bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation study of the collapse of linear and ring homopolymers
Authors: Ma Jianpeng , Straub John E. , Shakhnovich Eugene I.
Published: July 2002

The thermodynamic and kinetic properties of Lennard-Jones homopolymers are studied by computer simulation of the homopolymer at constant temperature and during collapse following an abrupt temperature drop. The homopolymer dynamics is simulated using both the Gaussian phase packet dynamics algorithm, which provides an approximate solution of the Liouville equation for the time dependent classical density distribution, and conventional molecular dynamics. Numerical simulations of both linear and ring polymers are analyzed and compared with theoretical predictions. The results support the idea that homopolymer collapse is a two-stage process.
Molecular dynamics simulation of a charged biological membrane
Authors: López Cascales J. J. , García de la Torre J. , Marrink S. J. , Berendsen H. J. C.
Published: July 2002

A molecular dynamics simulation of a membrane with net charge in its liquid-crystalline state was carried out. It was modeled by dipalmitoylphosphatidylserine lipids with net charge, sodium ions as counterions and water molecules. The behavior of this membrane differs from that was shown by other membranes without a net charge as a consequence of strong Coulomb interaction between atoms of adjacent phospholipids. The most remarkable effect produced by such interaction between neighboring lipids is a reduction of the surface area per phospholipid compared to an uncharged membrane. In addition, other properties of the membrane were also affected by this interaction between adjacent lipids such as the atom distribution across the membrane, the diffusion coefficient of the different components of the membrane and the order parameter of the phospholipid hydrocarbon region. Some comparisons of this membrane with dipalmitoylphosphatidylcholine membrane without net charge at similar conditions are presented.
Kinetic laws at the collapse transition of a homopolymer
Authors: Kuznetsov Yu. A. , Timoshenko E. G. , Dawson K. A.
Published: July 2002

We present results from numerical analysis of the equations derived in the Gaussian self-consistent method for kinetics at the collapse transition of a homopolymer in dilute solution. The kinetic laws are obtained with and without hydrodynamics for different quench depths and viscosities of the solvent. Some of our earlier analytical estimates are confirmed, and new ones generated. Thus the first kinetic stage for small quenches is described by a power law decrease in time of the squared radius of gyration with the universal exponent αi=9/11 (7/11) with (without) hydrodynamics. We find the scaling laws of the characteristic time of the coarsening stage, τm∼Nγm, and the final relaxation time, τf∼Nγf, as a function of the degree of polymerization N. These exponents are equal to γm=3/2, γf=1 in the regime of strong hydrodynamic interaction, and γm=2, γf=5/3 without hydrodynamics. We regard this paper as the completion of our work on the collapse kinetics of a bead and spring model of a homopolymer, but discuss the possibility of studying more complex systems.
High Rydberg states of DABCO: Spectroscopy, ionization potential, and comparison with mass analyzed threshold ionization
Authors: Boogaarts Maarten G. H. , Holleman Iwan , Jongma Rienk T. , Parker David H. , Meijer Gerard , Even Uzi
Published: July 2002

Doubly-resonant excitation/vibrational autoionization is used to accurately determine the ionization potential (IP) of the highly symmetric caged amine 1,4 diazabicyclo[2,2,2]octane (DABCO). The IP of DABCO excited with one quantum of the ν24(e′) vibration lies at (59 048.62±0.03) cm−1, based on fitting 56 components of the npxy Rydberg series (δ=0.406±0.002) to the Rydberg formula. Rydberg state transition energies and linewidths are determined using standard calibration and linefitting techniques. The IP determined from Rydberg state extrapolation is compared with that determined by mass analyzed threshold ionization (MATI). Effects of static electric fields on MATI signals measured for the high Rydberg states are discussed.
Full configuration–interaction and state of the art correlation calculations on water in a valence double-zeta basis with polarization functions
Authors: Olsen Jeppe , Jo/rgensen Poul , Koch Henrik , Balkova Anna , Bartlett Rodney J.
Published: July 2002

Using a valence double-zeta polarization basis, full configuration–interaction (FCI) calculations are carried out on water at its equilibrium geometry and at geometries where the OH bond lengths are stretched until dissociation. At the same geometries and with the same basis set configuration interaction calculations at excitation levels up to hextuples, multireference singles doubles configuration interaction calculations, coupled cluster calculations at excitation levels up to quadruples, Mo/ller–Plesset perturbation theory calculations through order fifteen, and complete active space second-order perturbation theory calculations are also carried out. The static correlation contribution increase with increasing bond length. The calculations show that the coupled cluster approach has a remarkable ability to describe even relatively large static correlation contributions. The single reference perturbation expansion breaks down for larger OH bond length, while the multireference approach preserves the accuracy for the whole potential curve. At the equilibrium geometry, FCI calculations have also been carried out for the lowest state of 2A1, 2B1, and 2B2 symmetry of H2O+, and the results compared with state of the art correlation results for total energies and ionization potentials (IP’s). Differential energies (IP’s) are obtained more accurately than absolute (total) energies in the size extensive coupled cluster and perturbation approaches. For the nonsize extensive configuration interaction method errors are obtained of the same size for differential and absolute energies.
Motion of ‘‘hot’’ oxygen adatoms on corrugated metal surfaces
Authors: Wahnström Göran , Lee Ann B. , Strömquist Johan
Published: July 2002

We have investigated the likelihood of a high transient mobility for hyperthermal adatoms resulting from dissociative adsorption of a diatomic molecule, in particular O2 on Al(111), using dynamical simulations on model potentials fitted to available first principle data. We find no evidence for a large transient mobility, compatible with the conclusions by Brune et al. [Phys. Rev. Lett. 68, 624 (1992)], for hyperthermal O atoms moving on the chemisorption potential energy surface for the O/Al(111) system. Our findings are more compatible with the STM results for O2 on Pt(111). We have also examined the possibility that one of the O atoms moves further away from the surface as a neutral species. That could result in that some O atoms leave the surface as neutral species (abstraction) or extended trajectories along the surface if a weakly bound neutral state for the O atom exists with sufficiently long lifetime.
Dynamics of the displacement of CO from Cu(111) by H atoms incident from the gas phase
Authors: Rettner C. T. , Auerbach D. J.
Published: July 2002

We find that CO is displaced from a ∼90 K Cu(111) surface by an incident H atom beam with a cross section of ∼10−16 cm2/H atom. As for a previous study of the ejection of O2 from Pt(111), our results indicate that part of the heat of adsorption of the incident species is carried away by the ejected molecule in a ‘‘dynamic displacement’’ process. We have determined the internal-state distribution of the ejected CO using quantum-state-specific laser ionization detection. We have also determined its angular and velocity distribution using a rotatable quadrupole mass spectrometer. The rotational distribution of molecules displaced in the v=0 and v=1 vibrational states are close to Boltzmann distributions at 390 K and 940 K, respectively. While the v=1 population is approximately proportional to the CO coverage, that for v=0 has a more complex coverage dependence, approximately following the presence of the CO α state, which gives a distinct temperature-programmed desorption peak for coverages above 1/3 ML. The equivalent vibration temperature ranges from 1500 K at low coverage to 800 K for a saturated surface. The velocity distribution of the ejected molecules is close to a Boltzmann distribution at 1300 K, corresponding to a translational energy of ∼0.22 eV. The angular distribution is symmetric about the normal and is close to a cos5 θf at small angles, desorption angles, θf, approximately following a cosine distribution for θf≳40°. We discuss the results in terms of the dynamic displacement model, where desorption of CO(v=0) is driven by a sudden switch from the chemisorption to physisorption wells. In the case of CO(v=1), we suggest that desorption may follow the formation of a temporary HCO intermediate.
Efficient pressure estimation in molecular simulations without evaluating the virial
Authors: Harismiadis V. I. , Vorholz J. , Panagiotopoulos A. Z.
Published: July 2002

A method for calculating the pressure in molecular simulations that does not require calculation of the virial is proposed, building on earlier work by [R. Eppenga and D. Frenkel, Mol. Phys. 52, 1303 (1984)]. The method requires ‘‘test’’ volume changes and it is particularly efficient for ensembles demanding volume fluctuations, such as the Gibbs ensemble. The method is validated by calculating the pressure in Gibbs ensemble simulations for monoatomic and polyatomic fluids.
Alignment of D2(v, J) desorbed from Cu(111): Low sensitivity of activated dissociative chemisorption to approach geometry
Authors: Gulding S. J. , Wodtke A. M. , Hou H. , Rettner C. T. , Michelsen H. A. , Auerbach D. J.
Published: July 2002

We have determined the alignment of D2(v,J) desorbed from Cu(111). The measurements reveal a small preference for ‘‘helicoptering’’ motion that increases with increasing J. At low J, the alignments are much smaller than predicted by recent calculations. We believe that the anisotropic potential may scramble the alignment as the molecules leave the surface.
Absolute cross sections for dissociative electron attachment to condensed CH3Cl and CH3Br: Effects of potential energy curve crossing and capture probability
Authors: Ayotte P. , Gamache J. , Bass A. D. , Fabrikant I. I. , Sanche L.
Published: July 2002

We report cross sections for the trapping of 0–10 eV electrons by CH3Cl and CH3Br physisorbed onto a Kr covered Pt substrate, measured as a function of Kr film thickness and methyl halide concentration. The molecules stabilize electrons incident at the surface by the dissociation of transient CH3Cl− and CH3Br− ions into an atomic anion and a neutral fragment [dissociative electron attachment DEA]. For CH3Cl, the condensed phase absolute DEA cross section at ≈0.5 eV, reaches 13×10−18 cm2±50%, which is 104–106 times larger than the gas phase cross section. At higher energies (5–10 eV) for CH3Cl, our measurements provide a lower limit for the DEA cross section. For CH3Br, the maximum DEA cross section occurs below the vacuum level; we measure an absolute magnitude of 3.0×10−16 cm2±50% near 0 eV, which is 100 times larger than the corresponding gas phase value. These enhancements in cross section arise from the lowering of the potential energy surfaces of intermediate anions due to polarization induced in the Kr layer and metal substrate. An increase in DEA cross section with a reduction in the distance of transient anions from the metal surface, is explained by the effect of image charges on the energy at which anion and neutral ground state potential energy curves cross. Below thicknesses of 5 ML of Kr, a decrease in DEA cross section is observed and attributed to a reduction in the electron capture probability of the halide due to competition with transfer to the metal substrate.
Vibrational cooling after ultrafast photoisomerization of azobenzene measured by femtosecond infrared spectroscopy
Authors: Hamm P. , Ohline S. M. , Zinth W.
Published: July 2002

The vibrational cooling of azobenzene after photoisomerization is investigated by time resolved IR spectroscopy with femtosecond time resolution. Transient difference spectra were obtained in a frequency range where phenyl ring modes and the central N=N-stretching mode absorbs. The experimental data are discussed in terms of a simple theoretical model which was derived in order to account for the off-diagonal anharmonicity between the investigated high-frequency modes and the bath of the remaining low-frequency modes in a polyatomic molecule. It is shown that these off-diagonal anharmonic constants dominate the observed transient absorbance changes while the anharmonicity of the high-frequency modes themselves (diagonal anharmonicity) causes only minor effects. Based on the transient IR spectra, the energy flow in the azobenzene molecule can be described as follows: After an initial ultrafast intramolecular energy redistribution process, the decay of the related intramolecular temperature occurs via intermolecular energy transfer to the solvent on a time scale of ca. 20 ps.
Quantum dynamical stereochemistry of atom–diatom reactions
Authors: de Miranda Marcelo P. , Clary David C.
Published: July 2002

We have used density matrix techniques and angular momentum algebra to obtain quantum–mechanical equations describing the dynamical stereochemistry of the atom–diatom reaction A+BC⇌AB+C. The relative motions of reagents and products are specified by four vectors: rotational angular momenta of diatomic molecules and relative velocities of reagents and products. Our equations show how the correlations between the spatial distributions of these four vectors are related to the scattering matrix determined in quantum scattering calculations. We present three different expressions for the four-vectors correlation. One of them is appropriate to the helicity representation of the scattering matrix, while the others are appropriate to the orbital angular momentum representation with either space-fixed or body-fixed reference frames. The formulation adopted allows for a rigorous comparison between theory and experiment. It takes mixed quantum–mechanical states and unobserved quantum-numbers into account, and all vector distributions are expressed in terms of measurable quantities (scattering angles and polarization moments of rotational angular momenta). Explicit expressions for most of the lower-order vector correlations obtained by direct reduction of the four-vectors correlation formulas are also presented.
On the appearance of resonances in reactive scattering: An experimental study of the H+D2→HD+D reaction at collision energies near 1.29 eV
Authors: Wrede E. , Schnieder L.
Published: July 2002

The hydrogen exchange reaction H+D2(v=0,j=0)→HD(v′,j′)+D was investigated at collision energies between 1.27 and 1.30 eV in a high resolution crossed beam experiment. The angle-velocity distribution of nascent D-atoms was measured using the technique of Rydberg atom time-of-flight spectroscopy. The resolution of this technique allows the identification of individual ro-vibrational states of the associated HD product molecule. Calculations done on the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential energy surface (PES) explicitly including the Geometric Phase effect predict a resonance in reactive scattering for collision energies close to 1.29 eV. The experimental data do not show signatures of this resonance in the energy range investigated. Instead of this a general good agreement between experiment and theory even on the basis of state-to-state differential cross sections is already found for calculations on the LSTH PES at a collision energy of 1.30 eV not including the Geometric Phase indicating that this effect does not play an important role at these collision energies.
Trapping-mediated and direct dissociative chemisorption of methane on Ir(110): A comparison of molecular beam and bulb experiments
Authors: Seets D. C. , Wheeler M. C. , Mullins C. B.
Published: July 2002

Molecular-beam and bulb gas techniques were employed to study dissociative chemisorption and physical adsorption of methane on Ir(110). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence. With this investigation, we provide the first unambiguous evidence of a trapping-mediated pathway for methane dissociation on any surface. This interpretation is supported by excellent quantitative agreement between our data at low kinetic energies and a simple kinetic model of the trapping-mediated mechanism. Additionally, this is the first molecular-beam study of any gas on any surface that is consistent with a simple trapping-mediated model in which the barrier to dissociation from the physically adsorbed state is greater than the barrier to desorption. At high-incident kinetic energies, the value of S0 increases with Ei indicative of a direct mechanism. The values of the reaction probability determined from the molecular-beam experiments are integrated over a Maxwell–Boltzmann energy distribution to predict the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. These calculations are in excellent agreement with the results obtained from bulb experiments conducted with room-temperature methane gas over Ir(110) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, a direct mechanism dominates reactive adsorption of methane over Ir(110).
Renner–Teller splitting in the C 1s→π* excited states of CS2, OCS, and CO2
Authors: Adachi Jun-ichi , Kosugi Nobuhiro , Shigemasa Eiji , Yagishita Akira
Published: July 2002

Fragment ions energetically emitted following the perpendicular (ΔΛ=+1) transitions of C 1s→π* of CS2, OCS, and CO2 are observed not only in the perpendicular (90°) direction but also in the parallel (0°) to the linear polarization; that is, ions have a momentum orthogonal to the linear molecule. This arises in the Renner–Teller (RT) vibronic coupling of bending vibrations in the C 1s→ in-plane π* excited state with a bent equilibrium geometry, though the RT splitting between the C 1s→ out-of-plane π* state with a linear geometry and the C 1s→ in-plane π* state is not visible directly due to the lifetime broadening. The 0° ion yield is relatively small in CS2 but is comparable to the 90° yield in CO2; in the latter the peak maximum at 0° is 0.06 eV lower than at 90° and the anisotropy parameter β is heavily dependent on the photon energy. In CO2 a great number of unresolved bending vibrations are coupled. The half-width at half-maximum on the lower energy side of the π* peak is much more sensitive to the RT splitting; 0.08, 0.11, and 0.29 eV for CS2, OCS, and CO2, compared with the full-width at half-maximum, 0.17, 0.39, and 0.64 eV. In CS2 the π* peak and β value are sharp and symmetric, indicating that the zero-point vibrational levels are only involved. In OCS three fine structures observed with separations of 0.21 eV are assigned to the ν3 mode, which is comparable to the stretching mode in CO.
Infrared spectroscopic study of rovibrational states of methane trapped in parahydrogen crystal
Authors: Momose Takamasa , Miki Masaaki , Wakabayashi Tomonari , Shida Tadamasa , Chan Man-Chor , Lee Steven S. , Oka Takeshi
Published: July 2002

The ν3 and ν4 vibrational transitions of methane trapped in solid parahydrogen have been observed by using Fourier transform infrared and high resolution laser spectroscopy. The observed spectrum is interpreted in terms of rovibrational states of the spherical rotor which are subjected to the crystal field splitting. The ν4 band shows extremely sharp lines of a width of ∼0.003 cm−1, while the ν3 band exhibits broader lines of a width of 1 cm−1. The infrared selection rules derived from an extended group theory to take into account the field effect are consistent with the observed spectra. The intermolecular interaction and the field effect in solid parahydrogen are analyzed quantitatively.
The convergence of basis set contractions: A case study of the molecular hyperfine structure of NH214
Authors: Kong Jing , Boyd Russell J.
Published: July 2002

The convergence of basis set contractions is examined at the multireference configuration interaction level with respect to the 14N isotropic hyperfine coupling constant in NH2, a case that is very sensitive to the choice of the basis set and the degree of correlation recovery. The contraction schemes studied include the atomic natural orbital (ANO) approach and two Hartree–Fock-based contraction methods: Raffenetti’s general contraction method and the segmented method. Contractions of a (13s8p2d) primitive set ranging from [4s3p2d] to [8s3p2d] in the s space and from [7s3p2d] to [7s6p2d] in the p space are employed. It is found that the ANO contraction yields the smoothest and fastest convergence, although all three contractions converge to the uncontracted results. Thus, the ANO contraction starts to converge at [6s3p2d] while the other two converge at [7s4p2d]. The correlation recovery of the core electrons versus that of the valence electrons is also studied. In contrast to previous speculation, it is found that the correlation due to core electrons is recovered well with the simple choice of the most populated ANOs.
Damping and higher multipole effects in the quantum electrodynamical model for electronic energy transfer in the condensed phase
Authors: Scholes Gregory D. , Andrews David L.
Published: July 2002

The interplay between electronic coupling, spectral linewidth, and rate of electronic energy transfer between chromophores is examined in the context of a quantum electrodynamical (QED) model. The QED framework properly allows us to identify the partitioning between the near and far zone mechanisms for transfer of energy between chromophores dispersed in condensed phase (liquid or solid) host media. The extent to which coupling is modified by the medium is investigated. A general QED treatment of higher multipole contributions to the coupling between transition moments is also derived, whence interactions involving electric dipole, quadrupole and octopole as well as magnetic dipole and quadrupole interactions are examined explicitly. A new formulation is presented wherein expressions for the multipolar coupling tensors are obtained in terms of spherical Bessel functions, providing a clear, compact representation of the retarded coupling interaction and its distance-dependence. The irreducible tensor formulation of the coupling is discussed, highlighting features concerning the exact form of the orientation factors that have often in the past escaped notice. The detailed method of implementing a rotational averaging of the resultant interaction tensors is demonstrated, finally leading to a novel and concise representation for multipolar couplings of arbitrary order. The coupling between bacteriochlorophyll a chromophores is discussed as an example.
The simulation of electronic absorption spectrum of a chromophore coupled to a condensed phase environment: Maximum entropy versus singular value decomposition approaches
Authors: Egorov S. A. , Gallicchio E. , Berne B. J.
Published: July 2002

We consider the problem of calculating the electronic absorption spectrum of a chromophore with intramolecular degrees of freedom coupled to a condensed phase environment. We approach this calculation in the framework of the imaginary-time path integral Monte Carlo techniques, and focus on the problem of the analytic continuation of the imaginary-time data to the real-time axis. Two alternative analytic continuation methods are considered: the maximum entropy method and the singular value decomposition method. An exactly solvable model is introduced to test the accuracy of these methods. Exact numerical results for the absorption spectra are compared to the spectra reconstructed by the analytic continuation methods; it is found that the singular value decomposition method gives systematically higher resolution than the maximum entropy method and is capable of reproducing the fine vibronic structure of the absorption spectrum.
Calculation of nuclear magnetic shieldings. XII. Relativistic no-pair equation
Authors: Fukui H. , Baba T.
Published: July 2002

A Schrödinger-Pauli type two-component perturbation theory has been presented for the calculation of relativistic effects of nuclear magnetic shieldings. The expression for the relativistic nuclear magnetic shieldings are derived from the Douglas-Kroll transformation of the no-pair equation for a molecule, which bears a nuclear magnetic dipole moment, and which is placed in an external magnetic field. The exact form of the relativistic kinetic energy is included in the eigenvalue equation which is solved variationally. We calculated the relativistic mass correction effect on the nuclear magnetic shieldings in the four hydrogen halide molecules, HF, HCl, HBr, and HI, at the coupled Hartree-Fock (CHF) level. It was shown that the mass correction effect increases the nuclear magnetic shieldings of the halogen nuclei. The increments in the shieldings are proportional to about the third power of the atomic numbers of the halogen nuclei. This increase in the shieldings results from the mass correction effect concentrating the electrons in the vicinity of the heavy nucleus, the so-called relativistic contraction.
Melting curves for neon calculated from pure theory
Authors: Solca Jan , Dyson Anthony J. , Steinebrunner Gerold , Kirchner Barbara , Huber Hanspeter
Published: July 2002

The melting curve of neon is determined from nonequilibrium molecular dynamics simulations performed at constant pressure, using ab initio pair potentials. The effects of various approximations on the predicted melting points are investigated through the use of pair potentials calculated at different levels of accuracy, and the inclusion of quantum effects on the motion via a Wigner–Kirkwood quantum effective potential. To avoid superheating of the model crystal, nucleation sites for melting are provided by creating clusters of void defects in the crystal prior to heating. The calculated melting curves are shown to be in good agreement with experimental measurements. Comparisons are made with similar calculations previously carried out for argon.
Sixth-order energy corrections with converged coupled cluster singles and doubles amplitudes
Authors: Kucharski Stanisław A. , Bartlett Rodney J.
Published: July 2002

A concise analysis of sixth-order perturbation theory and coupled cluster theory is presented, and several computational schemes correct through sixth order are constructed based upon an underlying coupled cluster singles and doubles calculation. The sixth-order corrections with respect to the coupled cluster singles and doubles wave function are identified and appropriate contributions are added to the energy using an n9 algorithm. Depending upon the type of the initial functional, several computational approaches can be developed. All of the methods considered have been tested for several small molecules and the results compared to the exact values. An average error of 0.3 mhartree is achieved compared to 0.1 mhartree for CCSDTQ (coupled cluster singles, doubles, triples, and quadruples).
A molecular-orbital derived polarization potential for liquid water
Authors: Gao Jiali
Published: July 2002

A molecular-orbital derived polarization (MP) model is developed and shown to yield good thermodynamic and structural results for liquid water. In this method, each solvent molecule is treated quantum-mechanically by the semiempirical AM1 model, and the charge polarization of the molecule is determined by electronic structure calculations, making use of a hybrid quantum mechanical and molecular mechanical (QM/MM) technique. The MP model is shown to be as successful as the best polarizable and nonpolarizable three-site potentials for water. In particular, the computed heat of vaporization and density for liquid water at 25 °C and 100 °C are within 1% of experimental data. In addition, the MP model gives good estimates for the radial distribution functions in comparison with neutron scattering results.
Crossed molecular beam studies of the O(1D)+CH4 reaction: Evidences for the CH2OH+H channel
Authors: Lin J. J. , Lee Y. T. , Yang X.
Published: July 2002

The O(1D)+CH4 reaction has been investigated using a new universal crossed molecular beam apparatus. Hydrogen atom elimination channel has been experimentally observed in this reaction. The pattern of dissociative ionization of the reaction products (from hydrogen loss channel) was compared with that of the methoxy (CH3O) radical produced from the photodissociation of CH3OH at 193 nm which has almost the same total energy deposition as the bimolecular reaction O(1D)+CH4. The experimental results suggest that the main hydrogen loss channel in the O(1D)+CH4 reaction should be CH2OH (hydroxymethyl)+H, while the CH3O (methoxy)+H channel is at most a minor reaction channel. This study provides an excellent experimental example of different dynamical behaviors exhibited in the unimolecular and bimolecular reactions of an essentially same chemical system (excited CH3OH) since the O(1D)+CH4 reaction likely occurs through the insertion mechanism.
Validity of Tolman’s equation: How large should a droplet be?
Authors: Koga Kenichiro , Zeng X. C. , Shchekin A. K.
Published: July 2002

Surface tension and the length δ (distance between the Gibbs surface of tension Rs and the equimolar surface Re) of simple liquid droplet (Lennard-Jones and Yukawa) are computed over a wide range of droplet sizes up to about 4×106 molecules. The study is based on the Gibbs theory of capillarity combined with the density–functional approach to gas–liquid nucleation. Since this method provides behavior of the surface tension fully consistent with the tension of the planner surface, the constant in Tolman’s equation δ∞ can be determined unequivocally from the asymptotic behavior of σs. Comparison of the tension given by Tolman’s equation against the result of exact thermodynamic relations reveals that Tolman’s equation is valid only when the droplet holds more than 106 molecules for the simple fluid systems near their triple points, in contrast to the conventional wisdom that Tolman’s equation may be applicable down to droplets holding a few hundreds of molecules.
Effective interactions between rigid polyelectrolytes and like-charged planar surfaces
Authors: Mashl Robert J. , Gro/nbech-Jensen Niels
Published: July 2002

We study the effective interaction between a planar array of uniformly negatively charged, stiff rods parallel to a negatively charged planar substrate in the absence of salt in a continuous, isotropic dielectric medium. Using Brownian dynamics simulations, we examine the general effects of counterion valence, rod spacing, macroion charge densities, and the rod size on the attractive rod-surface interaction force. At room temperature divalent as well as monovalent counterions mediate an interaction that can be repulsive or attractive upon adjusting either the macroion charge densities or the rod radius. Finally, we examine the effects of discretizing the surface charge as laterally mobile monovalent anions and of electrostatic images in the substrate.
Spin–orbit effects in the reaction of F(2P) with H2
Authors: Alexander Millard H. , Werner Hans-Joachim , Manolopoulos David E.
Published: July 2002

We report the first scattering calculations for the F+H2 reaction based on ab initio potential energy surfaces (PES’s) and an exact treatment of spin–orbit and Coriolis coupling. The probability of reaction of F*(2P1/2) is less than 10% of that for F(2P3/2) and the overall features of the scattering are well represented by calculations on the lowest electronic PES.
Charge-transfer exciton band structure in the fullerene crystal-model calculations
Authors: Pac Barbara , Petelenz Piotr , Eilmes Andrzej , Munn R. W.
Published: July 2002

A three-dimensional analog of the Merrifield model of the coupling between Frenkel and charge-transfer (CT) excitons of the C60 fullerene crystal is presented. The model is based on the nearest-neighbor approximation for the off-diagonal interactions and is essentially nonempirical. The parameters are estimated from free-molecule data and from the results of theoretical calculations published by other authors. The band structure of CT excitons in C60 is calculated and discussed in the context of available experimental evidence. Residual ambiguities in the parametrization are removed by calculation of the electroabsorption spectrum and comparison with experiment, as described in the accompanying paper.
Energy partitioning following photodissociation of methyl iodide in the A band: A velocity mapping study
Authors: Eppink A. T. J. B. , Parker D. H.
Published: July 2002

Translational and internal energy partitioning in the methyl and iodine fragments formed from photodissociation of methyl iodide in the A-band region is measured using velocity mapping. State-selective detection combined with the very good image quality afforded by the two-dimensional imaging technique allow a detailed analysis of the kinetic energy and angular distributions. Product vibrational energy is, as previously known, mainly partitioned into ν2, the umbrella mode of the methyl fragment, but a substantial fraction of molecules is also excited with one quantum of ν1, the symmetric C–H stretch, especially at higher dissociation energies. Preliminary evidence is also presented for excitation of several quanta of ν4, the asymmetric deformation mode. Rotational energy partitioning is similar for CH3 products formed in both the ground-state I(2P3/2) and the spin–orbit excited I*(2P1/2) channel for photodissociation across the full A-band spectrum. Dissociation of vibrationally excited molecules plays an increasingly important role at longer dissociation wavelengths. Two CH3I modes remain populated in the pulsed beam expansion, ν2(a1), the C–I stretch, and ν6(e), the methyl rock. Each reactant vibrational mode couples in a very specific manner into the I and I* dissociation channels. Trends in vibrational and rotational energy disposal are compared with recent theoretical predictions. Readjustment of many aspects of the ab initio multidimensional potential energy surfaces which have recently been calculated for CH3I appears to be necessary. The improved resolution offered by velocity mapping also allows a more accurate determination of the C–I bond energy. A dissociation energy of 2.41±0.02 eV is found.
Thermodynamic dissipation versus dynamical complexity
Authors: Nicolis G. , Nicolis C.
Published: July 2002

The thermodynamic entropy production is evaluated in different parts of the attractor of two representative reactive models giving rise to periodic and chaotic behavior. The variability patterns observed as the system runs over the attractor are compared with the variability of the indicators of the intrinsic complexity of the dynamics such as the local Lyapunov exponents, a local generalization of Kolmogorov entropy, and the linear velocity along the trajectory. The analysis reveals some unexpected correlations. In particular, it appears that the largest span of values of entropy production, including in many cases the maximum value itself, occurs at stages during which the dynamics is dominated by stability (local Kolmogorov entropy is close to zero). Strong correlations are also shown to exist between high dissipation and high values of the linear velocity.
The dynamics of the hydrogen exchange reaction at 2.20 eV collision energy: Comparison of experimental and theoretical differential cross sections
Authors: Wrede E. , Schnieder L. , Welge K. H. , Aoiz F. J. , Bañares L. , Castillo J. F. , Martı́nez-Haya B. , Herrero V. J.
Published: July 2002

The H+D2(v=0,j=0)→HD(v′,j′)+D isotopic variant of the hydrogen atom exchange reaction has been studied in a crossed molecular beam experiment at a collision energy of 2.20 eV. Kinetic energy spectra of the nascent D atoms were obtained by using the Rydberg atom time-of-flight technique. The extensive set of spectra collected has permitted the derivation of rovibrationally state-resolved differential cross sections in the center-of-mass frame for most of the internal states of the HD product molecules, allowing a direct comparison with theoretical predictions. Accurate 3D quantum mechanical calculations have been carried out on the refined version of the latest Boothroyd–Keogh–Martin–Peterson potential energy surface, yielding an excellent agreement with the experimentally determined differential cross sections. The comparison of the results from quasi-classical trajectory calculations on the same potential surface reveals some discrepancies with the measured data, but shows a good global accordance. The theoretical calculations demonstrate that, at this energy, reactive encounters are predominantly noncollinear and that collinear collisions lead mostly to nonreactive recrossing. The experimental results are satisfactorily accounted for by theoretical calculations without consideration of Geometric Phase effects.
Angular distributions of photoelectrons ejected from fixed-in-space molecules of C3v symmetry group
Authors: Kuznetsov V. V. , Cherepkov N. A. , Fecher G. H. , Schönhense G.
Published: July 2002

The angular distributions of photoelectrons from fixed-in-space molecules of C3v symmetry group are studied for the particular case when the light beam is incoming parallel to the C3 rotational axis of the molecule. The consideration is based on the expansion of photoelectron wave functions in symmetry-adapted spherical harmonics. The analytical equations for the angular distributions of photoelectrons ejected by linearly polarized, circularly polarized, and unpolarized light have been derived. The threefold symmetry of the target reveals itself most clearly for circularly polarized and unpolarized light when the dependence on the azimuthal angle is studied for a fixed polar angle. For linearly polarized light the photoelectron angular distributions have a more complicated dependence on the azimuthal angle. On the other hand, with linearly polarized light one can easily distinguish between the a1 and a2 initial states by observing the zeroes of photoelectron intensities in the symmetry plane ejected by light linearly polarized either in the symmetry plane or perpendicular to the symmetry plane. The numerical examples presented in the paper demonstrate the general properties of the angular distributions as well as properties of the circular and linear dichroism in the angular distribution.
Principal coordinate analysis on a protein model
Authors: Elmaci Nuran , Berry R. Stephen
Published: July 2002

A well-studied 46-bead protein model is the vehicle for examining principal coordinate analysis as a tool for interpreting topographies of complex potential surfaces. This study compares the effectiveness of several definitions of the comparison variable for revealing information about topographies. The extent of the information is ascertained by comparing the results of the various forms of principal coordinate analysis with results obtained from construction of interconnected monotonic sequences of linked stationary points (IMSLiSP) on the same surface. The conclusion is that the most powerful formulation of principal coordinate analyses for understanding protein folding and, in general, topographies of complex potentials, uses the changes in the set of interparticle distances as the definition of the comparison vector. However, even with this choice, the more efficient principal coordinate analysis is not able to reveal the extent of information contained in a more cumbersome IMSLiSP analysis.
Reconstruction of small Si cluster after ethylene adsorption: A full-potential linear-muffin-tin-orbital molecular-dynamics study
Authors: Qiu Min , Jiang Ming , Zhao Yu-Jun , Cao Pei-Lin
Published: July 2002

Using full-potential linear-muffin-tin-orbital method, we have performed molecular-dynamics simulations for the ethylene adsorption on the Si5–7-cluster surfaces. The calculations show that the most favored adsorption site is the short bridge site for Si5 cluster, with the adsorption energy 1.78 eV. The adsorption structure of ethylene molecule is similar to that of the dimer-maintained structure for C2H4+Si(100)-(2×1). It indicates that ethylene is di-σ bonded to the Si5 cluster. At the same time, the Si5 cluster reconstructs after ethylene adsorption. The three-center bond among side atoms breaks, and new bonds form. For Si6 cluster, the most favored adsorption site is the atop site on the side atom. After adsorption, Si6 cluster reconstructs from tetragonal bipyramid to edge-capped trigonal bipyramid. The short bridge is the only available site to adsorb ethylene for Si7 cluster.
Competing atomic and molecular hydrogen pathways in the photodissociation of methanol at 157 nm
Authors: Harich S. , Lin J. J. , Lee Y. T. , Yang X.
Published: July 2002

Photofragment translational spectra at m/e=1(H), 2(H2,D), 3(HD), and 4(D2) have been obtained for CH3OH, CH3OD, and CD3OH at 157 nm excitation. Analysis of the time-of-flight spectra reveals two different atomic H loss channels: hydroxyl H elimination, and methyl H elimination. While the hydroxyl H elimination seems to be a single fast process, the methyl H loss exhibits clearly two significantly different mechanisms: one fast and one slow. Experimental results also show two molecular hydrogen elimination channels: three-center elimination from the methyl group, which displays two different micropathways, and four-center elimination involving hydrogen atoms on both the C and O sites. The relative branching of the atomic versus molecular hydrogen elimination channels was found to be 1:0.15. These results present a uniquely clear picture of methanol photodissociation at 157 nm, and thus provide an excellent case for quantitative theoretical investigations.
Motion of interlamellar hydrated sodium ions in layered Cd0.75PS3Na0.5(H2O)2
Authors: Arun N. , Jeevanandam P. , Vasudevan S. , Ramanathan K. V.
Published: July 2002

The motion of hydrated Na ions in the galleries of layered Cd0.75PS3Na0.5(H2O)2 have been investigated by proton and Na23 NMR and impedance measurements as a function of temperature. The material is conducting and the frequency dependent conductivity exhibits a power law characteristic of non-Debye-like electrical conductivity relaxation, arising from Coulomb correlation among charge carriers. The NMR spectra indicates that the interlamellar Na ions are mobile and there is good agreement between the Na23 spin-lattice relaxation rates and the electrical conductivity relaxation. The proton NMR exhibits a well resolved orientation dependent dipolar splitting which has been rationalized by considering the anisotropic rotational motion of water molecules in the pseudo-octahedral hydration shell of the Na ion. Isotropically tumbling water as well as proton hopping among water molecules is absent in Cd0.75PS3Na0.5(H2O)2. Evidence is presented to show that within the galleries the Na ion moves along with its hydration shell, the motion being such that the orientation of the C⃗2 axis of the water molecule with respect to the normal to the layers is always preserved. The motion of the hydrated Na ions are correlated through their mutual Coulombic interaction but the rotational motion of the water molecule in the ions hydration shell may still be described by a single exponential Debye-like relaxation.
Structural, electronic, and bonding properties of liquid water from first principles
Authors: Silvestrelli Pier Luigi , Parrinello Michele
Published: July 2002

We study, from first principles, structural, electronic, and bonding properties of liquid water. Our system is twice as large as that used in previous ab initio simulations and our computed structural properties are in good agreement with the most recent neutron scattering experiments. Moreover, the use of a novel technique, based on the generation of maximally localized Wannier functions, allowed us to describe the molecular charge distribution and the polarization effects in liquid water with a degree of accuracy not previously possible. We find that, in the liquid phase, the water molecule dipole moment has a broad distribution around an average value of about 3.0 D. This value is 60% higher than that of the gas phase and significantly larger than most previous estimates. A considerable increase is also observed in the magnitude of the average eigenvalues of the quadrupole moment tensor. We also find that the anisotropy of the electronic charge distribution of the water molecule is reduced in the liquid. The relevance of these results for current modeling of liquid water is discussed.
Ground state potential energy curves for He2, Ne2, Ar2, He–Ne, He–Ar, and Ne–Ar: A coupled-cluster study
Authors: Cybulski Sławomir M. , Toczyłowski Rafał R.
Published: July 2002

Potential energy curves for three homonuclear (He2, Ne2, Ar2) and three heteronuclear (He–Ne, He–Ar, Ne–Ar) rare gas dimers are presented. The curves were calculated using several correlation consistent basis sets and the supermolecule single and double excitation coupled-cluster theory with noniterative perturbational treatment of triple excitations, CCSD(T). The most accurate results were obtained with the aug-cc-pV5Z basis set supplemented with an additional (3s3p2d2f1g) set of bond functions. The results obtained with a smaller aug-cc-pVQZ+(3s3p2d2f1g) basis set are almost as accurate. Both basis sets give results in better agreement with potentials based on experiments than the recent results obtained with larger d-aug-cc-pV6Z and t-aug-cc-pV6Z basis sets but without bond functions. For each complex and each basis set a fitted potential energy curve is given. In addition, for each complex, with the exception of He2, the values of Re, De, B0, D0, and 〈R〉0 are given. For He2 no bound states were found so only the values of Re and De are presented. For Ne2, Ar2, and Ne–Ar the calculated frequencies of vibrational and pure rotational transitions are shown to be in good agreement with the experimental results.
Solubilities of small molecules in polyethylene evaluated by a test-particle-insertion method
Authors: Fukuda Mitsuhiro
Published: July 2002

Free-energy related properties for penetrant sorption (water, methane, argon, oxygen, nitrogen, ethane, propane, and carbon dioxide) in amorphous polyethylene have been evaluated by a test-particle-insertion method with the excluded volume map sampling (EVMS). Two model systems, one a long chain model composed of 4 linear chains each having 1002 carbon atoms and another a short chain model composed of 167 molecules of C24H50, were compared in relation to the penetrant solubility at 298 K. The time-dependent molecular configurations were obtained by a molecular dynamics (MD) simulation under constant particle number, constant pressure, and constant temperature (NPT) conditions. The solubility coefficient obtained by the EVMS method was larger in the order, C3H8>C2H6>CO2>CH4>Ar>O2>N2, similar to the experimental results, and the absolute values were also in agreement with the experimental value. In the long chain host-molecules, reflecting the higher density, the solubility was about 0.85–0.90 times smaller for most penetrants tested, while ethane and propane showed higher solubility in contrast. Analysis of the averaged unoccupied volume fraction and their distribution in both systems revealed that big holes, in which a sphere with a diameter larger than 5.0 Å can be introduced, were formed in the long chain model irrespective of the denser structure. These findings resulted in higher solubility of a larger penetrant such as ethane and propane in the long chain matrix.