bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initioand density functional study on the mechanism of the C2H2++methanol reaction
Authors: Irle Stephan , Morokuma Keiji
Published: July 2002

High level ab initio (G2MS and CASSCF) and density functional (B3LYP) calculations were carried out to study the mechanism of the ion–molecule reaction C2H2++CH3OH for four reaction channels: hydride abstraction from methanol (HA), proton transfer from acetylene cation (PT), charge transfer (CT), and covalent complex formation (CC) channel. For the CT channel, two pathways have been found: a usual nonadiabatic pathway via A′/A″ seam of crossing, and a low-energy adiabatic pathway through an initial intermediate; the latter may be the dominant process with favorable energies and a large impact parameter. The HA process involves a low-energy direct intermediate and a very low barrier to form C2H3+CH2OH+ and is also energetically favorable. The PT processes require passage over a high-energy transition state (TS) and are not important. One of the experimentally unobserved CC channels, formation of the COCC skeleton, is energetically favorable and there is no energetic reason for it not to take place; a “dynamic bottleneck” argument may have to be invoked to explain the experiment. The increase in reaction efficiency with the C–C stretch excitation may be justified by considering the TSs for two CT pathways, where the C–C distance changed substantially from that in the reactant C2H2+. Very qualitatively, the C2H2++CH3OH potential energy surface looks more like that of the C2H2++NH3 system than the C2H2++CH4 system, because of the differences in the ionization potentials: NH3∼CH3OH<C2H2<CH4.
Ab initio potential energy surface for the ground (2A′) state of H+SiO and rotationally inelastic collision cross sections for circumstellar H+SiO collisions
Authors: Jimeno P. , Gray M. D. , Balint-Kurti G. G.
Published: July 2002

An ab initio potential energy surface has been calculated for the ground electronic state of the H+SiO system. The calculations were performed by first characterizing all the critical points of the system using ab initio complete active space-self consistent field (CAS-SCF) methods. These preliminary calculations were followed by multireference configuration interaction (CI) calculations covering a grid of 1748 nuclear geometries. The surface has been fitted to a global analytic form, using the recently proposed Reproducing Kernel Hilbert Space Method of Ho et al., [J. Chem. Phys. 104, 2584 (1996)] and preliminary vibrationally-rotationally inelastic scattering cross sections have been computed using infinite order sudden quantum scattering methods. The motivation of the research is the evaluation of these inelastic cross sections which are needed in the modeling of circumstellar SiO maser radiation. Significant structure has been observed in the computed inelastic cross sections. This structure is of the form which might be expected to give rise to the vibrational–rotational quantum state population inversions which in turn leads to the observed circumstellar maser action. An interesting feature of the computed potential energy surface is a line of maxima in nonlinear geometries. These maxima appear to arise from a nonsymmetry related conical intersection.
The outer valence photoionization of acetylene
Authors: Wells M. C. , Lucchese Robert R.
Published: July 2002

We report fixed-nuclei photoionization cross-sections and asymmetry parameters for photoionization leading to the X 2Πu, A 2Σg+, B 2Σu+, and C 2Σg+ states of acetylene with emphasis on the first four states. The magnitude of the photoionization cross-sections calculated here is in excellent agreement with recent experiment at both low and high photon energy. Further, as a result of the multichannel scattering methodology used to perform the calculation, the partial channel cross-sections and asymmetry parameters reported here resolve significant structure arising from indirect photoionization processes such as autoionization. Although vibrational degrees of freedom are not included within the fixed nuclei framework employed here, we find that, even without vibrational degrees of freedom, the present theoretical results generally exhibit the same detailed features as the experimental results, both for conventional photoionization spectra and, as a result of vibrational autoionization, for threshold photoionization spectra. This general agreement suggests that a large part of the structure in the low energy or outer valence photoionization spectrum of acetylene is explicable solely in terms of Rydberg transitions. This study also predicts that dark states may cause some appreciable distortion of the profile of the photoionization asymmetry parameter in the photon region of ∼20–21.5 eV as a result of final state correlations with more intense states.
Reaction dynamics of chlorine atom with methane: Dual-level ab initio analytic potential energy surface and isotope effects
Authors: Yu Hua-Gen , Nyman Gunnar
Published: July 2002

An analytic potential energy surface for the Cl+CH4⇌HCl+CH3 reaction in C3V symmetry has been obtained by fitting to 1136 energy points from a dual-level MP2/SAC (Mo/ller–Plesset second order perturbation/scaling all correlation) calculation using the 6-311G(2d,d,p) basis set. A zero-point energy correction is made to account for all modes not explicitly treated with the time-independent quantum scattering rotating line umbrella (RLU) model, which is used for the dynamics calculations. The effective potential gives a vibrationally adiabatic ground-state barrier height of 3.36 kcal/mol and an endothermicity (0 K) of 1.19 kcal/mol for the Cl+CH4 reaction, and 4.43 kcal/mol and 2.29 kcal/mol, respectively, for Cl+CD4. Thermal rate constants, tunneling and kinetic isotope effects have been investigated in detail. Calculated differential cross sections for Cl+CD4→DCl+CD3, with reactants and products in their vibrational ground states, show that the DCl product is strongly backward scattered. Further, ground state Cl+CD4 reacts to give the CD3 product predominantly unexcited at collision energies ranging from 0.15 eV to 0.25 eV. Generally, good agreement with experimental measurements and previous theoretical work is obtained.
The glass transition of charged and hard sphere silica colloids
Authors: Beck Ch. , Härtl W. , Hempelmann R.
Published: July 2002

Dynamic and static light scattering is applied to concentrated suspensions of silica nanoparticles with surface functionalizations causing highly charged or hard sphere interaction potentials, respectively. The index of refraction of the dispersion medium was matched to that of the particles using a mixture of water/glycerol for the charged particles and toluene/ethanol for the hard spheres. The static structure factors correspond to the appropriate theoretical models, Percus–Yevick and rescaled mean spherical approximation. At volume fractions φ=0.18 a glass transition for the charged systems and at φ=0.53 for the hard spheres can be observed, as evident from the nondecaying components of the intermediate scattering functions. In the glassy state the experimental correlation functions agree with the predictions of the mode-coupling theory over several orders of magnitude in time. Using the fitted experimental structure factors as input for the mode-coupling theory we find good agreement between the theoretical nonergodicity parameters and the measured Debye–Waller factors. In the liquid state close to the glass transition the experimental intermediate scattering functions and the predictions of the mode-coupling theory for the α and β relaxation are compared within a large Q range. Using an exponent parameter λ=0.74 for the charged particles and λ=0.76 for the hard spheres a good agreement between theory and experiment can be obtained, both for the liquidlike and the glassy state.
Theory and ab initio calculations of 2p photoabsorption spectra: The lowest Rydberg resonances in HCl
Authors: Fink Reinhold F. , Kivilompolo Mika , Aksela Helena
Published: July 2002

We present a theory for 2p core excitation spectra of linear molecules that explicitly includes the spin-orbit splitting of the 2p core orbitals as well as all nonrelativistic effects. This is applied to the absorption spectrum of HCl at the 2p→6σ*, 4s, 4p, 3d, and 5s resonance energies. All input data for the spectrum are obtained from ab initio calculations. These are (i) the nonrelativistic energies and transition probabilities of the contributing states, (ii) the spin-orbit coupling parameter, and (iii) the total Auger transition rates of the core excited states. The prior theoretical finding that the total Auger decay rate depends substantially on the core hole orientation is supported by a comparison of the theoretical and experimental absorption spectra. Furthermore, the preferred orientation of the core excited HCl molecule with respect to the molecular bond axis is discussed. General considerations about intensity ratios between corresponding states with 2p3/2 and 2p1/2 holes are given on the basis of the theoretical framework.
Spectroscopy of O3 trapped in rare gas matrices. II. Vibrational energies and transition moments of low-lying levels
Authors: Chabbi H. , Dahoo P. R. , Lakhlifi A.
Published: July 2002

The theoretical model described in part I is applied to calculate the vibrational energies and transition moments for low-lying levels of O3 trapped in rare gas matrices. Results are given for molecules trapped in distorted face-centered-cubic (fcc) and hexagonal-closed-packed (hcp) lattice structures. New harmonic and anharmonic constants are determined that lead to matrix dependent calculated energy levels. Changes are significant for harmonic and third-order anharmonic ones. Moreover the symmetry of the potential in which the ozone oxygen nuclei move is shown to be altered. Calculated energy levels compare well with observed ones and allow predictions of unobserved ones. The 2ν3→ν3 fluorescence observed in different rare gas matrices is confirmed. Transition moments hardly differ from one matrix to the other for 2ν3→ν3 and ν1+ν3 transitions although for the latter, it is one order-of-magnitude higher in a double than in a single substitutional site.
Clathrate hydrates in the system H2O–Ar at pressures and temperatures up to 30 kbar and 140 °C
Authors: Lotz H. T. , Schouten J. A.
Published: July 2002

Studies of the binary mixture H2O–Ar—by means of a quasi-isochoric scanning method—have revealed the range of stability of clathrate hydrates in the high-pressure and high-temperature region. The results obtained show an extension of the decomposition curve above the melting curve of pure argon and up to the melting curve of pure ice VII. At low pressures the argon decomposition temperature first increases and then decreases with pressure, showing a local maximum and minimum temperature. At higher pressures the slope of the decomposition curve remains positive but undergoes three more breaks. Two new quadruple points have been determined; at 105 °C, 20 kbar (Q4) and at 136 °C, 29 kbar (Q5). In the lower region (below 10 kbar) the decomposition curve shows two breaks; at 31.5 °C, 6.2 kbar (Q2) and at 37.5 °C, 9.6 kbar (Q3), indicating that three different clathrate structures are formed in this region. The positions of the three-phase lines separating the two clathrate structures at lower pressure could be obtained via Raman spectroscopy since the pressure and temperature dependence of the vibrational frequencies of the coupled O–H oscillations is different for each clathrate structure.
On the direct evaluation of the equilibrium distribution of clusters by simulation
Authors: Kusaka Isamu , Oxtoby David W. , Wang Zhen-Gang
Published: July 2002

An expression is derived that relates the average population of a particular type of cluster in a metastable vapor phase of volume Vtot to the probability, estimated by simulation, of finding this cluster in a system of volume V taken inside Vtot, where V≪Vtot. Correct treatment of the translational free energy of the cluster is crucial for this purpose. We show that the problem reduces to one of devising the proper boundary condition for the simulation. We then verify the result obtained previously for a low vapor density limit [J. Chem. Phys. 108, 3416 (1998)]. The difficulty implicit in our recent calculation [J. Chem. Phys. 110, 5249 (1999)], in which the approach in the former was generalized to higher vapor densities, is shown to be resolved by a method already suggested in that paper.
Ionization reactions of ion complexes in mesoscopic water clusters
Authors: Consta Styliani , Kapral Raymond
Published: July 2002

The free energy and dynamics of the dissociation reactions of the [Na+(Cl−)2] ion complex in mesoscopic water clusters are examined. The free energy surface shows the existence of stable single and double solvent-separated complex species formed from ionization of the stable double-contact ion complex. The reaction occurs on the cluster surface for mesoscale clusters composed of tens of water molecules. Passage between stable species is an activated process but barrier crossing has a large diffusive component so that dynamical corrections to transition state theory are large. The structure of the decay of the time-dependent rate constant reflects the diffusive character of the recrossing dynamics so that a plateau is not established on a 10 ps time scale in contrast to ionization dynamics in bulk fluids.
Dynamic properties of simple liquids: Dependence on the softness of the potential core
Authors: Anento N. , Padró J. A. , Canales M.
Published: July 2002

The dependence of both individual and collective dynamic properties of simple liquids on the steepness of the potential cores is analyzed. Molecular dynamics simulations of liquids in identical conditions but assuming two repulsive interaction potentials with very different softness have been carried out. Two liquids at very different densities (with volume packing fractions η=0.5 and η=0.3) have been considered for each potential. The study of the dynamic collective properties includes the dynamic structure factor and the longitudinal and transverse current correlations. It has been corroborated that longitudinal modes associated with density fluctuations propagate up to higher wave numbers in liquids with softer potential cores. However, the propagating transversal modes in dense liquids are weakly influenced by the hardness of the potential core. Other properties such as the sound velocity in liquids at high density or the dynamic structure factor at the kinetic regime are rather insensitive to the details of the repulsive interactions. The velocity correlation functions for liquids with soft repulsive potential walls are markedly oscillatory. These oscillations are associated with the coupling of the atomic velocity to collective density fluctuations.
Isotopic branching in (He, HD+) collisions: A time-dependent quantum mechanical study in three dimensions
Authors: Kalyanaraman C. , Clary David C. , Sathyamurthy N.
Published: July 2002

A time-dependent quantum mechanical approach has been used to investigate the reaction He+HD+(v=0–4,j=0–3)→HeH++D; HeD++H in three dimensions for total angular momentum J=0. The vib-rotation (v,j) state-selected reaction probability (Pv,jR) is shown to increase with v over the collision energy (Etrans) range (0.95–2.25 eV) investigated for both the exchange channels, in accord with the experimental results. The isotopic branching ratio Γ=PR(HeH+)/PR(HeD+) generally remains less than unity for different v states at different Etrans in agreement with experiment. But at Etrans=1.0 eV, for v=4, Γ obtained from our calculations for j=0 of HD+ is ∼0.8, in excellent agreement with the earlier quasiclassical trajectory calculations, but a factor of 2 less than that obtained from experiment. This difference could arise from the inclusion of nonzero j states in the experimental study, as Pv,jR is found to be j dependent for both the channels. While Pv,jR (HeH+) decreases initially with increase in j from 0 to 2 and then increases when j is increased further to 3, Pv,jR (HeD+) reveals an unusual j dependence; it is larger for even j states of HD+ than for odd j. As a result, Γ is strongly dependent on j, in contrast to the marginal dependence shown by the earlier quasiclassical trajectory calculations.
Spherulites: A new vesicular system with promising applications. An example: Enzyme microencapsulation
Authors: Bernheim-Grosswasser A. , Ugazio S. , Gauffre F. , Viratelle O. , Mahy P. , Roux D.
Published: July 2002

A new technology is tested for enzyme encapsulation. The capsules are small multilamellar vesicles of surfactant called spherulites which are produced by shearing a lamellar phase under well-controlled conditions. Encapsulation of alkaline phosphatase into spherulites is studied here as an example. Once encapsulated, the enzyme is shown to be unable to develop any enzymatic activity on its substrate, the p-nitrophenylphosphate. This is due to the absence of contact between the enzyme and the substrate. Interestingly, the whole enzymatic activity is recovered after destruction of the vesicles. Encapsulation efficiency ranges between 70% and 95% depending upon the enzyme over phospholipids ratio. Beyond the example of alkaline phosphatase, many applications of spherulites in the medical or in the biotechnology fields seem now at hand.
Interaction of the water dimer with π-systems: A theoretical investigation of structures, energies, and vibrational frequencies
Authors: Tarakeshwar P. , Kim Kwang S. , Brutschy B.
Published: July 2002

The interaction of the water dimer with both the olefinic and aromatic π systems (ethene, benzene, toluene, fluorobenzene, and p-difluorobenzene) has been investigated using both the supermolecular [second-order Møller–Plesset (MP2)] and perturbational (symmetry adapted perturbation theory) approaches. The geometry optimizations, harmonic vibrational frequencies, and the components of the binding energy were evaluated using fairly large basis sets (6-31+G* and aug-cc-pVDZ). The minimum energy structures obtained at the MP2/6-31+G* and MP2/aug-cc-pVDZ levels of theory indicate that the water dimer exhibits a π-type of interaction with ethene, benzene, and toluene and a σ-type of interaction with both fluorobenzene and p-difluorobenzene. This is demonstrated from the vibrational frequencies which are in good agreement with the experimentally determined numbers. Our calculations indicate that the nature and strength of the interaction of the donor water molecule (water dimer) with the π system has a significant bearing on the total binding energy of the complex. Apart from the interaction of the water dimer with the π system, we also show how this interaction influences the hydrogen bond characteristics of the water dimer by evaluating the hydrogen bond strengths.
Multicanonical jump walk annealing: An efficient method for geometric optimization
Authors: Xu Huafeng , Berne B. J.
Published: July 2002

A new global optimization method, multicanonical jump walk annealing (MJWA), is proposed and applied to the geometric optimization of Lennard-Jones and Morse clusters and the hydrophobic (B), hydrophilic (L), and neutral (N) (BLN) protein model. The method efficiently finds the global minima of these systems. In four comparative studies, MJWA greatly outperforms the conventional simulated annealing in locating the global minima. Theoretical comparison with other global optimization methods is discussed. Through this paper, we demonstrate a criterion for devising stochastic global optimization schemes. Namely, a stochastic global optimization method must favor the global minimum thermodynamically and at the same time be able to cross the high energy barriers.
Acetone n-radical cation internal rotation spectrum: The torsional potential surface
Authors: Shea Dana A. , Goodman Lionel , White Michael G.
Published: July 2002

The one color REMPI and two color ZEKE-PFI spectra of acetone-d3 have been recorded. The 3px Rydberg state of acetone-d3 lies at 59 362.3 cm−1 and both of the torsional modes are visible in this spectrum. The antigearing Rydberg (a2) mode, v12*, has a frequency of 62.5 cm−1, while the previously unobserved gearing (b1) mode, v17*, is found at 119.1 cm−1. An ionization potential of 78 299.6 cm−1 for acetone-d3 has been measured. In acetone-d3 n-radical cation ground state, the fundamentals of both of the torsional modes have been observed, v12+ at 51.0 cm−1 and v17+ at 110.4 cm−1, while the first overtone of v12+ has been measured at 122.4 cm−1. Deuterium shifts show that v12+ behaves like a local C3v rotor, but that v17+ is canonical. Combining this data with that for acetone-d0 and aacetone-d6 has allowed us to fit the observed frequencies to a torsional potential energy surface based on an ab initio C2v cation ground state geometry. This potential energy surface allows for prediction of the v17 vibration in acetone-d0 and acetone-d6. The barrier to synchronous rotation is higher in the cation ground state than in the neutral ground state, but significantly lower than in the 3s Rydberg state. The 3px Rydberg and cation ground state potential energy surfaces are found to be very similar to each other, strongly supporting the contention that the 3px Rydberg state has C2v geometry and is a good model for the ion core. The altered 3s Rydberg state potential surface suggests this state has significant valence character.
Crystallization of a Yukawa fluid via a modified weighted density approximation with a solid reference state
Authors: Wang Dean C. , Gast Alice P.
Published: July 2002

We investigate the freezing behavior of particles interacting with a Yukawa potential using extensions of the Denton and Ashcroft modified weighted density approximation (MWDA) model of density functional theory [A. R. Denton and N. W. Ashcroft, Phys. Rev. A 39, 470 (1989)]. An attempt is made to incorporate properties of the static solid into the fluid-based MWDA model via our previous model for the crystallization of inverse nth-power fluids [D. C. Wang and A. P. Gast, J. Chem. Phys. 110, 2522 (1999)], as well as a model that includes the Einstein vibrations of the localized particles. Both extensions yield improvements over the MWDA model in terms of coexisting densities and the ability to stabilize a body-centered cubic solid compared with computer simulation data. The fractional change in density upon freezing also compares favorably with results from available simulation studies and those for the inverse nth-power system. Reasons for the differences in results obtained for freezing properties of the Yukawa system among computer simulation data, theoretical approaches, and experimental studies are discussed.
De-wetting of lubricants on hard disks
Authors: Xu Lei , Ogletree D. Frank , Salmeron Miquel , Tang Huan , Gui Jing , Marchon Bruno
Published: July 2002

The de-wetting process of Zdol-type lubricants on hard disks has been studied at the molecular scale with scanning polarization force microscopy (SPFM) and ellipsometry. It was found that de-wetting occurred over hours to days when the initial lubricant layer was thicker than one monolayer (∼20 Å thick). De-wetting was found to take place in a layered fashion for Zdol-TX and Z-Tetraol, with weakly bound second layers accreting into droplets, however, no layering was seen for Zdol during de-wetting, only droplets. The droplets eventually grew to the point where they could be observed by optical microscopy. In all cases, the first monolayer remained bound to the surface. Although the atomic force microscopy (AFM) tip could easily penetrate this first layer, it did not remove it.
Diffusion and surface reaction in random fibrous media
Authors: Strieder William
Published: July 2002

In the classic “trapping” problem, the reactant is uniformly generated at a constant rate across a matrix phase, and diffuses to the interface of a dispersed cylinder phase, where it instantly reacts. Prager’s upper bound on the effective reaction rate constant k (a “best” bound based on void-point nearest-neighbor-surface statistics) is derived for randomly placed, freely overlapping, infinitely long cylinders of radius a, where each cylinder has an arbitrary orientation with respect to the others. To compare with simulation data for an isotropic bed of overlapping spheres of radius a, the variational upper bound is considered for the case of the three-dimensional, isotropic overlapping cylinder bed with totally random mutual cylinder orientations. Once a correction factor of 3/2 for the surface area ratio of the overlapping sphere to overlapping cylinder beds is applied, the analytical variational cylinder bed bound is nearly coincident with the sphere dispersion simulation curve. The small differences observed at lower dispersion densities are consistent with the expected diffusion flux differences in the sphere and cylinder geometry. By comparing the variational upper bound with other simulation results for randomly overlapping, aligned, elongated, prolate spheroids, a maximum range for the k change due to mutual rotation between neighboring cylindrical reaction sites (29% and 32%, respectively, at solid volume fractions of 0.30 and 0.50) is obtained. The necessary distribution moments for the corresponding “relaxation time” lower bound are also given. The Doi lower bound on the effective reaction rate (a “best” bound based on two-point void–void Fvv, void–surface Fvs, and surface–surface Fss correlations) is addressed. The two- point correlations for an isotropic bed of overlapping cylinders with random mutual orientation in three dimensions are presented. Their behavior is discussed, and a logarithmic singularity in Fss at the two-point distance of 2a is pointed out, that renders the Doi bound indeterminate.
Optimized partitioning in perturbation theory: Comparison to related approaches
Authors: Surján P. R. , Szabados Á.
Published: July 2002

A generalized Epstein–Nesbet type perturbation theory is introduced by a unique, “optimal” determination of level shift parameters. As a result, a new partitioning emerges in which third order energies are identically zero, most fifth order terms also vanish, and low (2nd, 4th) order corrections are quite accurate. Moreover, the results are invariant to unitary transformations within the zero order excited states. Applying the new partitioning to many-body perturbation theory, the perturbed energies exhibit appealing features: (i) they become orbital invariant if all level shifts are optimized in an excitation subspace; and (ii) meet the size-consistency requirement if no artificial truncations in the excitation space is used. As to the numerical results, low order corrections do better than those of Mo/ller–Plesset partitioning. At the second order, if the single determinantal Hartree–Fock reference state is used, the CEPA-0 (=LCCD) energies are recovered. Higher order corrections provide a systematic way of improving this scheme, numerical studies showing favorable convergence properties. The theory is tested on the anharmonic linear oscillator and on the electron correlation energies of some selected small molecules.
Surplus function variational quantum Monte Carlo approach: Excited state processing
Authors: Huang Hongxin , Zeng Xianbiao , Ming Lei
Published: July 2002

Surplus function variational quantum Monte Carlo approach used for the excited state of electrons has been proposed. If the initial wave functions of both the excited state and ground state belong to the irreducible representation with different symmetries, the surplus function variational quantum Monte Carlo approach of the excited state is identical to that of the ground state, and if they belong to the irreducible representation with an identical symmetry, the surplus function variational quantum Monte Carlo approach of the ground state can also be applied to the excited state only by orthogonal revision for the initial wave function of the excited state. The detailed calculations formulas on the surplus function variational quantum Monte Carlo approach of the excited state have been deduced. A complete set of the surplus function variational quantum Monte Carlo approach for both the ground state and excited state has been established. The energy values of X 3B1, 1 1A1, and 2 1A1 states of CH2 have been calculated using this approach. The calculation results show that the precision of the surplus function variational quantum Monte Carlo approach is comparable to that of FCI approach only by iteration to the third-order energy value for three states of CH2. The correlation energy percentages of the fourth- and the fifth-order energy obtained by the surplus function variational quantum Monte Carlo approach are over 80% and 90%, respectively. This indicates that the approach is successful both in the ground state and in the excited state.
Potential energy surfaces for and energetics of the weakly-bound Al–H2 and B–H2 complexes
Authors: Williams Jason , Alexander Millard H.
Published: July 2002

Following an earlier investigation [M. H. Alexander, J. Chem. Phys. 99, 6014 (1993)] of the B(2s22p 2P)–H2 complex, we report new ground state potential energy surfaces (PESs) for the Al(2s22p 2P)–H2 and B(2s22p 2P)–H2 van der Waals complexes, based on multireference configuration-interaction calculations with an extrapolation to the complete basis set limit. The degeneracy of the Al 3p and B 2p orbitals gives rise to three adiabatic PESs (two of A′ symmetry and one of A″ symmetry in Cs geometry). By viewing the two adiabatic states of A′ symmetry as an orthogonal transformation of the in-plane Al 3p and B 2p orbitals, we transform the PESs to an approximate diabatic representation, which involves four potential energy functions. Both molecules have energy minima in C2v geometry with an electronic symmetry 2B2. The dissociation energies are De=231 cm−1 for Al–H2 and 142 cm−1 for B–H2. The energies of the lowest bend–stretch levels of the complexes of Al (2P) and B (2P) with either o- or pH2 are determined from the diabatic PESs. The predicted zero-point-corrected dissociation energies (D0) are 49.2 and 74.3 cm−1 for Al with pH2 and oH2, respectively, and 39.7 and 59.8 cm−1 for B with pH2 and oH2, respectively.
Structure and properties of the weakly bound trimer (H2O)2HCl observed by rotational spectroscopy
Authors: Kisiel Z. , Białkowska-Jaworska E. , Pszczółkowski L. , Milet A. , Struniewicz C. , Moszynski R. , Sadlej J.
Published: July 2002

The weakly bound trimer between two water molecules and one hydrogen chloride molecule was identified from its rotational spectrum observed at conditions of supersonic expansion. The spectrum was measured with a cavity, Fourier transform microwave spectrometer and the rotational, quartic, and chlorine hyperfine coupling constants were determined for six isotopomers of the complex: (H216O)2H35Cl, (H216O)2H37Cl, (H16OD⋅⋅⋅16OH2)H35Cl, (H18OH⋅⋅⋅16OH2)H35Cl, (H16OH⋅⋅⋅18OH2)H35Cl, and (H218O)2H35Cl. The rotational spectrum consists of transitions allowed by the μa and the, much smaller, μb dipole moment components. The rotational transitions are further split into components belonging to four low-lying vibration-rotation-tunneling substrates. The trimer is bound strongly enough to attenuate the dynamics of the water subunits to a level such that the differences in rotational constants between the four states are below 1 MHz. The complex is near planar as evidenced by inertial defect of −0.5 uÅ2. Analysis of experimental and theoretical results reveals that the three monomers are bound in a triangular arrangement through O⋅⋅⋅HO and O⋅⋅⋅HCl hydrogen bonds and a primarily dispersive OH⋅⋅⋅Cl bond. All atoms, with the exception of two nonbonded hydrogens, lie near a common plane. The rs, r0, and r* structural parameters have been evaluated and the two hydrogen bonds r*(OH⋅⋅⋅O)=2.8151(8), and r*(O⋅⋅⋅HCl)=3.0840(11) Å are appreciably shorter than analogous bonds in (H2O)2 and H2O⋅⋅⋅HCl, whereas r*(OH⋅⋅⋅Cl)=3.4152(12) Å is close to the sum of the van der Waals radii for O and Cl. The experimental results concerning all observables determinable from the rotational spectrum are found to be in very good agreement with theoretical predictions.
Mixed quantum-classical surface hopping dynamics
Authors: Nielsen Steve , Kapral Raymond , Ciccotti Giovanni
Published: July 2002

An algorithm is presented for the exact solution of the evolution of the density matrix of a mixed quantum-classical system in terms of an ensemble of surface hopping trajectories. The system comprises a quantum subsystem coupled to a classical bath whose evolution is governed by a mixed quantum-classical Liouville equation. The integral solution of the evolution equation is formulated in terms of a concatenation of classical evolution segments for the bath phase space coordinates separated by operators that change the quantum state and bath momenta. A hybrid Molecular Dynamics–Monte Carlo scheme which follows a branching tree of trajectories arising from the action of momentum derivatives is constructed to solve the integral equation. We also consider a simpler scheme where changes in the bath momenta are approximated by momentum jumps. These schemes are illustrated by considering the computation of the evolution of the density matrix for a two-level system coupled to a low dimensional classical bath.
Maximum radius of convergence perturbation theory
Authors: Finley James P.
Published: July 2002

An ab initio method is introduced, called the maximum radius of convergence (MAXRc) perturbation theory, that exploits the added degrees of freedom permitted with flexible energy denominator perturbation theory [J. Chem. Phys. 109, 7725 (1998)] by defining the energy-denominator factors of a Rayleigh–Schrödinger perturbative expansion to be (approximately) optimal. This method can yield rapid convergence as long as there is no quasidegeneracies in first order between the reference-space state and one of the orthogonal-space states.
Manipulating and enhancing chemical reactions in helium droplets
Authors: Lugovoj Evgeni , Toennies J. Peter , Vilesov Andrey
Published: July 2002

The rate of the chemiluminescent Ba+N2O→BaO*+N2 reaction is greatly enhanced inside cold (0.38 K) helium droplets. If Xe atoms are added to the droplets, the “hot” surface channel is suppressed and all the emission is in a vibrational progression of sharp lines, indicating that the reaction occurs only inside the droplets.
A time-dependent quantum mechanical investigation of dynamical resonances in three-dimensional HeH2+ and HeHD+ systems
Authors: Maiti B. , Mahapatra S. , Sathyamurthy N.
Published: July 2002

Bound and quasibound states of HeH2+ and HeHD+ in three dimensions, for zero total angular momentum, have been computed using a time-dependent quantum mechanical approach. Time evolution of a carefully chosen wave packet in the interaction region is followed and the time correlation function evaluated and its Fourier transform obtained. The resulting eigenvalue spectrum and the corresponding eigenfunctions are examined to characterize the nature of the dynamical resonances for the system. It becomes clear that at low energies the quasibound states can be assigned readily in terms of local modes. While some of the higher energy state eigenfunctions resemble the hyperspherical modes, a large number of them cannot be assigned easily, suggesting irregular dynamics, in keeping with a large number of unstable periodic orbits known for the system.
Explicitly correlated coupled cluster R12 calculations of the dipole polarizability of He2
Authors: Jaszuński Michał , Klopper Wim , Noga Jozef
Published: July 2002

The effect of the interatomic interaction on the static dipole polarizability has been reexamined in detail for the He dimer at the equilibrium internuclear distance of RHe… He=5.6 a0 by using high-precision explicitly correlated R12 approaches. Our most accurate CCSD(T)-R12 results for the increments of the polarizability are 61.79×10−3 a.u. for the anisotropy and −1.04×10−3 a.u. for the trace, not significantly different from recent literature CCSD data and confirming their high accuracy.
S–T
conversion induced by external magnetic field in gaseous oxalylfluoride excited to the 7151-level of the Ã 1Au state
Authors: Makarov Vladimir I. , Kochubei Sergei A. , Ishchenko Valerii N. , Khmelinskii Igor V.
Published: July 2002

Fluorescence decay of gaseous oxalylfluoride (COF)2 excited to the Ã 1Au(7151) level was measured as a function of gas pressure in absence and in presence of magnetic field, B=0.3 T. On excitation to this level, the dynamics in both zero and nonzero fields may be described in the intermediate-molecule limit, with the fluorescence exhibiting biexponential and triexponential decay, respectively. The fast component decay rate constant kf=(5.31±0.22)⋅107 s−1 is independent on the (COF)2 gas pressure or magnetic field strength, while that of the first slow component depends on both. The second slow component lifetime is independent of the field strength, while dependent on the gas pressure. The Stern–Volmer dependence, measured with a field in the 0.3–12 mTorr pressure range, is nonlinear. The results obtained were explained by the indirect (electronic- and nuclear-spin-decoupling) mechanism, proposed earlier.
Structure and dynamics of N2–IH
Authors: Jabs Wolfgang , McIntosh Avery L. , Lucchese Robert R. , Bevan John W. , Brugh Dale J. , Suenram Richard D.
Published: July 2002

The ground state rotational spectra and internal dynamics in five isotopomers N214–IH, N214–ID, N215–IH, N14N15–IH, and N15N14–IH of the nitrogen–hydrogen iodine dimer have been investigated by pulsed-nozzle, Fourier-transform microwave spectroscopy. Analysis of the recorded spectra yields rotational, centrifugal distortion, nuclear quadrupole and spin-rotation coupling constants for the five isotopomers. The spectroscopic constants are interpreted in terms of a ground-state NN–IH structure using the parameters rcm, kσ, and the oscillation angles θN2 and θHI of the N2 and IH moieties. For N214–IH, the values rcm=4.197 872(67) Å, kσ=1.461 52(32) N/m, θN2=25.61(22)°, and θHI=23.454 6(91)° are obtained. The rotational transitions of N214–IH, N214–ID, and N215–IH show a doubling, which is a consequence of tunneling associated with the interchange of the two nitrogen nuclei. The experimental findings are compared with results from ab initio calculations.
Dynamics of methane trapped in C60 interstices
Authors: Kwei George H. , Trouw Frans , Morosin Bruno , King Harry F.
Published: July 2002

We have carried out inelastic neutron scattering studies of the methane/C60 system in order to understand the hindered rotational and vibrational dynamics of methane trapped in C60 interstices and to determine the structure around the interstitial site. At temperatures of 20 K and below, we observe inelastic peaks from rotational transitions of the CH4. These transitions allow unambiguous assignment of the hindered rotational energy levels and a determination of the interaction potential. The appearance of two peaks for one of the J=0→3 transitions implies the existence of two distinct kinds of interstitial sites and the measured transition energies suggest rotational barriers of about 26 and 16 meV for these sites. Time-dependent changes in peak heights indicate slow (t1/2≈2.6 h) triplet→quintet nuclear spin conversion that necessarily accompanies the J=1→0 rotational relaxation. We also have observed an inelastic peak at 10.9 meV in a region where there is a gap in the calculated, and otherwise dense, rotational spectrum. We believe that this must correspond to a local vibrational mode of CH4 rattling in its cage at ∼2.6 THz. Other peaks involving higher-energy vibrational excitations in CD4/C60 correspond in energy to assigned peaks in the inelastic neutron scattering spectra of C60, albeit sometimes with different intensities. Taken together, these findings suggest the rotation of CH4 is hindered by the C60 cage and that the vibrations of CH4 and C60 are loosely coupled.
Time-resolved photoelectron angular distributions as a probe of intramolecular dynamics: Connecting the molecular frame and the laboratory frame
Authors: Underwood Jonathan G. , Reid Katharine L.
Published: July 2002

A formalism is presented in which the laboratory frame photoelectron angular distribution (PAD) is expressed as a convolution of the molecular frame PAD with the laboratory frame molecular axis distribution. Molecular and laboratory frame PADs are discussed in the context of probing intramolecular dynamics in the time domain. Model calculations for a C3v molecule are presented as an illustration of the differences between measurements in these two reference frames, and the effect of the degree of molecular alignment upon the laboratory frame measurements. Different symmetries of the orbital undergoing ionization are also considered in order to illustrate the sensitivity of PADs to nonadiabatic processes.
Grassmann analogs of classical matrix groups
Authors: Backhouse N. B. , Fellouris A. G.
Published: July 2002

In general, if the parameters of a real or complex algebraic matrix group are replaced by Grassmann parameters, without changing the algebraic constraints, the resulting set fails to form a group. It is shown how to remedy this defect of naive Grassmannification by generalizing the constraint relations. In particular, it is shown how to define Grassmann analogs of the orthogonal, unitary, and symplectic groups.
Symmetries of the higher-order KP equations
Authors: Case K. M.
Published: July 2002

Symmetry generators T(l)n=tIn+L(l)n are constructed for the lth-order Kadomtsev–Petviashvili (KP) equation for all n≥l−2. In the case of the ordinary KP equation (l=2) these symmetries are those found by Chen et al. [Physica D 9, 439 (1983)].
Theorem on linearized Hamiltonian systems
Authors: Case K. M.
Published: July 2002

Many nonlinear field equations can be written in Hamiltonian form. Thus the equation ∂tu=K(u) can be written ∂tu =[u, H], where H is an appropriate functional and [ , ] is a Poisson bracket. Frequently one is interested in the solution of the equation linearized about a given solution, i.e., the equation ∂t τ=K′(τ), where K′(τ)=(d/dε) K(u+ετ)‖ε=0. It is known that if a functional I is a constant of motion then τ=[u, I] is a solution. Recently, more general solutions of this form have been found. To prove these results, it is very useful to have the answer to the following question: If Ii, Ij are two functionals, and Ki =[u, Ii], what is K′i(Kj)? The answer is Ki(Kj) =[[u, Ii], Ij]. Previously, this was proved assuming that canonical coordinates can be introduced. Here a proof is given without any such assumption.
Bifurcations in the slow-fluctuation technique
Authors: Augusteijn Marijke F. , Breitenberger Ernst
Published: July 2002

In the slow-fluctuation method, the bifurcations between solutions of a conservative, autonomous, nonlinear oscillatory system can be found by an essentially algebraic, systematic study of constant-amplitude motions and root parities. This is shown for simple systems with sufficient detail to indicate the extensions needed in the treatment of more complex systems. An example from celestial mechanics is given.
Minimal acceleration requirements for ‘‘time travel’’ in Gödel space-time
Authors: Malament David B.
Published: July 2002

It is demonstrated that the total integrated acceleration of any closed timelike curve in Gödel space-time must be at least ln(2+(5)1/2). This answers a question posed by Geroch.
Isotropic homogeneous universe with viscous fluid
Authors: Santos N. O. , Dias R. S. , Banerjee A.
Published: July 2002

Exact solutions are obtained for the isotropic homogeneous cosmological model with viscous fluid. The fluid has only bulk viscosity and the viscosity coefficient is taken to be a power function of the mass density. The equation of state assumed obeys a linear relation between mass density and pressure. The models satisfying Hawking’s energy conditions are discussed. Murphy’s model is only a special case of this general set of solutions and it is shown that Murphy’s conclusion that the introduciton of bulk viscosity can avoid the occurrence of space-time singularity at finite past is not, in general, valid.
The well-posedness of (N=1) classical supergravity
Authors: Bao David , Choquet-Bruhat Yvonne , Isenberg James , Yasskin Philip B.
Published: July 2002

In this paper we investigate whether classical (N=1) supergravity has a well-posed locally causal Cauchy problem. We define well-posedness to mean that any choice of initial data (from an appropriate function space) which satisfies the supergravity constraint equations and a set of gauge conditions can be continuously developed into a space-time solution of the supergravity field equations around the initial surface. Local causality means that the domains of dependence of the evolution equations coincide with those determined by the light cones. We show that when the fields of classical supergravity are treated as formal objects, the field equations are (under certain gauge conditions) equivalent to a coupled system of quasilinear nondiagonal second-order partial differential equations which is formally nonstrictly hyperbolic (in the sense of Leray–Ohya). Hence, if the fields were numerical valued, there would be an applicable existence theorem leading to well-posedness. We shall observe that well-posedness is assured if the fields are taken to be Grassmann (i.e., exterior algebra) valued, for then the second-order system decouples into the vacuum Einstein equation and a sequence of numerical valued linear diagonal strictly hyperbolic partial differential equations which can be solved successively.
Comments on the perturbed sine–Gordon equation
Authors: Papastamatiou N. J. , Matsumoto H. , Umezawa H.
Published: July 2002

We examine the sine–Gordon equation with a perturbation λΔV. We derive necessary conditions on ΔV such that the perturbed equation has solutions with finite energy, analytic in λ, and which reduce to the static soliton when the perturbation is removed (λ→0). Several examples illustrating these conditions are presented.
Static gravitational field in generalized scalar tensor theory
Authors: Duttachoudhury S. B. , Banerjee N.
Published: July 2002

A useful relation between the metric component g00 and the scalar field φ is obtained for a static gravitational field in Nordtvedt’s general scalar tensor theory where the parameter ω is a variable. A simple method for generating the solutions of Nordtvedt’s field equations from the corresponding Einstein’s vacuum solutions is obtained. Static axially symmetric field equations are solved in prolate and oblate spheroidal coordinates. Simple cylindrically symmetric solutions are also given.
Bound states, resonances, and symmetries of a neutral Dirac particle with anomalous magnetic moment, coupled to a fixed monopole
Authors: Barut A. O. , Bracken A. J.
Published: July 2002

For the system consisting of a neutral Dirac particle with anomalous magnetic moment, interacting with a fixed magnetic monopole, zero-energy bound states are constructed for each possible value of the total angular momentum. Results of Kazama and Yang for the charge–monopole system are used to deduce the existence of other bound states for this system, when the mass of the bound particle is nonzero. In the zero-mass case, there are no other bound states, but there are resonant states, and these are determined exactly. A noncompact, so(3,2) symmetry algebra of the zero-energy bound states is given for the finite-mass case and for the zero-mass case. In each case the infinite number of such states is associated with an irreducible Majorana representation of the algebra.
On a particular solution of the equation of Ernst with n fields, parametrized by an arbitrary harmonic function
Authors: Léauté B. , Marcilhacy G.
Published: July 2002

We determine a particular solution, dependent upon an arbitrary harmonic function in cylindrical coordinates, of the system of n partial differential equations, which characterizes both the axially symmetric field solution of the Einstein (n−1)–Maxwell equations and one class of axially symmetric static self-dual SU(n+1) Yang–Mills fields.
Separation of variables for the Rarita–Schwinger equation on all type D vacuum backgrounds
Authors: Kamran N.
Published: July 2002

We present a separable master equation governing Rarita–Schwinger spin- (3)/(2) fields, valid in the whole class of type D vacuum backgrounds.