Two cases of the non-RRKM behaviors have been studied. One is a nonstatistical behavior in the low energy region such as the so-called coexistence phase. The other is the short-time behavior. It is well established [M. Berblinger and C. Schlier, J. Chem. Phys. 101, 4750 (1994)] that in a relatively simple and small system such as H+3, the so-called direct paths, which lead to dissociation before the phase-space mixing is completed, increase the probability of short-time passage. In contrast, we have found in our Ar7-like molecules that trajectories of short passage time are fewer than expected by the statistical theory. It is conceived that somewhat a long time in the initial stage of the isomerization is spent by a trajectory to find its ways out to the next basins.
Benchmark calculations with correlated molecular wave functions. X. Comparison with “exact” MP2 calculations on Ne, HF, H2O, and N2
Authors: Wilson Angela K. , Dunning, Jr. Thom H.
Published: July 2002

The convergence of the MP2 valence correlation energy and pair energies for the correlation consistent basis sets has been investigated. Ne, HF, H2O, and N2 were studied. For all of these molecules, accurate MP2 correlation and pair energies are available from the recent MP2-R12 calculations of W. Klopper [J. Chem. Phys. 102, 6168 (1995)]. The magnitudes of the calculated MP2 valence correlation and pair energies are found to increase systematically with increasing basis set size, with the cc-pV6Z basis set yielding 97.4%–98.3% of the MP2 valence correlation energy. A detailed analysis of the results for Ne reveals that the error due to truncation of the radial functions in the cc-pV6Z set is comparable to that due to neglect of higher angular momentum functions. Procedures for extrapolating the results to the complete basis set limit have also been investigated.
Dynamics of hydrogen dissociation on Pt{100}: Steering, screening and thermal roughening effects
Authors: Pasteur A. T. , Dixon-Warren St. J. , Ge Q. , King D. A.
Published: July 2002

The normal translational energy (Ei) and surface temperature (Ts) dependence of the initial D2 sticking probability (s0) on Pt{100} has been measured using molecular beam techniques. On the hex phase s0 is found to decrease sharply with Ei between 5 and 10 meV, and to increase more gradually over the range 20⩽Ei⩽400 meV. This is interpreted as dynamical steering at low incident energies, giving way to weak translational activation at higher energies. The variation of the barrier to dissociation (Ea) across the unit cell gives rise to screening of dissociation with the impact parameter. There is less variation of Ea across the smaller (1×1) unit cell, leading to a stronger Ei dependence on this surface. On the hex phase s0 has been measured over a wide Ts range, from 150 to 1100 K. Weak thermal activation of dissociation is observed. This is well explained by a roughening model in which s is assumed to be proportional to the linear thermal displacement of surface metal atoms. The model is supported by density functional theory calculations of potential energy surfaces for a modelled rough surface. The absolute value of s0 under conditions of zero-point motion is estimated as 0.03 at Ei=69 meV and 0.06 at 342 meV.
The ammonia dimer spectrum in cold helium clusters
Authors: Behrens Michael , Buck Udo , Fröchtenicht Ralf , Hartmann Matthias , Havenith Martina
Published: July 2002

Infrared molecular beam depletion spectroscopy has been used to measure the spectrum of ammonia dimers embedded in large helium clusters after the excitation of the umbrella mode. The spectral region from 940 to 1010 cm−1 was covered by a line-tunable CO2-laser. By measuring the dependence of the depletion signal on the pressure of the pick-up cell, we confirmed that all lines are caused by the dimer. The spectrum exhibits two groups of two lines at 978/983 and 992/997 cm−1 separated by 14 cm−1 with a line splitting of 5 cm−1 each. The infrared transitions are only marginally shifted compared to the free ammonia dimer. Also the inversion tunneling splittings are equal to those of the free dimer. Differences in the amplitudes are attributed to relaxation effects of the different inversion components of the same symmetry at the low temperature of 0.4 K. The interchange tunneling splitting, however, which dominates the line splittings in the free dimer is considerably quenched in the helium environment. In addition, the difference in the excitation of the two non-equivalent monomers is remarkably reduced in support of a more cyclic structure in the helium environment.
Simple reversible molecular dynamics algorithms for Nosé–Hoover chain dynamics
Authors: Jang Seogjoo , Voth Gregory A.
Published: July 2002

Reversible algorithms for Nosé–Hoover chain (NHC) dynamics are developed by simple extensions of Verlet-type algorithms: leap frog, position Verlet, and velocity Verlet. Tests for a model one dimensional harmonic oscillator show that they generate proper canonical distributions and are stable even with a large time step. Using these algorithms, the effects of the Nosé mass and chain length are examined. For a chain length of two, the sampling efficiency is much more sensitive to the Nosé mass than for a longer chain of length four. This indicates that the chain length in general should be longer than two. The noniterative nature of the algorithms allows them to be easily adapted for constraint dynamics. For the most general case where multiple NHC’s are coupled to a system with constraints, a correction of the first Nosé acceleration is required, which is derived from the continuity equation on a constrained hypersurface of the phase space. Tests for model systems of two and three coupled harmonic oscillators with one normal mode constrained show that these algorithms, in combination with the corrected dynamical equations, sample the canonical distributions for the unconstrained degrees of freedom.
Quantum localization and dynamical tunneling of quasiseparatrix wave functions for molecular vibration
Authors: Hashimoto Naoyuki , Takatsuka Kazuo
Published: July 2002

We report a new kind of “dynamical tunneling” that can be observed in chaotic molecular vibration. The present phenomenon has been found in eigenfunctions quantized in a thin quasiseparatrix (chaotic zone) in phase space. On the classical Poincaré section corresponding to this situation, two or more unstable (hyperbolic) fixed points coexist and are connected through the so-called heteroclinic crossings, whereby the entire quasiseparatrix is generated. When the quasiseparatrix is thin enough, each of the hyperbolic fixed points is surrounded by the relatively “wide lake” of chaos due to the infinite and violent crossings between the stable and unstable manifolds, and these lakes are in turn connected by “narrow canals.” Our finding is, in spite of the fact that the narrow canals are classically allowed for the trajectories to pass through fast, wave packets can be quantized predominantly as “quasistanding-waves” in each lake area and hence can be mostly localized to remain there for much longer time than the corresponding classical trajectories do. In other words, the wave packets are localized in the vicinity of the classically unstable fixed points due to the quantum effect. However, a pair of these “localized” wave packets are eventually delocalized into the other lakes, and thereby form a pair of eigenfunctions (purely standing waves) with a small level splitting. Thus the present phenomenon can be characterized as a tunneling between the states of quantum localization in an oscillator problem.
Theoretical investigation of weakly-bound complexes of O(3P) with H2
Authors: Alexander Millard H.
Published: July 2002

We report multireference configuration interaction (CI), as well as coupled-cluster, calculations for the three O(3P)H2 potential-energy surfaces (two of A″ symmetry and one of A′ symmetry in Cs geometry) in the region relevant to the weakly bound O(3P)⋯H2 complex. The two electronically adiabatic states of A″ symmetry correspond to an orthogonal transformation of two orthogonal electronic occupations of the O 2p orbitals. The transformation of the three electronically adiabatic states to an approximate diabatic representation, which involves four potential-energy functions, can be obtained, either from calculated matrix elements of the electronic orbital angular momentum or from analysis of the expansion coefficients of the CI wave functions. An exact treatment of the nuclear motion including spin–orbit coupling, based on the diabatic PES’s (potential energy surfaces), is used to determine the energies of the lowest bend–stretch levels of complexes of O(3P) with both nuclear spin isomers of H2 and D2. The predicted dissociation energies (D0) are 15.4 and 22.4 cm−1 for the complexes with pH2 and oH2, respectively, and 22.3 and 31.4 cm−1 for the complexes with oD2 and pD2, respectively.
On the importance of higher order anharmonic molecular couplings
Authors: Pearman R. , Gruebele M.
Published: July 2002

The dynamics of molecular vibrational dephasing (IVR) are governed by the coupling structure of the vibrational Hamiltonian. We use a recently developed factorization model for normal coordinate potentials to compare the importance of direct high order couplings to both direct low order couplings and low-order coupling chains. The importance of high order couplings is characterized as a function of the vibrational energy and of molecular size. Particular attention is paid to phase cancellation effects in coupling chains, and to correlations between vibrational modes depending on their localization length. To account properly for IVR at high densities of states and long times, high order couplings must be taken into account.
The structure and adsorption of the four bonding sites model for associating fluids in disordered porous media from replica Ornstein–Zernike integral equation theory
Authors: Kovalenko Andriy , Pizio Orest
Published: July 2002

A model for a network-forming associating fluid in which each of the particles have four sites available for bonding is considered. The model possesses liquid–gas transition in the absence of attractive long-range nonassociative interactions. We have studied the adsorption of the fluid in a disordered porous media that corresponds to an equilibrium configuration of hard spheres. The associative replica Ornstein–Zernike (ROZ) equations are solved with the Percus–Yevick (PY) and hypernetted chain (HNC) closures and with the ideal network approximation. The pair distribution functions and the structure factors have been obtained. The adsorption isotherms have been calculated using a system of hard spheres adsorbed in a hard-sphere matrix as a reference. The associative contribution to the chemical potential follows from Wertheim’s thermodynamic perturbation theory, however, with monomer fraction from the solution of the ROZ equations. The liquid–vapor coexistence curve has been evaluated. We have observed shrinking of the coexistence envelope with increasing matrix density. The critical temperature and the critical density are sensitive to the density of adsorbent. Both decrease with increasing matrix density.
Numerical and theoretical analysis of multiquantum magic-angle spinning experiments
Authors: Charpentier Thibault , Fermon Claude , Virlet Joseph
Published: July 2002

Using a recent investigation of the Floquet’s theorem for magic-angle spinning nuclear magnetic resonance simulations (NMR), a procedure for computing multiquantum magic-angle spinning spectra is derived. The general formalism which is introduced here can be applied more generally to any solid-state NMR two-dimensional experiments. All interactions and their time dependency are considered during the pulses. Furthermore, for powder patterns, a formal average is possible on γ (the third component of the Euler angle describing the orientation of the crystallite) which leads to great simplifications and to an improved computing efficiency. As an application, the intensity of the spinning sidebands in the two-dimensional multiquantum magic-angle spinning spectrum is investigated. The recently reported appearance of numerous spinning sidebands in the multiquantum dimension is discussed. Such effects appear naturally in the present formalism which provides a theoretical framework for further investigations. Simulations of two-dimensional spectra are compared with experimental data.
Development and assessment of new exchange-correlation functionals
Authors: Hamprecht Fred A. , Cohen Aron J. , Tozer David J. , Handy Nicholas C.
Published: July 2002

We recently presented a new method for developing generalized gradient approximation (GGA) exchange-correlation energy functionals, using a least-squares procedure involving numerical exchange-correlation potentials and experimental energetics and nuclear gradients. In this paper we use the same method to develop a new GGA functional, denoted HCTH, based on an expansion recently suggested by Becke [J. Chem. Phys. 107, 8554 (1997)]. For our extensive training set, the new functional yields improved energetics compared to both the BLYP and B3LYP functionals [Phys. Rev. A 38, 3098 (1988); Phys. Rev. B 37, 785 (1988); J. Chem. Phys. 98, 5648 (1993); J. Phys. Chem. 98, 11623 (1994)]. The geometries of these systems, together with those of a set of transition metal compounds, are shown to be an improvement over the BLYP functional, while the reaction barriers for six hydrogen abstraction reactions are comparable to those of B3LYP. These improvements are achieved without introducing any fraction of exact orbital exchange into the new functional. We have also re-optimized the functional of Becke—which does involve exact exchange—for use in self-consistent calculations.
Molecular dynamics of homogeneous nucleation in the vapor phase. II. Water
Authors: Yasuoka Kenji , Matsumoto Mitsuhiro
Published: July 2002

Homogeneous nucleation process in the vapor phase of water is investigated with a molecular dynamics computer simulation at 350 K under supersaturation ratio 7.3. Using a method similar to Lennard-Jones fluid (Part I), the nucleation rate is three orders of magnitude smaller than prediction of a classical nucleation theory. The kinetically defined critical nucleus size is 30–45, much larger than the thermodynamically defined value of 1.0 estimated with the classical theory. Free energy of cluster formation is estimated from the cluster size distribution in steady state time region. The predicted nucleation rate with this free energy agrees with the simulation result. It is concluded that considering the cluster size dependence of surface tension is very important.
Molecular simulation of hydrogen adsorption in single-walled carbon nanotubes and idealized carbon slit pores
Authors: Wang Qinyu , Johnson J. Karl
Published: July 2002

The adsorption of hydrogen gas into single-walled carbon nanotubes (SWNTs) and idealized carbon slit pores is studied by computer simulation. Hydrogen-hydrogen interactions are modeled with the Silvera-Goldman potential. The Crowell-Brown potential is used to model the hydrogen-carbon interactions. Calculations include adsorption inside the tubes, in the interstitial regions of tube arrays, and on the outside surface of isolated tubes. Quantum effects are included through implementation of the path integral formalism. Comparison with classical simulations gives an indication of the importance of quantum effects for hydrogen adsorption. Quantum effects are important even at 298 K for adsorption in tube interstices. We compare our simulations with experimental data for SWNTs, graphitic nanofibers, and activated carbon. Adsorption isotherms from simulations are in reasonable agreement with experimental data for activated carbon, but do not confirm the large uptake reported for SWNTs and nanofibers. Although the adsorption potential for hydrogen in SWNTs is enhanced relative to slit pores of the same size, our calculations show that the storage capacity of an array of tubes is less than that for idealized slit pore geometries, except at very low pressures. Ambient temperature isotherms indicate that an array of nanotubes is not a suitable sorbent material for achieving DOE targets for vehicular hydrogen storage.
On the convergence of simulation of asymmetric electrolytes with charge asymmetry 60:1
Authors: Linse Per
Published: July 2002

The efficiency of molecular dynamics (MD) and Monte Carlo (MC) simulations of highly asymmetric electrolytes modeling aqueous solution of ionic surfactant micelles has been investigated by using a soft-sphere (SS) and a hard-sphere (HS) short-range potential. The simulations were performed with standard simulation techniques, including Ewald summation for handling the long-range Coulombic interactions. Both the macroions and the small ions entered the model explicitly, and the macroion carried 60 elementary charges. Comparison of the efficiency of (i) MD simulation of the SS model, (ii) MC simulation of the SS model, and (iii) MC simulation of the HS model was made at (a) optimal balance of the computational load in the real and Fourier spaces, (b) optimal radial extension of the interaction cutoff for the preparation of the neighbor list, and (c) in the MC simulations, optimal translational displacement parameter of the macroions. For the SS model, it was found that the MD and MC methods gave the same result in accordance with the ergodic hypothesis. The two methods gave the energy with the same efficiency (same precision for a given computational effort), whereas the MD method predicted the macroion structure more efficiently. Finally, it was found that the MC simulation of the HS system was less efficient as compared to the MC simulation of the SS system, due to a smaller optimal translational displacement parameter, which in turn was related to the harsher potential and larger accumulation of counterions in the close vicinity of the macroion. In the MC simulations, the optimal macroion displacement corresponded to an acceptance rate of the macroion moves of ∼5%.
Toward a unified view of stress in small-molecular and in macromolecular liquids
Authors: Picu R. C. , Loriot G. , Weiner J. H.
Published: July 2002

We consider a family of molecular liquids, each consisting of linear molecules with N covalent bonds, focusing specifically on N=1 (diatomic liquid), N=3 (four-atom molecular liquid), and N=200 (macromolecular liquid). The bonded and nonbonded potentials, ub(r) and unb(r), are the same for each system, with ub representing stiff linear springs and unb corresponding to the repulsive portion of the Lennard-Jones potential. The relaxation of the stress difference σ, following a constant-volume elongation of the system, is determined in terms of interatomic interactions by nonequilibrium molecular dynamics simulations. It is found that the nonbonded interactions make the principal contribution to σ while the bonds make a negative contribution. For all systems studied it is found that, following a short induction period after the start of relaxation, σ=C〈〈P2(θb)〉〉, where 〈〈P2(θb)〉〉(t) is a measure of the individual bond orientation and the proportionality constant C=3.5 within simulation accuracy, for N⩾3. The principal difference between the behavior for small and large N is the rate at which 〈〈P2(θb)〉〉(t) decays. An explanation of the broad applicability of the relation σ=C〈〈P2(θb)〉〉 is presented in terms of the concepts of steric shielding, intrinsic interaction distributions, and intrinsic stresses. The failure of this relation during the short induction period is explained in terms of anisotropies in atom distributions present immediately after deformation.
Geometry optimization in delocalized internal coordinates: An efficient quadratically scaling algorithm for large molecules
Authors: Baker Jon , Kinghorn Don , Pulay Peter
Published: July 2002

Using a Z-matrix-like approach for generating new Cartesian coordinates from a new geometry defined in terms of delocalized internal coordinates, we eliminate the costly O(N3) iterative back-transformation required in standard geometry optimizations using delocalized (or natural/redundant) internals, replacing it with a procedure which is only O(N). By replacing the gradient transformation with an iterative solution of a set of linear equations, we also reduce this step from O(N3) to roughly O(N2). This allows a very efficient method for geometry optimization of large molecules in internal coordinates. Several optimizations on systems containing up to 500 atoms are presented, comparing the performance of the new algorithm with its predecessor, and demonstrating the practical utility and efficiency of our approach.
The core excitation of pyridine and pyridazine: An electron spectroscopy and ab initio study
Authors: Hannay C. , Duflot D. , Flament J.-P. , Hubin-Franskin M.-J.
Published: July 2002

The carbon and nitrogen K-shell excitation spectra of gaseous pyridine and pyridazine were recorded using the electron-energy loss spectroscopy under electric-dipole conditions (2 keV, small angle) with a resolution of 0.2 eV. Ab initio Configuration interaction calculations in the frame of the equivalent core model were performed in order to help in the assignment of the spectral features. The spectra are dominated by the transitions to the 1π* and σ* type orbitals. The C1s spectra of both molecules are close to that of benzene: The intensity of Rydberg transitions are enhanced by an important valence σC–H* character; the 1s→3π* transition is mixed with double excitations and give rise to several states, some of them lying above the ionization thresholds. Finally, the N1s spectra of both molecules are similar to the s-triazine one.