bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiclassical initial value representation for rotational degrees of freedom: The tunneling dynamics of HCl dimer
Authors: Sun Xiong , Miller William H.
Published: July 2002

The semiclassical initial value representation (SC-IVR) is emerging as a practical way of generalizing classical trajectory simulation methods to include (approximately) the effects of quantum mechanics (i.e., interference and tunneling). This paper describes the application of the SC-IVR approach to determine the low lying vibrational states of the HCl dimer on a realistic potential energy surface. Overall agreement of the semiclassical energy levels with accurate quantum values is very good, including a good description of the tunneling splitting in the ground state. Issues regarding the applications of the SC-IVR methodology to the angular variables related to rotational degrees of freedom are explicitly discussed.
Molecular dynamics simulations of stretched water: Local structure and spectral signatures
Authors: Parker M. E. , Heyes D. M.
Published: July 2002

Molecular dynamics simulations have been performed on a system of flexible extended simple point charge (SPC/E) model water molecules which include intramolecular stretching and bending terms. A series of densities, 0.70, 0.80, 0.90, and 1.00 g cm−3 was simulated at 298 K, thereby investigating the early stages of bulk water “stretching” leading to cavitation. The local structural changes were followed using atom-resolved pair radial distribution functions which revealed that the local water structure deforms inhomogeneously as the density decreases below 1.00 g  cm−3. Snapshots of the configurations and the radial distribution functions revealed that the decrease in density was accommodated in the sample by the appearance of “cavities” coexisting with clusters of more bulklike water but which were increasingly strained as the density decreased. Power spectra in the region of 0–5000 cm−1 were obtained from bond length, bond angle, and atom velocity autocorrelation functions. Combined with the power spectrum derived from the hydrogen atom velocity autocorrelation function, the vibrational frequencies of the gas and liquid states and the spectral features have been more clearly assigned. The power spectra are largely dominated by these near normal bulklike regions, which explains their observed insensitivity to density in the wide range studied. Another sequence of simulations was carried out at temperatures of 259, 273, and 350 K at a density of 1.00 g cm−3.
Comment on “Ionization potentials and electron affinities from the extended Koopmans’ theorem applied to energy derivative density matrices: The EKTMPn and EKTQCISD methods” [J. Chem. Phys. 107, 6804 (1997)]
Authors: Chen Edward S. , Chen Edward C. M. , Kozanecki N.
Published: July 2002

In the subject article, it is stated that “the EKTMP2 formalism is certain to replace the Koopmans’ theorem as the ‘black box’ estimator of ionization potentials” and by inference, for electron affinities. It is shown that semiempirical MINDO/3 multiconfiguration interaction ionization potentials for the five aromatic hydrocarbons considered in the article are as accurate as the EKT values. No theoretical EKT electron affinities are presented in the subject article. Experimental and theoretical electron affinities for a number of aromatic hydrocarbons calculated using MINDO/3 with MCCI are presented to be matched by any other estimator.
Evaluation of the many-body contributions to the interionic interactions in MgO
Authors: Rowley Adrian J. , J̈emmer Patrick , Wilson Mark , Madden Paul A.
Published: July 2002

The predictions of an “extended” ionic interaction model for the phonons and other properties of crystalline MgO are considered. In the extended model, the polarization interactions resulting from the dipoles and quadrupoles induced on the oxide ions by both Coulombic and short-range interactions with other ions are explicitly represented. Furthermore, the shapes of the oxide ions, as perceived through the short-range repulsive interactions with their immediate neighbors, are allowed to respond to their instantaneous coordination environment. Shape deformations of spherical (breathing), dipolar and quadrupolar symmetry are allowed. Each of these many-body effects is independently represented in the simulation model, so that the effect of each term on observed properties can be separately evaluated. It is shown how the polarization and breathing effects may be directly parametrized from ab initio electronic structure calculations. By consideration of the symmetry of the individual phonons at high symmetry points, it can be shown that the remaining effects may contribute only to particular phonons. Consequently, they may be assigned values in an unambiguous manner. The importance of each many-body effect for the different phonon branches is clearly delineated.
Vapor–liquid equilibria and heat effects of hydrogen fluoride from molecular simulation
Authors: Visco Donald P. , Kofke David A.
Published: July 2002

The vapor–liquid coexistence densities, vapor pressure, and heat of vaporization of hydrogen fluoride (HF) is calculated via Monte Carlo simulation from three intermolecular potential models that are found in the literature. The first is a pure pair potential based solely on ab initio data, the second is a semi-empirical pair potential which uses an ab initio derived surface fitted with dimer spectroscopic data, and the third is an effective pair potential that was fit to experimental data for the condensed phase. As expected, the effective potential reproduces the saturated liquid densities more accurately than the others do, while all the potential models predict the wrong slope and curvature in the vapor pressure curve. The inability to reproduce the vapor pressure dependence on temperature is connected to the models’ poor prediction of the heat of vaporization at temperatures below 400 K. A biasing algorithm is introduced to study the superheated-vapor heat capacity, density, association number, and oligomer distribution along three low-pressure isobars using both the semi-empirical and effective pair potentials. It is found that both these potential models do predict a peak in the heat capacity, however, they are at cooler temperatures and only about half the magnitude relative to the experiment. When comparing the potential models to each other, it is found that the semi-empirical pair potential predicts the onset of near-ideal gas conditions at about 30 K cooler than the effective pair potential. Additionally, the percentage of ring oligomers predicted by both models is considerable at all but the highest temperatures. Both models also agree that the monomer and cyclic tetramer are the two most important species at the nonideal states.
Compression of klockmannite, CuSe
Authors: Peiris Suhithi M. , Pearson Tania T. , Heinz Dion L.
Published: July 2002

Copper selenide (CuSe) was compressed in a diamond anvil cell at room temperature up to a pressure of 52 GPa and studied using energy dispersive x-ray diffraction and Raman spectroscopy. CuSe is nearly isostructural with copper sulfide (CuS), and a previous study indicates that copper sulfide undergoes reversible pressure-induced amorphization at 18 GPa. The intensity of the x-ray diffraction peaks for CuSe decrease slowly, however, they never completely disappear up to a pressure of 52 GPa. The third-order Birch–Murnaghan equation of state fit to the data yields K0=96.9±5.3 GPa and K0′=4.1±0.5. Vinet’s universal equation of state yields essentially identical parameters. Raman spectroscopy demonstrates that upon compression, the S–S bond in CuS compresses differently than the Se–Se bond in CuSe, possibly accounting for the different high pressure behavior of these two very similar compounds.
RG+Cl
(
2
P)
(RG=He, Ne, Ar) interactions: Ab initio potentials and collision properties
Authors: Burcl R. , Krems R. V. , Buchachenko A. A. , Szczȩśniak M. M. , Chal/asiński G. , Cybulski S. M.
Published: July 2002

The lowest states of Σ and Π symmetry of Rg⋯Cl (Rg=He, Ne, Ar) complexes were investigated using the coupled cluster approach with single, double, and noniterative triple excitations (CCSD(T)) in an extended basis set including bond functions. The Σ states possess deeper minima at shorter interatomic distances than the corresponding Π states. The Σ-Π splittings, which for He⋯Cl and Ar⋯Cl are significantly larger than previously deduced, are mainly due to differences in the exchange repulsion terms. The total energies were dissected into electrostatic, exchange, induction, and dispersion components. The calculated potentials have been used in the calculations of collision properties of Rg⋯Cl systems. Absolute total cross sections, spin-orbit quenching rate constants, and diffusion coefficients were evaluated from both the ab initio and previously available empirical potentials. The performance of ab initio potentials in these calculations proved to be very reasonable.
Dynamical structure of water in NaCl aqueous solution
Authors: Mizoguchi Kohji , Ujike Tomoko , Tominaga Yasunori
Published: July 2002

We report on the dynamical structure of water in NaCl aqueous solutions as functions of temperature and concentration by a low-frequency Raman spectroscopy. The spectral profiles in the frequency range from −50 to 250 cm−1 have been analyzed with a superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes. The distributions of the characteristic frequencies of the intermolecular vibrational modes observed around 50 and 180 cm−1 in NaCl aqueous solutions are always wider than those in pure water. In NaCl aqueous solutions with various concentrations, the spectral line width g1=1/(2πcτ) of a central component, which corresponds to the reciprocal relaxation time, linearly changes with temperature from about 255 K to 300 K, while the relaxation time above 300 K holds an Arrhenius-type behavior. The slope of the spectral linewidth against temperature below 300 K decreases with increasing concentration. The distribution parameter of the relaxation time in the Cole–Cole type relaxation formula decreases with decreasing temperature and it becomes smaller with increasing concentration.
Temperature-dependent infrared spectroscopic and crystallographic study of α,α′-dimethyl oligothiophenes in neutral form
Authors: Ramı́rez F. J. , Aranda M. A. G. , Hernández V. , Casado J. , Hotta S. , López Navarrete J. T.
Published: July 2002

We have studied structural and spectroscopic characteristics on a series of α,α′-dimethyl end-capped oligothiophenes (from two to six thiophene units) by varying temperature. Powder x-ray diffraction and infrared spectra at different temperatures between −170 and 150 °C were recorded. The diffractometric study shows that the thermal expansion coefficients of these materials are highly anisotropic. The expansion in the ab plane, which is perpendicular to the chain direction, is an order of magnitude bigger than that along the chain direction. Corresponding to these structural features, the infrared spectra show interesting temperature dependence. For instance, the separation between adjacent components of the multiplets near 800 cm−1, that are assigned to the C–H out-of-plane bending vibrations, increases with decreasing temperatures. This behavior reflects the change in the intermolecular forces caused by the thermal expansion. Furthermore, the shifts in the C=C stretching vibrations indicate that the extent of the π-electron delocalization increases with increasing temperatures. The origin for this is briefly discussed.
Nonadiabatic effects in the photodissociation of vibrationally excited HNCO: The branching between singlet (a 1Δ) and triplet (X 3Σ−) NH
Authors: Berghout H. Laine , Brown Steven S. , Delgado Ruben , Fleming Crim F.
Published: July 2002

Initial vibrational excitation of a state containing three quanta of N–H stretch (3ν1) decreases the fractional photolysis yield of NH (a 1Δ) relative to NH (X 3Σ−) by a factor of approximately two compared to the isoenergetic photodissociation of a 300 K thermal sample of HNCO. At a total energy of 43 480 cm−1, NH (a 1Δ) accounts for 24% of the total NH yield in the direct photolysis but only 10% in the photodissociation of 3ν1. At 44 440 cm−1, the NH (a 1Δ) yields are 65% and 32% in the single photon and two-step photodissociations, respectively. The variation in branching ratio may arise from dynamical behavior that is closely related to the preferential production of NCO in the photolysis of vibrationally excited HNCO. The initial vibrational excitation has no influence on the rotational and vibrational distributions of NH (X 3Σ−), but it significantly increases the amount of energy in rotation of NH (a 1Δ). These results, along with several recent experimental and theoretical studies, suggest the participation of at least three different potential energy surfaces in the photodissociation of isocyanic acid.
On the use of the quasi-Gaussian entropy theory in noncanonical ensembles. II. Prediction of density dependence of thermodynamic properties
Authors: Apol M. E. F. , Amadei A. , Berendsen H. J. C.
Published: July 2002

In previous articles we derived and tested the quasi-Gaussian entropy theory, a description of the excess free energy in terms of the potential or full internal energy or enthalpy probability distribution, instead of the (configurational) partition function. We obtained in this way the temperature dependence of thermodynamic functions in the NVT, NpT and μVT ensembles assuming a Gaussian, Gamma or Inverse Gaussian distribution. In this article we extend the theory to describe the density dependence of thermodynamic properties, using the distribution of volume and number of particles in the isothermal-isobaric and grand canonical ensemble, respectively. In both ensembles pressure-density expressions for a Gaussian and various Gamma distributions are derived and applied to water. A Gamma description for the volume distribution turns out to be a good model in the gas range, which is in accordance with the volume distribution of an ideal gas. A Gamma description for the particle number distribution works well for liquid densities.
A variational Monte Carlo study of the 2s-2p near degeneracy in beryllium, boron, and carbon atoms
Authors: Sarsa A. , Gálvez F. J. , Buendía E.
Published: July 2002

We apply the variational Monte Carlo method to study the beryllium, boron, and carbon atoms. An explicitly correlated wave function is used in order to include the dynamic correlation among the electrons. The nondynamic correlation due to the 2s-2p near degeneracy effect present in these atoms is taken into account by using a multideterminant wave function.
Theory for the conformational changes of double-stranded chain molecules
Authors: Chen Shi-Jie , Dill Ken A.
Published: July 2002

We develop statistical mechanical theory to predict the thermodynamic properties of chain molecules having noncovalent double-stranded conformations, as in RNA molecules and β-sheets in proteins. Sequence dependence and excluded volume interactions are explicitly taken into account. We classify conformations by their polymer graphs and enumerate all the conformations corresponding to each graph by a recently developed matrix method [S-J. Chen and K. A. Dill, J. Chem. Phys. 103, 5802 (1995)]. All such graphs are summed by a recursive method. Tests against exact computer enumeration for short chains on a 2D lattice show that the density of states and partition function are given quite accurately. So far, we have explored two classes of conformations; hairpins, which model small β-sheets, and RNA secondary structures. The main folding transition is predicted to be quite different for these two conformational classes: the hairpin transition is two-state while the RNA secondary structure transition is one-state for homopolymeric chains.
Kinetics of a dimer–dimer irreversible catalytic surface reaction
Authors: Khan K. M. , Yaldram K. , Ahmad N.
Published: July 2002

The kinetics of irreversible dimer–dimer surface reactions of the type A2+B2→2AB with desorption of dimer B2 has been investigated by means of Monte Carlo simulation. For a desorption probability (P) of B2 equal to zero, a single discontinuous transition separating an A+vacancies saturated surface from a B+vacancies saturated surface is obtained at yB2=0.5 (yB2 is the partial pressure of B2 in the gas phase). With the increase in P, a steady reactive state which is separated from the poisoned states by two continuous transitions is obtained for this system. The position of the transition points depends upon the value of P. For P equal to one, a single continuous transition at yB2=0.538 separates a poisoned state from a steady reactive state. The window width, which defines the steady reactive state, shows an exponential behavior with P. Preliminary studies have been under taken to determine the universality class of the system for different P values.
Comment on “Atomic stress isobaric scaling for systems subjected to holonomic constraints” [J. Chem. Phys. 106, 195 (1997)]
Authors: Kneller G. R. , Mülders T.
Published: July 2002

We show that the molecular dynamics simulation scheme for geometrically constrained systems at constant pressure proposed by Melchionna and Ciccotti [J. Chem. Phys. 106, 195 (1997)] is not consistent with the desired isobaric ensemble. The extended system method necessitates to impose the standard geometrical constraints for the physical momenta explicitly. A solution to this problem has been recently presented by Kneller and Mülders [Phys. Rev. E, 54, 6825 (1996)].
Pump–probe spectra of linear molecular aggregates: Effect of exciton–exciton interaction and higher molecular levels
Authors: Juzeliūnas Gediminas , Reineker Peter
Published: July 2002

The paper considers the differential pump–probe spectra due to excitons in linear molecular aggregates taking into account simultaneously effects of both exciton–exciton interaction and higher molecular levels. The theoretical analysis, carried out in terms of the Green function technique, provides analytical expressions for the line shape of the pump–probe spectrum valid for an arbitrary number N of molecules forming the aggregate. Furthermore, the theory can accommodate any number of molecular states with higher energies. This includes, inter alia, the most common situation in which the higher lying states form a dense set of sublevels of electronic, vibrational, etc. origin. It has been demonstrated that incorporation of such higher molecular levels introduces widths to biexciton peaks formed below the two-exciton continuum. In addition, the indirect interaction between the excitons via the higher molecular levels can facilitate formation of a biexciton at lower than usual values of the direct exciton–exciton coupling γ, in extreme cases even for negative γ values characterizing repulsion rather than attraction between the excitons. On the other hand, in the region around the exciton band-edge, the differential spectrum can be described reasonably well in terms of the model of noninteracting excitons for a wide range of parameters of the system, subject to the replacement of an actual number of molecules per aggregate N by the effective one Neff. The latter Neff is shown to be influenced both by the direct coupling between the excitons and also by the indirect coupling via the higher molecular levels.
Application of the quasi-Gaussian entropy theory to molecular dynamics simulations of Lennard-Jones fluids
Authors: Roccatano D. , Amadei A. , Apol M. E. F. , Di Nola A. , Berendsen H. J. C.
Published: July 2002

The quasi-Gaussian entropy theory has been applied to reproduce the temperature dependence of the internal energy, pressure and isochoric heat capacity of a molecular dynamics simulated Lennard-Jones (LJ) fluid at density ρ=1 (reduced units). The results show that the gamma state level of the theory is an excellent approximation, able to predict the behavior of these properties over a large temperature range. This application of the theory to the simulated LJ fluid confirms previous results, obtained using experimental fluid data, and shows that the gamma state level of the theory, in combination with molecular simulation techniques, can be used as a general model to obtain accurate and physically consistent equations of state for fluid systems.
Slow-motion theory of nuclear spin relaxation in paramagnetic complexes (S=1) of arbitrary symmetry
Authors: Nilsson T. , Svoboda J. , Westlund P.-O. , Kowalewski J.
Published: July 2002

A generalization of the slow-motion theory of nuclear spin relaxation in paramagnetic systems (S=1) is developed. The new model takes into account the effects of rhombic symmetry in the static zero-field splitting tensor. We also allow the principal axis system of the static zero-field splitting tensor to deviate from the molecule-fixed frame of the dipole–dipole tensor between the nuclear and electron spins. These symmetry-breaking properties have profound effects on the nuclear spin–lattice relaxation rate for some cases. Specifically, the relaxivity is reduced substantially at low magnetic field. Nuclear magnetic relaxation dispersion profiles for a large number of cases are discussed, ranging from slightly asymmetric [low static zero-field splitting (ZFS)] weakly deformable (low transient ZFS) to asymmetric (large static ZFS) highly deformable (large transient ZFS) transition-metal complexes. The dynamical regimes covered for the electron spin range from within the Redfield limit into the slow-motion region. One of the main objectives of this investigation is to provide a standard set of essentially exact calculations using the general slow-motion theory, against which simplified models may be tested.
A comparison of exact quantum mechanical and various semiclassical treatments for the vibronic absorption spectrum: The case of fast vibrational relaxation
Authors: Rabani Eran , Egorov S. A. , Berne B. J.
Published: July 2002

We have extended our study of the vibronic absorption spectrum in condensed matter [S. A. Egorov, E. Rabani, and B. J. Berne, J. Chem. Phys. 108, 1407 (1998)] to the case when the electronic dephasing rate is slow compared to the vibrational relaxation rate in both electronic states. We find that under such circumstances, unlike the case of fast electronic dephasing, treating all nuclear degrees of freedom classically provides better agreement with the exact quantum treatment than the mixed quantum-classical approximation. These results are consistent with the conclusions reached by Bader and Berne in their study of mixed quantum-classical treatments of vibrational relaxation processes.
Exact thermodynamic formulation of chemical association
Authors: Fisher Michael E. , Zuckerman Daniel M.
Published: July 2002

The thermodynamics of interacting systems of two species of particles, A and B, may be specified in “physical” terms using only the two densities ρa and ρb or, alternatively, in a “chemical picture” using three densities ρa, ρb, and ρc related by a mass-action law corresponding to the “reaction” A+B⇌C, where C denotes a “compound,” “complex,” “cluster,” or “associated pair.” We present exact methods for generating associative or “chemical” thermodynamics from an arbitrary physical specification. Both explicit order-by-order matching conditions and a variety of thermodynamically stable, closed-form solutions are derived. The analysis elucidates precisely the freedom available to choose the association constant, the definition of a cluster, and the interactions of a cluster (or “pair”) with other clusters and with unpaired (i.e., dissociated, or “free”) species. A single-species system of, say, X particles described chemically by 2X⇌Y is analyzed similarly. Various examples, including purely hard-core liquids and the van der Waals fluid, demonstrate applications of the theory, which should aid in improving approximate treatments for electrolytes and other systems. The precautions necessary in selecting a physically acceptable association constant are discussed.
Protein folding and models of dynamics on the lattice
Authors: Hoang Trinh Xuan , Cieplak Marek
Published: July 2002

We study folding in 16-monomer heteropolymers on the square lattice. For a given sequence, thermodynamic properties and stability of the native state are unique. However, the kinetics of folding depends on the model of dynamics adopted for the time evolution of the system. We consider three such models: Rouse-like dynamics with either single monomer moves or with single and double monomer moves, and the “slithering snake” dynamics. Usually, the snake dynamics has poorer folding properties compared to the Rouse-like dynamics, but examples of opposite behavior can also be found. This behavior relates to which conformations act as local energy minima when their stability is checked against the moves of a particular dynamics. A characteristic temperature related to the combined probability, PL, to stay in the non-native minima during folding coincides with the temperature of the fastest folding. Studies of PL yield an easy numerical way to determine conditions of the optimal folding.
Comment on “Polarization effects in time resolved incoherent anti-Stokes Raman spectroscopy” [J. Chem. Phys. 105, 6141 (1996)]
Authors: Mizutani Yasuhisa
Published: July 2002

The theoretical inspection on anti-Stokes Raman measurements following strong vibrational excitation by Hofmann et al. [M. Hofmann, R. Zürl, and H. Graener, J. Chem. Phys. 105, 6141 (1996)] was reexamined. It is found that the depolarized intensity is dependent on the incident angle of the pump pulse in the case of perpendicularly polarized pumping. Therefore, one can observe “rotation-free” depolarized scattering only with some particular incident angle, which was not described in their paper. In addition to that, it is suggested to use the definition of depolarization ratio for polarized probe light instead of that for unpolarized light, since currently laser is a light source for this kind of experiments.
Excited-state reversible geminate reaction. I. Two different lifetimes
Authors: Gopich Irina V. , Solntsev Kyril M. , Agmon Noam
Published: July 2002

We obtain an accurate long-time solution for the diffusive kinetics of an excited geminate pair dissociating reversibly in solution, where the excited-state lifetimes of the bound and unbound states may be nonequal. We analyze the long-time asymptotic behavior, finding a transition between two types of behaviors. In region A, the excited bound pair decays as t−3/2 exp(−t/τ0′), where τ0′ is the lifetime of the unbound state. In region AB, the decay is a pure exponential. At the critical value of the parameters where the transition occurs, we find a t−1/2 exp(−t/τ0′) decay. These predictions are tested against an exact numerical solution of the corresponding coupled partial differential equations. For region A, we supply an experimental example involving time-resolved excited-state proton transfer from 5-cyano-2-naphthol to dimethyl sulfoxide.
Nitrate ion detection in aerosols using morphology-dependent stimulated Raman scattering
Authors: Aker Pamela M. , Zhang Jian-Xiang , Nichols William
Published: July 2002

A nitrate ion concentration of 5×10−5 M has been detected in ∼180 μm diam aqueous aerosols using morphology-dependent stimulated Raman scattering (MDSRS). This low concentration was detected by allowing the droplet size to be tuned during an experiment. Comparison of the experimental results with the MDSRS gain equation shows that it may be possible to detect concentrations a factor of ten lower.
Gallium compounds, a possible problem for the G2 approaches
Authors: Bauschlicher Charles W. , Melius Carl F. , Allendorf Mark D.
Published: July 2002

The G2 atomization energies of fluorine and oxygen containing Ga compounds are greatly in error. This arises from an inversion of the Ga 3d core orbital and the F 2s or O 2s valence orbitals. Adding the Ga 3d orbital to the correlation treatment or removing the F 2s orbitals from the correlation treatment are shown to eliminate the problem. Removing the O 2s orbital from the correlation treatment reduces the error, but it can still be more than 6 kcal/mol. It is concluded that the experimental atomization energy of GaF2 is too large.
High resolution electronic spectroscopy of Kr⋅OH/D and an empirical potential energy surface
Authors: Carter Christopher C. , Miller Terry A. , Lee Hee-Seung , Korambath Prakashan P. , McCoy Anne B. , Hayes Edward F.
Published: July 2002

The high resolution laser-induced fluorescence spectra of the Kr⋅OH van der Waals complex and its deuterated analog are reported. The rotational analysis provides information about the observed fine, hyperfine, spin-rotation, and parity interactions in these two complexes. The molecular parameters allow a direct comparison with previously reported results on the analogous Kr⋅SH/D complexes. Additionally, lower resolution scans have revealed vibronic bands that have not been reported in the previous work of Fei, Zheng, and Heaven [J. Chem. Phys. 97, 1655 (1992)], while high resolution results of the Kr·OH/D86,Kr·OH/D84, and Kr·OH/D82 isomers confirmed the previous vibrational quantum number assignment. The results of the high resolution analysis are used in conjunction with measured vibrational intervals to develop an empirical potential energy surface for Kr⋅OH/D. This is compared to the recently reported potentials by Korambath et al. [J. Chem. Phys. 107, 3460 (1997)] for the other R⋅SH/D (R=Ar, Kr) complexes.
Binary nucleation kinetics. V. Φ lines and evaporation rate surfaces
Authors: Wyslouzil Barbara E. , Wilemski Gerald
Published: July 2002

We explore the quasiuniversal behavior of the function Φ(i,j,t)=f(i,j,t)/N(i,j,t) in binary nucleation, where f(i,j,t) and N(i,j,t) are the nonequilibrium and equilibrium cluster concentrations, respectively. The simple, regular patterns that are formed by this function during both the transient period and at steady state suggest that the contour lines of constant Φ form one half of a natural curvilinear coordinate system that underlies the binary nucleation process. In this paper we present the Φ-line patterns for binary systems that display a wide range of liquid phase nonideality. Quantitative comparisons between analytical expressions for the angle that ∇Φ makes with the component A axis and for the spacing of the Φ contour lines give good agreement with the values derived from the numerical solution of the binary kinetics equations. The insensitivity of the Φ-line patterns to changes in the gas phase activities of the nucleating species can be better understood by writing the binary kinetics equations with the evaporation rate coefficients as the “diffusion coefficients.” In this form it is easy to see that the equations only depend weakly on the actual gas phase compositions.
Structural and vibrational properties of carbon monoxide adlayers on the copper (001) surface
Authors: Lewis Steven P. , Rappe Andrew M.
Published: July 2002

The structure and vibrational states of a prototype adsorbate-substrate system—carbon monoxide on the copper (001) surface—have been calculated from first principles within local density functional theory. Three CO coverages have been examined: θ=0 (bare surface), 0.5, and 1. These systems are represented by a well converged slab model within which all atomic degrees of freedom are treated on an equal footing. The computed structural relaxations and vibrational frequencies are generally in excellent quantitative agreement with the available experimental measurements. The full monolayer is found to be energetically favorable to the half monolayer plus free CO molecule. This indicates that the maximum stable coverage is greater than θ=0.5, in agreement with experiment. The vibrational analysis reveals that resonant coupling between adsorbate and substrate motions has a profound effect on the vibrational spectra, for example, the low-frequency, in-plane frustrated translational motion of the CO molecules mixes with long-wavelength copper phonons to form a broad resonance peak. This implies a finite lifetime which, for the half-monolayer system, is computed to be 3.0 ps, in excellent agreement with the measured value of 2.3±0.4 ps. For the full-monolayer system, the predicted lifetime is 0.7 ps; however this system is presently inaccessible to experiment. Resonant coupling is also found to affect the Rayleigh wave of the copper (001) surface. At half-monolayer CO coverage, this mode resonantly mixes with bulk copper phonons developing a finite lifetime, which is predicted to be 5.2 ps. To our knowledge, the lifetime of this mode has never been measured. For the fully covered surface, the Rayleigh wave does not form a resonance because the phonon coupling is forbidden by symmetry.
Polyelectrolyte solutions with stickiness between polyions and counterions
Authors: Jiang Jianwen , Liu Honglai , Hu Ying
Published: July 2002

Polyelectrolyte solutions are modeled as linear tangent-jointed charged hard-sphere chains and counterions embedded in a continuum medium; the stickiness between the polyions and the counterions beyond the territorial binding is taken into account as the short-range non-Coulombic perturbation. Expressions of thermodynamic properties derived are then consisted of two contributions. The chain contribution concerns the formation of polyion chains from the monomers. The sticky contribution accounts for the additional non-Coulombic stickiness, which is derived by solving the Ornstein–Zernike integral equation through the mean-spherical approximation (MSA) and the hypernetted-chain approximation (HNC). For the model solutions without stickiness between the polyions and the counterions, effects of polyion chain lengths, counterion sizes, counterion charges, and the dielectric constants on the thermodynamic properties are extensively studied. Comparisons of the osmotic pressures for the model solutions with those predicted from Manning’s limiting law (LL), and where possible, with computer simulations are presented. For practical synthetic polyelectrolyte solutions, involving an adjustable parameter responsible for the additional stickiness, experimental thermodynamic properties can be described satisfactorily.
Thermal isomerization in isolated cesium–halide cluster anions
Authors: Fatemi F. K. , Fatemi D. J. , Bloomfield L. A.
Published: July 2002

We have used photoelectron spectroscopy to observe thermal isomerization in cesium–halide cluster anions. In many of the (CsX)nCsm− (X=Cl, Br, I; n=2–7; m=0, 1) systems we have studied, small changes in the source nozzle temperature produce dramatic changes in the distributions of cluster isomers. There is also evidence that the isomer distributions are in dynamic equilibrium, with their temperatures controlling the rates and probabilities of isomerization processes. We present data and discussion of several cluster families that exhibit this behavior.
Photodynamic control using field optimized initial state: A mechanistic investigation of selective control with application to IBr and HI photodissociation
Authors: Vandana K. , Mishra Manoj K.
Published: July 2002

The probability density profiles from the optimal superpositions of the field free vibrational eigenstates which maximize flux out of the desired photodissociation channels are examined for IBr and HI molecules. Analysis of the structure in these optimal superposition states obtained by applying the Rayleigh–Ritz variational procedure to the time integrated flux operator shows that the transfer of probability density to appropriate areas of the Franck–Condon region on the excited surfaces is responsible for selective flux maximization out of different channels. Localizing the wave packet on the more repulsive part of the higher curve facilitates fast diabatic exit out of the upper channel and transition to the less repulsive part promotes slow adiabatic exit out of the lower channel. This mechanism is further probed by utilizing time dependent wave packet dynamics to obtain absorption spectra and branching ratios using full Fourier transform of the autocorrelation functions for these field optimized initial states. The results corroborate the central role of altered spatial profile of the initial state in selective control of photodissociation.
Measurement of the dissociation energies of anionic silver clusters (Agn−, n=2–11) by collision-induced dissociation
Authors: Spasov Vassil A. , Lee Taeck Hong , Maberry John P. , Ervin Kent M.
Published: July 2002

The energy-resolved collision-induced dissociation method is applied to measure the fragmentation patterns, cross sections, and dissociation energies of small anionic silver clusters (Agn−,n=2–11). The main reaction channels are found to be loss of atom and loss of dimer, with dimer loss favored for odd n values. The dissociation energies for the loss of atom, D0(Agn−1−–Ag), show strong even–odd alternation. Threshold models that account for collisional activation efficiency, kinetic shifts, and competitive shifts are employed to obtain dissociation energies. A critical examination of the models is performed using a thermochemical cycle comparing sequential atom loss with dimer loss.
Estimation of the quadrupole and hexadecapole moments of N2 from the far-infrared spectrum of a N2–Xe gaseous mixture
Authors: Roco J. M. M. , Calvo Hernández A. , Velasco S. , Medina A.
Published: July 2002

A previously reported quantum spectral theory for evaluating the far-infrared absorption spectrum of diatomic molecules in rare-gas fluids [J. Chem. Phys. 103, 9161 (1995)] is applied to the case of a N2–Xe gaseous mixture at room temperature and a very low concentration of N2 (0.85%). The theoretical line shape is built by considering the two leading electric multipolar induced contributions. From comparison between theoretical and experimental spectra an estimation of the quadrupole and hexadecapole moments of N2 is given.
Intramolecular dynamics diffusion theory approach to complex unimolecular reactions
Authors: Guo Yin , Shalashilin Dmitrii V. , Krouse Justin A. , Thompson Donald L.
Published: July 2002

A further development of the intramolecular dynamics diffusion theory (IDDT) [J. Chem. Phys. 107, 6204 (1997)] for computing unimolecular reaction rate constants in the IVR-controlled regime is described. The approach is based on Kramers’ energy diffusion theory, with the reaction coordinate taken as the subsystem and the rest of the vibrational modes as the bath. The method provides a practical means of obtaining the rate constants in the IVR-controlled regime at considerable savings of computer time compared to the usual classical trajectory simulations. Its accuracy has been demonstrated in our earlier applications to some simple bond-fission reactions. In the study described here the idea of intrinsic reaction coordinate (IRC) is used to extend the IDDT approach to more complicated systems for which simple reaction coordinates are not easily identifiable. The basic idea is to take the IRC as the subsystem and the transverse vibrational modes as the bath. The method is applied to the unimolecular dissociation of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and the rate constants calculated using IDDT are in good agreement with classical trajectory simulations over a wide range of energies, suggesting that the approach may be generally applicable to large polyatomic systems.
Correlated two-electron momentum properties for helium to neon atoms
Authors: Sarsa A. , Gálvez F. J. , Buendı́a E.
Published: July 2002

Two-electron properties in momentum space for the atoms helium to neon have been calculated starting from explicitly correlated wave functions. The different integrals involved in the calculation have been evaluated by using the Monte Carlo algorithm. In particular, the spherically averaged interelectronic momentum distribution, γ(2)(p12),its radial moments 〈p12n〉, with n=−2 to +3, the expectation value 〈p1⋅p2〉, and both the electron–electron coalescence, γ(2)(0), and counterbalance, Γ(2)(0), densities have been calculated. A systematic study of the electronic correlation has been performed by comparing the correlated results with the corresponding Hartree–Fock ones. Finally an analysis of the structure of the interelectronic momentum distribution in terms of its parallel and antiparallel components has been carried out.
Direction of the steady state nucleation flux in the whole size space for binary systems
Authors: Li Jin-Song , Nishioka Kazumi
Published: July 2002

For steady state binary nucleation, the directions of the nucleation flux and the nucleation force are obtained in the whole size space. The derivation of the directions is made possible by introducing a local coordinate axis system along the streamlines of the nucleation-force vector and by employing the assumption of local uniformity for the direction of the streamlines. For an ideal hexanol-ethanol system, the results predicted by our theory agree with the numerical results. For a specific nonideal system (PD2), our theory fails to apply to the region where our assumption is posteriorly invalid. In this case it may be necessary to consider the variation of the direction of the force.
A density functional study of liquid–liquid interfaces in partially miscible systems
Authors: Napari Ismo , Laaksonen Ari , Talanquer Vicente , Oxtoby David W.
Published: July 2002

Liquid–liquid interfaces and nucleation in partially miscible Lennard–Jones (LJ) mixtures are considered using density functional theory. We present phase diagrams, interfacial liquid–vapor and liquid–liquid profiles, and gas–liquid as well as liquid–liquid surface tensions for two types of mixtures having different mixing rules for the LJ energy parameter. A simple local density approximation does not give oscillatory behavior at the liquid–liquid interface, but a more realistic weighted density approximation does show this behavior. Both approaches also give a total density minimum near the interface, comparable to that found in molecular dynamics and integral equation studies. Finally, we calculate the density profiles and free energies for critical nuclei in liquid–liquid phase separation.
An interatomic potential for reactive ion etching of Si by Cl ions
Authors: Hanson D. E. , Kress J. D. , Voter A. F.
Published: July 2002

An interatomic potential has been developed to describe the dynamics of Si/Cl systems, with particular relevance to reactive ion etching of Si by energetic Cl ions. We have modified the Stillinger–Weber (SW) potential of Feil et al. by adding two new terms: (1) an embedding term that corrects for the variation in Si–Cl bond strength as a function of the number of neighbors, and (2) a four-body term to describe the variation of the Si–Si bond strength as a function of the number of neighbors of each Si atom and the atom types (a bond order correction). Calculated Si etch rates obtained from molecular dynamics simulations using the new potential are in better agreement with recent experimental results than those obtained with the unmodified potential. Predictions of the stoichiometry of the etch products are also markedly different between the two potentials.
Valence correlation bands of model oligomers of polyethylene: A Green’s function study by the band-Lanczos approach
Authors: Golod A. , Deleuze M. S. , Cederbaum L. S.
Published: July 2002

The innermost valence electron levels of large molecular chains are subject to a strong breakdown of the orbital picture of ionization into particularly dense sets of shake-up lines with exceedingly small intensities, which give rise to correlation bands in the ionization spectrum. A suitable algorithm is required for a size-consistent investigation of such sets. In this work, we report the inclusion of the band-Lanczos approach in the final diagonalization step of a one-electron Green’s function calculation in the algebraic diagrammatic construction scheme. From a first application to a series of oligomers CnH2n+2 (n=2–9) converging to polyethylene, the C2s correlation bands of large saturated hydrocarbons are shown to extend beyond 60 eV, and to represent, regardless of cross section effects, at least 20% of the total valence ionization intensity, and about 60% of the intensity found in the inner valence region [εb=18–70 eV]. With the band-Lanczos approach, the distribution of spectral intensity is nearly entirely recovered, with the exception of a tiny violation (2% for the n-nonane compound).
Modeling the Monte Carlo simulation of associating fluids
Authors: Visco Donald P. , Kofke David A.
Published: July 2002

We model the Monte Carlo simulation of a simple associating system to understand and quantify the ability of the simulation to provide measurements of the energy and heat capacity. The simulation is examined for its convergence to the equilibrium degree of association, and separately for the precision of its measurements. The molecular model used for the study was proposed by van Roij; it is very simple, and allows the formation of only chain-like association structures while exhibiting only association and no repulsion or dispersion interactions. However, the model captures the essential features that make the simulation of associating systems difficult, and it yields to various types of analysis. Unbiased and biased simulation methods are studied. Modeling is facilitated by coarse-graining the system in terms of the total number of association bonds. A bond-balance kinetic model is used to examine the convergence behavior of the simulation. An unexpected finding is that the strength of association is the sole determinant of the rate of convergence, and that the size (volume) of the association sites does not separately impact the convergence rate. The precision of the calculation is quantified via a variance that is obtained from a Markov model of the simulation. We present contours that quantify the precision to be expected in the energy and the heat capacity, from which one can estimate the confidence limits to be expected when simulating a particular system at a given state with a given number of Monte Carlo trials. The results apply rigorously only to the model used in the study, but they should prove useful in gauging the difficulty to be expected in simulating realistic model systems. The biasing Monte Carlo algorithm offers dramatic improvement in both convergence and precision when simulating strongly associating systems, but it is not particularly helpful for weakly associating systems. The analysis also uncovers a finite-size effect that is manifested when measuring the heat capacity in strongly associating systems.
Hydrogen bonding and cooperative effects in mixed dimers and trimers of methanol and trifluoromethanol: An ab initio study
Authors: Parra Rubén D. , Zeng X. C.
Published: July 2002

Ab initio calculations are used to provide information on the mixed dimers and cyclic trimers of the methanol-trifluoromethanol system. In order to better understand the systems, the monomers and their corresponding dimers and trimers are also investigated. Molecular structures and harmonic frequencies are obtained at the B3LYP/6-311++G(d,p) level. Interaction energies are calculated with the MP2 and B3LYP methods using the 6-311++G(d,p), 6-311++G(2d,2p), and 6-311++G(3d,2p) basis sets for the dimers and heterodimers. The 6-311++G(d,p) basis set was used to calculate the interaction energies for the trimers and heterotrimers. Because the primary goal of this study is to examine cooperative effects, particular attention is given to parameters such as O…O distances, electronic charge densities at the bond critical points, enhanced dipole moments, shifts in the stretching frequencies of the donor O–H bond, and the length of the donor O–H bond.