bluuebunny/crossref_metadata_2025 huggingface dataset:
Argon metastables in a high density processing plasma
Authors: Leonhardt D. , Eddy C. R. , Shamamian V. A. , Fernsler R. F. , Butler J. E.
Published: July 2002

Absolute densities of metastable argon atoms (Paschen 1s5, 1s3) and the intermediate resonant state (1s4) were measured in a high density plasma etching environment. Excited species densities were measured ranging from 108 to 3×109 cm−3, depending on the particular atomic state. A straightforward reaction rate formalism consisting of only two competing electron-atom collision rates accurately predicts such densities. Because of the low densities of these long-lived excited state species, all excited argon species need to be considered only as energy loss channels in modeling high density (1011–1012 cm−3), low pressure (∼1 mTorr) plasma sources. Metastable production rates were also used to identify energy transfer mechanisms under etching conditions of Cl2/Ar mixtures and substrate biasing in the reactor.
Thermal quenching of F-center luminescence in Al2O3:C
Authors: Akselrod M. S. , Agersnap Larsen N. , Whitley V. , McKeever S. W. S.
Published: July 2002

Using time-resolved photoluminescence spectroscopy we have measured the lifetime of the F-center luminescence from α-Al2O3:C. The measurements reveal a lifetime of 35–36 ms at room temperature, decreasing to <2 ms over the temperature range from 370 to 500 K. The decrease in the lifetime is shown to follow a classical Mott-Seitz dependence for thermal quenching of luminescence, with an activation energy W of ∼1.08±0.03 eV and a corresponding frequency factor ν of ∼1014 s−1. Similar values for the energy and frequency factor were also obtained from an analysis of thermoluminescence (TL) glow curves measured at different heating rates, when the TL is measured over a wavelength range corresponding to the F-center luminescence emission (centered at 420 nm). Furthermore, the parameters obtained were independent of the glow curve shape, the degree of trap filling, or the specific conditions under which the crystals were grown. This is interpreted as a demonstration that the well-known heating rate dependence of the TL from this material is a result of thermal quenching of the F-center emission. Whereas the thermal quenching parameters obtained from measurement of the luminescence lifetime and from the heating rate analysis of the TL glow curves are independent of the sample type, the degree of trap filling, and the heating or cooling rate, measurements of the photoluminescence intensity, induced by absorption of F-band light, were found to be dependent upon all of the above conditions. This difference in behavior is attributed to a phosphorescence signal from traps associated with the ∼265, ∼310, and ∼450 K TL peaks.
Potential distribution measurement of surface discharge by Pockels sensing technique
Authors: Kumada A. , Chiba M. , Hidaka K.
Published: July 2002

A new technique for measuring the transient potential distribution of electrical discharge is proposed. The measuring system consists of a Bi4Ge3O12 Pockels crystal, an Ar ion laser, a beam expander, a streak camera and a charge-coupled device camera. The potential profile of the discharge along a linear path of 50 mm in length is measured in the range of 0∼±36 kV. The temporal and spatial resolutions of this system are estimated to reach 2 ns and 0.1 mm, respectively. The potential distribution of a surface discharge creeping on a dielectric material is measured to clarify its developing mechanism. The tip of the propagating surface discharge has a high potential close to an applied potential. The maximum value of electric fields of a negative surface streamer exceeds 16 kV/cm in the direction of the streamer propagation.
Magnetocaloric effect from indirect measurements: Magnetization and heat capacity
Authors: Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

Accurate values for the magnetocaloric effect can be obtained from both magnetization and heat-capacity data. A reliable estimate of the experimental errors in the calculated magnetocaloric effect can be made from the known experimental errors of the measured physical properties. Attempts in the past to simplify the basic thermodynamic relation to allow the calculation of the adiabatic temperature change from the heat capacity at constant field and the magnetic entropy change calculated from the magnetization data fail because the assumption that heat capacity is magnetic-field independent is erroneous. A suitable approach to carry out these calculations from the combined heat capacity and magnetization data is suggested.
A physically based model of electromigration and stress-induced void formation in microelectronic interconnects
Authors: Gleixner R. J. , Nix W. D.
Published: July 2002

In this article, a model of electromigration and stress-induced void formation in microelectronic interconnects is presented. This model solves the equations governing atomic diffusion and stress evolution in two dimensions, and can therefore account for the complex grain structures present in typical metal lines. A combined analytical and numerical solution scheme is developed to calculate the atomic fluxes and the evolution of mechanical stress, while avoiding the difficulties associated with finite element based approaches. Once a void has formed, growth is modeled by calculating the flux of atoms away from the void site. By combining models of atomic diffusion, stress evolution, void nucleation, and void growth, the complete void formation process can be simulated. To demonstrate this approach, void growth is calculated in interconnects where electromigration and thermal stress-induced damage have been experimentally observed. The results confirm that the model can quantitatively simulate void formation in realistic grain structures.
Mechanisms for microstructure evolution in electroplated copper thin films near room temperature
Authors: Harper J. M. E. , Cabral C. , Andricacos P. C. , Gignac L. , Noyan I. C. , Rodbell K. P. , Hu C. K.
Published: July 2002

We present a model which accounts for the dramatic evolution in the microstructure of electroplated copper thin films near room temperature. Microstructure evolution occurs during a transient period of hours following deposition, and includes an increase in grain size, changes in preferred crystallographic texture, and decreases in resistivity, hardness, and compressive stress. The model is based on grain boundary energy in the fine-grained as-deposited films providing the underlying energy density which drives abnormal grain growth. As the grain size increases from the as-deposited value of 0.05–0.1 μm up to several microns, the model predicts a decreasing grain boundary contribution to electron scattering which allows the resistivity to decrease by tens of a percent to near-bulk values, as is observed. Concurrently, as the volume of the dilute grain boundary regions decreases, the stress is shown to change in the tensile direction by tens of a mega pascal, consistent with the measured values. The small as-deposited grain size is shown to be consistent with grain boundary pinning by a fine dispersion of particles or other pinning sites. In addition, room temperature diffusion of the pinning species along copper grain boundaries is shown to be adequate to allow the onset of abnormal grain growth after an initial incubation time, with a transient time inversely proportional to film thickness.
Nitrogen and hydrogen in thick diamond films grown by microwave plasma enhanced chemical vapor deposition at variable H2 flow rates
Authors: Nistor S. V. , Stefan M. , Ralchenko V. , Khomich A. , Schoemaker D.
Published: July 2002

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H2 gas flow rates, keeping a constant [CH4]:[H2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic atomic species, is directly related to the concentration of the extended lattice defects.
Classical-quantum correspondence in the distribution dynamics of integrable systems
Authors: Jaffé Charles , Brumer Paul
Published: July 2002

The classical-quantum correspondence is developed in its most natural setting, i.e., that of phase space distributions. Specifically, the Wigner–Weyl representation of the two fundamental quantum densities ‖n≳<n‖ and ‖n≳<m‖, which are eigenstates of the quantum Liouville operator, are shown, for integrable systems, to yield stationary and nonstationary classical Liouville eigenstates in the ℏ→0 limit. The former are uniform distributions on semiclassically quantized tori, whereas the latter are nonuniform distributions on similar tori at intermediate actions. Applications to semiclassical mechanics in action-angle variable, the Heisenberg correspondence for matrix elements and the computation of vibrational intensities are provided.
The formation and reactivity of HOC+: Interstellar implications
Authors: Wagner-Redeker Winfried , Kemper Paul R. , Jarrold Martin F. , Bowers Michael T.
Published: July 2002

CHO+ ions are made by electron impact on CD3OH in the source (ICR1) of a tandem ion cyclotron resonance spectrometer. These ions are injected into a differentially pumped analysis cell (ICR2) where they are reacted with a number of small molecules. The internal energy distribution in the CHO+ ions is obtained using total reactivity studies with neutral molecules of varying proton affinities. About 40% of the CHO+ ions react with D2 either by proton transfer to form D2H+ or isotopic exchange to form CDO+ ions. A series of experiments are performed that conclusively show these ions are the HOC+ isomer and the exchange is due to the catalytic isomerization reaction HOC++D2→DCO++HD which is approximately 37 kcal/mol exothermic. The product DCO+ ions are vibrationally cool indicating the reaction releases most of its energy as kinetic energy. Absolute rate constants for reactions of CHO+ ions with the neutrals 13CO, 15N2, CO2, O2, D2, and Ar are reported. HOC+ reacts with D2 at about 30% of the collision rate. This rate decreases with increasing HOC+ kinetic energy in the range 0.025 to 0.44 eV. The two products of the reaction are D2H+/CO and DCO+/HD with DCO+/HD comprising ∼42% of the products for the HOC+ ions formed from CD3OH. Phase space theory calculations are performed to determine the barrier to catalytic isomerization. The data are best fit with a barrier between 0.0 and −0.05 eV relative to the HOC+/D2 asymptotic energy. A barrier of this magnitude yields reaction rate constants at interstellar temperatures of at least 10% of the collision rate constant (i.e., k>1×10−10 cm3/s), and could explain why so little HOC+ is observed in interstellar clouds.
Diode laser spectroscopy of the ν3 and ν2 fundamental bands of DO2
Authors: Uehara Hiromichi , Kawaguchi Kentarou , Hirota Eizi
Published: July 2002

The diode laser spectrum and the IR LMR spectrum of the ν3 fundamental band (O–O stretch) of DO2 were observed, as well as the diode laser spectrum of the ν2 fundamental band (D–O–O bend). Simultaneous analysis of the ν3 and ν2 bands has yielded the molecular constants for the (001) and (010) states and the Coriolis coupling constant. These, together with other available molecular constants for DO2 and HO2 were used to derive the equilibrium molecular structure of the HO2 radical, as already reported [K. G. Lubic et al., J. Chem. Phys. 81, 4826 (1984)]. The harmonic force field obtained in the present study has explained well the general features of the Coriolis coupling constant, quartic centrifugal distortion constants, and the inertia defects for DO2.
Impulsive stimulated scattering: General importance in femtosecond laser pulse interactions with matter, and spectroscopic applications
Authors: Yan Yong-Xin , Gamble Edward B. , Nelson Keith A.
Published: July 2002

It is shown that ‘‘impulsive’’ stimulated Raman scattering (ISRS) should occur, with no laser intensity threshold, when a sufficiently short laser pulse passes through many types of matter. ISRS excitation of coherent optic phonons, molecular vibrations, and other excitations (including rotational, electronic, and spin) may play important roles in femtosecond pulse interactions with molecules, crystals, glasses (including optical fibers), semiconductors, and metals. Spectroscopic applications of ISRS, including time-resolved spectroscopy of vibrationally distorted molecules and crystals, are discussed.
Density functional theory of nonuniform polyatomic systems. II. Rational closures for integral equations
Authors: Chandler David , McCoy John D. , Singer Sherwin J.
Published: July 2002

With the density functional theory outlined in paper I, we address and formally solve the nonlinear inversion problem associated with identifying the entropy density functional for systems with bonding constraints. With this development, we derive a nonlinear integral equation for the average site density fields of a polyatomic system. When external potential fields are set to zero, the integral equation represents a mean field theory for symmetry breaking and thus phase transformations of polyatomic systems. In the united atom limit where the intramolecular interaction sites become coincident, the mean field theory becomes identical to that developed for simple atomic systems by Ramakrishnan, Yussouff, and others. When the external potential fields are particle producing fields (in the sense introduced long ago by Percus), the integral equation represents a theory for the solvation of a simple spherical solute by a polyatomic solvent. In the united atom limit for the solvent, the theory reduces to the hypernetted chain (HNC) integral equation. This reduction is not found with the so-called ‘‘extended’’ RISM equation; indeed, the extended RISM equation—the theory in which the HNC closure of simple systems is inserted directly into the Chandler–Andersen (i.e., RISM or SSOZ) equation—behaves poorly in the united atom limit. The integral equation derived herein with the density functional approach however suggests a rational closure of the RISM equation which does pass over to the HNC theory in the united atom limit. The new integral equation for pair correlation functions arising from this suggested closure is presented and discussed.
Dimensional scaling as a symmetry operation
Authors: Kais S. , Herschbach D. R. , Levine R. D.
Published: July 2002

The scaling of the Schrödinger equation with spatial dimension D is studied by an algebraic approach. For any spherically symmetric potential, the Hamiltonian is invariant under such scaling to order 1/D2. For the special family of potentials that are homogeneous functions of the radial coordinate, the scaling invariance is exact to all orders in 1/D. Explicit algebraic expressions are derived for the operators which shift D up or down. These ladder operators form an SU(1,1) algebra. The spectrum generating algebra to order 1/D2 corresponds to harmonic motion. In the D→∞ limit the ladder operators commute and yield a classical-like continuous energy spectrum. The relation of supersymmetry and D scaling is also illustrated by deriving an analytic solution for the Hooke’s law model of a two-electron atom, subject to a constraint linking the harmonic frequency to the nuclear charge and the dimension.
Computer simulation of ammonia on graphite. I. Low temperature structure of monolayer and bilayer films
Authors: Cheng Ailan , Steele W. A.
Published: July 2002

Molecular dynamics computer simulations are reported here for the structures and energies of monolayer and bilayer films of ammonia adsorbed on the basal plane of graphite. A pair-wise additive intermolecular potential function is used that includes electrostatic interactions by distributions of discrete charges within each molecule. The nonelectrostatic energy is assumed to be given by a sum of site–site energies over the four atoms in each molecule. Parameters for this potential have been chosen to give agreement with experimental data for the bulk NH3. The NH3/graphite potential was also assumed to be site–site potentials summed over the four atoms per molecule and over the C sites in the solid. The simulations yield translational and orientationally ordered layers. A unit cell for the monolayer is deduced which is triangular with an NH3–NH3 spacing in good agreement with experiment. Because of a relatively complex orientational ordering, the unit cell contains four molecules. The orientational ordering is dominated by the electrostatic energies, but differs considerably from that for a purely dipolar monolayer due to the presence of important quadrupolar interactions. The orientational structures of the individual layers in the bilayer films are different from that of the monolayer. The relationship between these observations and the intermolecular potential functions is discussed.
Molecular diffusion and latex film formation: An analysis of direct nonradiative energy transfer experiments
Authors: Wang Yongcai , Zhao Cheng-Le , Winnik Mitchell A.
Published: July 2002

The diffusion of polymer molecules across an interface is an important topic in polymer physics. Measuring this diffusion process requires a labeling experiment so that the mixing of otherwise identical molecules can be followed. Here we examine the use of nonradiative energy transfer to follow the interdiffusion of polymer molecules during the annealing of latex films. Those films are prepared from a mixture of phenanthrene (Phe, the donor)- and anthracene(An, the acceptor)-labeled poly(butyl methacrylate) (PBMA) latex particles. We examine in some detail the kinetics of direct energy transfer (DET) at an interface and the evolution of the DET kinetics as the components mix. Time resolved and steady state measurement approaches are compared, and the former is employed to examine molecular diffusion in a series PBMA latex films. Various diffusion models are considered. Diffusion coefficients are calculated by treating the data in terms of a Fickian spherical diffusion model. A film from one set of samples is similar in molecular weight to that studied by small angle neutron scattering (SANS) [K. Hahn, G. Ley, H. Schuller, and R. Oberthur, Colloid Polym. Sci. 264, 1029 (1986)]. Diffusion coefficients calculated from both sets of experiments using similar Fickian diffusion models are very similar in magnitude.
Generalized correlations in terms of polarizability for van der Waals interaction potential parameter calculations
Authors: Cambi Roberto , Cappelletti David , Liuti Giorgio , Pirani Fernando
Published: July 2002

General correlations between van der Waals interaction potential parameters and polarizabilities of the interacting neutral–neutral partners of any nature are presented and discussed. To ensure the full applicability of the correlations, an evaluation of the long-range interaction constants is performed in terms of the Slater–Kirkwood approximation whose numerical coefficients, having the meaning of effective electron numbers, are estimated interpolating the values deduced by theoretical considerations. The values of the long-range constants so obtained are compared satisfactorily with the available experimental ones. The correlations are tested successfully over practically all systems characterized experimentally. Their use to predict the parameters of unknown systems is suggested.
Ab initio investigation of potential-energy surfaces involved in the photophysics of benzene and pyrazine
Authors: Sobolewski A. L. , Woywod C. , Domcke W.
Published: July 2002

Potential-energy surfaces of the lowest singlet and triplet excited states of benzene and pyrazine have been calculated using complete-active-space self-consistent-field and multireference configuration interaction (MRCI) techniques. We have focused our attention on the saddle points and surface intersections associated with the reaction path to a biradical form called prefulvene. The barrier heights separating the prefulvenic minimum from the minimum of the planar aromatic form on the ππ* excited singlet surface and on the ground-state surface have been estimated by large-scale MRCI calculations. The conical intersection of the lowest ππ* excited singlet surface with the S0 surface has been mapped out in two dimensions, the reaction coordinate to prefulvene and the coordinate of maximum coupling perpendicular to it. The relevance of these ab initio potential-energy data for the interpretation of photophysical relaxation pathways in benzene and pyrazine (‘‘channel-three’’ effect) is discussed.
Density functional theory for inhomogeneous polymer solutions
Authors: Woodward Clifford E. , Yethiraj Arun
Published: July 2002

The structure of polymer solutions confined between hard walls is investigated via density functional theory. A generalized Flory theory for polymer mixtures incorporating an equation of state for dimers is implemented in the theory without introducing any adjustable parameters. The structure predicted by the theory is in good agreement with computer simulations for confined melts, with polymer lengths ranging from 3 mers to 20 mers and for several volume fractions. Similarly good agreement is found for 8-mer/monomer mixtures.
Solvation potentials for macromolecules
Authors: Grayce Christopher J. , Schweizer Kenneth S.
Published: July 2002

A solvation potential replaces the problem of the conformation of a molecule in solution with the problem of the conformation of a single molecule in an external field. Practical solvation potentials for macromolecules treated with an interaction-site model must be site-pairwise additive. A key requirement for accuracy of a pairwise additive potential is the inclusion to some degree of the intramolecular screening of the solvent-mediated interactions between sites. We analyze this requirement from a diagrammatic point of view and suggest it explains the accuracy of the ‘‘Gaussian-fluctuation’’ approximate potential relative to the superposition approximation. Our analysis suggests a new approximate potential that we call a ‘‘Percus–Yevick-style’’ solvation potential, the presentation of which is the concrete contribution here. In the following paper this new potential is tested for a model linear nonpolar polymer.
Density functional study of phosphorus and arsenic clusters using local and nonlocal energy functionals
Authors: Ballone P. , Jones R. O.
Published: July 2002

Previous calculations of the structures of isomers of phosphorus clusters up to P11 (density functional calculations with simulated annealing, local spin density approximation to the exchange-correlation energy) have been extended to arsenic clusters. The structures of Asn clusters are characterized by an almost uniform expansion (∼ 9%) of the corresponding Pn isomers. All cluster isomers have also been studied using a nonlocal, gradient corrected (Becke–Perdew) energy functional. While the structures are almost unchanged, there are significant improvements in the cohesive energies of all clusters. We present a simple picture to show that the improvements arise from contributions both in the atoms and near the ‘‘surface’’ of the clusters.
Successive capture and coagulation of atoms and molecules to small clusters in large liquid helium clusters
Authors: Lewerenz M. , Schilling B. , Toennies J. P.
Published: July 2002

The multiple pickup of up to 30 foreign particles by large (N≳103 atoms) liquid helium clusters and the formation of clusters of the foreign particles within the helium cluster has been observed in a molecular beam experiment for Ar, Kr, Xe, H2O, and SF6. Evaporation of helium atoms due to the pickup process has been measured quantitatively by two methods. The measured apparent cross sections for capture and coagulation of these particles are significantly smaller than the total scattering cross sections of the clusters. After electron impact ionization the size distribution of the foreign particle cluster ion fragments can be well described by a Poisson distribution. This is interpreted as indicating negligible fragmentation of the coagulated foreign particle clusters. The measured coagulation cross sections are smaller than the capture cross sections. This is explained through a model which involves only partial coagulation of the foreign particles. The observed processes allow the preparation of foreign particle clusters with well-defined size distributions inside large helium clusters.
Reactivity indices and fluctuation formulas in density functional theory: Isomorphic ensembles and a new measure of local hardness
Authors: Baekelandt Bart G. , Cedillo Andrés , Parr Robert G.
Published: July 2002

Fluctuation formulas for the external potential v(r) are introduced in a modified Legendre-transformed representation of the density functional theory of electronic structure (isomorphic ensemble). A new (nuclear/geometric) reactivity index h(r), having the same status as the electronic Fukui function in the canonical ensemble, is thereby identified, h(r)=(1/N)[δμ/δσ(r)]N,T=(1/kT) [〈μ⋅v(r)〉−〈μ〉〈v(r)〉], where μ is the electronic chemical potential, σ is the shape factor of the electron density distribution, N is the number of electrons, 〈...〉 denotes the ensemble average of a quantity, and 〈v(r)〉 is the ensemble averaged external potential. This new local quantity is shown to be an inverse of the local softness, and to provide a useful definition of a local hardness.
Gaussian basis sets for use in correlated molecular calculations. V. Core-valence basis sets for boron through neon
Authors: Woon David E. , Dunning Thom H.
Published: July 2002

The correlation-consistent polarized valence basis sets (cc-pVXZ) for the atoms boron through neon have been extended to treat core and core-valence correlation effects. Basis functions were added to the existing cc-pVXZ sets to form correlation-consistent polarized core-valence sets (cc-pCVXZ) in the usual pattern: Double zeta added (1s1p), triple zeta added (2s2p1d), quadruple zeta added (3s3p2d1f), and quintuple zeta added (4s4p3d2f1g). The exponents of the core functions were determined by minimizing the difference between all-electron and valence-only correlation energies obtained from HF+1+2 calculations on the ground states of the atoms. With the cc-pCVXZ sets, core, core-valence, and valence correlation energies all converge exponentially toward apparent complete basis set (CBS) limits, as do the corresponding all-electron singles and doubles CI energies. Several test applications of the new sets are presented: The first two ionization potentials of boron, the 3P–5S separation in carbon, and the X̃ 3B1–ã 1A1 state separation in CH2. As expected, correlation effects involving the core electrons of the first row atoms, B–Ne, are small but must be included if high accuracy is required.
Intermolecular vibrations of the phenol dimer revealed by spectral hole burning and dispersed fluorescence spectroscopy
Authors: Schmitt M. , Henrichs U. , Müller H. , Kleinermanns K.
Published: July 2002

Inter- and intramolecular vibrations of the Phenol dimer have been examined using spectral hole burning and dispersed fluorescence spectroscopy. The dispersed fluorescence spectra have been recorded via excitation of the electronic donor origin and all intermolecular vibrational transitions. The Franck–Condon intensity pattern allowed a straightforward assignment of the ground state vibrational frequencies to the excited state frequencies, which were examined by hole-burning spectroscopy. The existence of another conformer that possibly absorbs in the region of interest was ruled out by hole-burning spectroscopy. For obtaining the hole-burning spectra, the electronic origin of the donor chromophore was analyzed, while the hole-burning laser was scanned over the region of interest. As both acceptor and donor part belong to one molecule the hole-burning signal could be analyzed via fluorescence from one of the chromophores.
Classical numerical integrators for wave-packet dynamics
Authors: Sanz-Serna J. M. , Portillo A.
Published: July 2002

Gray and Verosky have recently studied the reformulation of the N-state matrix representation of the time-dependent Schrödinger equation as an N-degrees of freedom classical Hamiltonian system. This opens the possibility of using in quantum dynamics numerical integrators originally devised for classical mechanics. When the Hamiltonian matrix is time-dependent, Gray and Verosky suggest the use of a Magnus approximation before reducing the quantum system to its classical format. We show that Magnus approximations are not necessary and suggest an alternative technique. With the new technique it is possible to obtain simple integrators of arbitrarily high orders of accuracy that can be applied to all matrix Schrödinger problems with a, possibly time-dependent, real Hamiltonian matrix. The connection between the new approach and high-order split-operator methods is studied.
Ab initio path integral molecular dynamics: Basic ideas
Authors: Marx Dominik , Parrinello Michele
Published: July 2002

The essential ideas underlying ab initio path integral molecular dynamics and its efficient numerical implementation are discussed. In this approach the nuclei are treated as quantum particles within the path integral formulation of quantum statistical mechanics. The electronic degrees of freedom are treated explicitly based on state-of-the-art electronic structure theory. This renders ab initio simulations of quantum systems possible without recourse to model potentials. A combined extended Lagrangian for both quantum nuclei and electrons defines a dynamical system and yields molecular dynamics trajectories that can be analyzed to obtain quantum statistical expectation values of time-independent operators. The methodology can be applied to a wide range of fields addressing problems in molecular and condensed matter chemistry and physics.
Partial third-order quasiparticle theory: Comparisons for closed-shell ionization energies and an application to the Borazine photoelectron spectrum
Authors: Ortiz J. V.
Published: July 2002

Valence ionization energies of a set closed-shell molecules calculated in a partial third-order (P3) quasiparticle approximation of the electron propagator have an average absolute error of 0.19 eV. Diagonal elements of the self-energy matrix include all second-order and some third-order self-energy diagrams. Because of its fifth power dependence on basis set size and its independence from electron repulsion integrals with four virtual indices, this method has considerable potential for large molecules. Formal and computational comparisons with other electron propagator techniques illustrate the advantages of the P3 procedure. Additional applications to benzene and borazine display the efficacy of the P3 propagator in assigning photoelectron spectra. In the borazine spectrum, 2E′ and 2A2′ final states are responsible for an observed feature at 14.76 eV. Another peak at 17.47 eV is assigned to a 2E′ final state.
CH
5
+
:
The story goes on. An explicitly correlated coupled-cluster study
Authors: Müller Hendrik , Kutzelnigg Werner , Noga Jozef , Klopper Wim
Published: July 2002

CCSD(T)-R12 calculations (as well as R12 calculations at lower CC and MPn level) with large basis sets have been performed for the three lowest stationary structures of CH5+, for CH4, and for CH3+ + H2. The basis set limit of the total energy at all levels of the treatment of correlation is reached within ∼1 kcal/mol. At the CCSD(T)-R12 level the same accuracy is achieved for CH4 with respect to its “experimental’’ non-relativistic energy. Accordingly, energy differences should be correct to within a small fraction of a kcal/mol. In agreement with the most accurate previous calculations, the Cs(1) structure of CH5+ is lowest, followed by Cs(2) and C2v. For the relative energies of the two latter structures we predict energy differences of 0.1 and 0.8 kcal/mol respectively. The possibility of a vibrational stabilization of the C2v -structure is discussed. Accurate predictions of the proton affinity of CH4 and the binding energy between CH3+ and H2 are also given.
Continuum solvation models: A new approach to the problem of solute’s charge distribution and cavity boundaries
Authors: Mennucci Benedetta , Tomasi Jacopo
Published: July 2002

In continuum solvation models the definition of a cavity that embeds the solute molecule leads to problems related to the portion of solute’s electronic charge lying outside its boundaries (charge tails). The correction strategies developed so far can be shown to work insufficiently, since they only correct the global charge defect, but lead to considerable local errors. The present paper will be focused on the theoretical and technical aspects of this problem, and it will present in detail a new method which allows a very refined treatment of solute’s charge tails in the outer space; some numerical results of solutes in water will be shown and discussed. As further analyses, the introduction of Pauli repulsion term will be considered, and the implications all these effects have on molecular properties, such as (hyper)polarizabilities, numerically evaluated. The new approach has been implemented within the framework of the polarizable continuum model (PCM).
Chemical potential, hardness, hardness and softness kernel and local hardness in the isomorphic ensemble of density functional theory
Authors: De Proft Frank , Liu Shubin , Parr Robert G.
Published: July 2002

New relations among reactivity descriptors are provided within the recently introduced modified isomorphic ensemble of density functional theory. In addition, expressions for the softness and hardness kernel are derived in the canonical, grand canonical, isomorphic, and grand isomorphic ensemble. There results a new definition for the local hardness, η(r)=[∂u(r)/∂N]σ=ησ−g(r), where g(r)=[∂υ(r)/∂N]σ and σ=ρ/N is the shape factor. This identifies the local hardness as a function measuring the response of the system’s external potential to a perturbation in electron number at a constant shape factor. Furthermore, it is shown that one cannot represent both local softness and local hardness unambiguously with one representation.
Assessment of Gaussian-2 and density functional theories for the computation of ionization potentials and electron affinities
Authors: Curtiss Larry A. , Redfern Paul C. , Raghavachari Krishnan , Pople John A.
Published: July 2002

A set of 146 well-established ionization potentials and electron affinities is presented. This set, referred to as the G2 ion test set, includes the 63 atoms and molecules whose ionization potentials and electron affinities were used to test Gaussian-2 (G2) theory [J. Chem. Phys. 94, 7221 (1991)] and 83 new atoms and molecules. It is hoped that this new test set combined with the recently published test set of enthalpies of neutral molecules [J. Chem. Phys. 106, 1063 (1997)] will provide a means for assessing and improving theoretical models. From an assessment of G2 and density functional theories on this test set, it is found that G2 theory is the most reliable method. It has an average absolute deviation of 0.06 eV for both ionization potentials and electron affinities. The two modified versions of G2 theory, G2(MP2,SVP) and G2(MP2) theory, have average absolute deviations of 0.08–0.09 eV for both ionization potentials and electron affinities. The hybrid B3LYP density functional method has the smallest average absolute deviation (0.18 eV) of the seven density functional methods tested for ionization potentials. The largest deviation for the density functional methods is for the ionization potential of CN (>1 eV). The BLYP density functional method has the smallest average absolute deviation (0.11 eV) of the seven density functional methods tested for electron affinities, while the BPW91, B3LYP, and B3PW91 methods also do quite well.
Quantum cluster equilibrium theory of liquids: Illustrative application to water
Authors: Weinhold F.
Published: July 2002

The Quantum Cluster Equilibrium (QCE) theory, developed in a previous paper, is illustrated with a simple 7-cluster RHF/3-21G model for liquid water. We first describe a simplified QCE(0) model that corresponds to an ideal gas cluster mixture (neglect of all cluster–cluster interactions), but is nevertheless able to exhibit many characteristic features of a true gas/liquid phase transition (macro-clustering, volume collapse, specific heat increase, Clausius–Clapeyron pressure dependence). We employ the QCE(0) model to illuminate the interplay between macroscopic and microscopic properties (dependence on particular cluster species, cooperative binding energetics, vibrational entropy factors, etc.) in a purely ab initio framework. We then introduce two simple corrections for the neglected cluster–cluster interactions: (i) a mean-field correction for residual inter-cluster attractions and (ii) a van der Waals-like correction for excluded volume effects. The numerical accuracy of the resulting QCE model is demonstrated for a wide range of thermodynamic properties of water, complementing recent comparisons with experimental quadrupole coupling data. Some shortcomings of the current implementation are noted, and prospects for future QCE applications to more complex liquids and liquid mixtures are discussed.
Density pair correlation functions for molecular liquids: Approximations for polymers
Authors: Donley James P. , Rajasekaran J. J. , Liu Andrea J.
Published: July 2002

We present a simple, physically motivated equation for the radial distribution function g(r) for molecular liquids, valid for polymers interacting via soft potentials. It is constructed to perform properly at low density for polyelectrolyte solutions. However, it also accounts for intermolecular correlations at both the molecular and monomer level, above those contained in the random phase approximation. We show that this theory reduces in various limits to some well-known polymer theories for g(r). In a preliminary analysis, we apply the equation for soft potentials to two very different systems: a solution of rod polyelectrolytes and a solution of flexible van der Waals chains.
Free energy calculations on systems of rigid molecules: An application to the TIP4P model of H2O
Authors: Vlot Margot J. , Huinink Jan , van der Eerden Jan P.
Published: July 2002

Free energy calculations of different phases are necessary to establish the thermodynamically stable phase in simulations. A new method is proposed to calculate the free energy of a crystal of rigid molecules, which is slightly different from the method [L. A. Báez and P. Clancy (Mol. Phys. 86, 385, (1995)]. The new method is applied to the ice phase of the TIP4P model for H2O [W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, and M. L. Klein, J. Chem. Phys. 79, 926 (1983)]. The free energy of the liquid and gas phase are calculated as well, using different methods as the Widom method, overlapping distribution method, and thermodynamic integration. The melting point of the proton ordered ice Ih of the TIP4P model at atmospheric pressure is found to be Tm=214(±6) K and the boiling point Tb=363(±3) K.
Conjugated polymer aggregates in solution: Control of interchain interactions
Authors: Nguyen Thuc-Quyen , Doan Vinh , Schwartz Benjamin J.
Published: July 2002

We present evidence that the degree of interchain interactions and morphology in conjugated polymer films can be controlled by altering the chain conformation in the solution from which the film is cast. Light scattering experiments show that the physical size of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) chains can vary by a factor of 2 in different solvents such as chlorobenzene (CB) or tetrahydrofuran (THF). Photoluminescence and wavelength-dependent excitation indicate that MEH-PPV forms aggregate species with an absorption and luminescence spectra that are distinctly red-shifted from the intrachain exciton. The degree of aggregation is both concentration and solvent dependent; for solutions with concentrations typical of those used in spin casting, aggregates comprise a significant fraction of the total number of excited state species. The overall photoluminescence quantum yield is found to depend both on how restricted the polymer conformation is due to the choice of solvent and on aggregation due to polymer concentration. The excited state aggregates have a longer lifetime than their intrachain exciton counterparts, as evidenced by a near-infrared transient absorption in femtosecond pump-probe and anisotropy measurements. Memory of the chain conformation and the extent of aggregation of MEH-PPV in solution is carried into cast films. Thus, many conflicting results presented on the degree of interchain interactions can be explained by noting that the film samples in different studies were cast from precursor solutions with different solvents and concentrations. Overall, a careful choice of the solution (both solvent and concentration) can be used to produce MEH-PPV films with desired interchain interactions for particular device applications.
Isomerization dynamics in viscous liquids: Microscopic investigation of the coupling and decoupling of the rate to and from solvent viscosity and dependence on the intermolecular potential
Authors: Murarka Rajesh K. , Bhattacharyya Sarika , Biswas Ranjit , Bagchi Biman
Published: July 2002

A detailed investigation of viscosity dependence of the isomerization rate is carried out for continuous potentials by using a fully microscopic, self-consistent mode-coupling theory calculation of both the friction on the reactant and the viscosity of the medium. In this calculation we avoid approximating the short time response by the Enskog limit, which overestimates the friction at high frequencies. The isomerization rate is obtained by using the Grote–Hynes formula. The viscosity dependence of the rate has been investigated for a large number of thermodynamic state points. Since the activated barrier crossing dynamics probes the high-frequency frictional response of the liquid, the barrier crossing rate is found to be sensitive to the nature of the reactant–solvent interaction potential. When the solute–solvent interaction is modeled by a 6–12 Lennard-Jones potential, we find that over a large variation of viscosity (η), the rate (k) can indeed be fitted very well to a fractional viscosity dependence: (k∼η−α), with the exponent α in the range 1⩾α>0. The calculated values of the exponent appear to be in very good agreement with many experimental results. In particular, the theory, for the first time, explains the experimentally observed high value of α, even at the barrier frequency, ωb≃9×1012 s−1 for the isomerization reaction of 2-(2′-propenyl)anthracene in liquid n-alkanes. The present study can also explain the reason for the very low value of ωb observed in another study for the isomerization reaction of trans-stilbene in liquid n-alkanes. For ωb⩾2.0×1013 s−1, we obtain α≃0, which implies that the barrier crossing rate becomes identical to the transition-state theory predictions. A careful analysis of isomerization reaction dynamics involving large amplitude motion suggests that the barrier crossing dynamics itself may become irrelevant in highly viscous liquids and the rate might again be coupled directly to the viscosity. This crossover is predicted to be strongly temperature dependent and could be studied by changing the solvent viscosity by the application of pressure.
Measurements of the lifetimes of electron bubbles in large size selected HeN−4 droplets
Authors: Fárnı́k Michal , Samelin Björn , Toennies J. Peter
Published: July 2002

The lifetimes of electron bubbles in large size selected HeN−4 droplets have been measured in a molecular beam experiment to lie between 2⋅10−3 and 0.2 s in droplets with sizes between N≈105 and 107 atoms, respectively. The dependence of the lifetimes on the electric fields used to size select the droplets, on the energies of the incident electrons, and on the infrared radiation emitted from the apparatus walls were investigated and taken into account. The experimental lifetimes are many orders of magnitude smaller than predicted by earlier theories but consistent with a recent theory published by the authors in Phys. Rev. Lett. 81, 3892 (1998).
Ab initio calculation of anharmonic vibrational states of polyatomic systems: Electronic structure combined with vibrational self-consistent field
Authors: Chaban Galina M. , Jung Joon O. , Gerber R. Benny
Published: July 2002

An algorithm for first-principles calculation of vibrational spectroscopy of polyatomic molecules is proposed, which combines electronic ab initio codes with the vibrational self-consistent field (VSCF) method, and with a perturbation-theoretic extension of VSCF. The integrated method directly uses points on the potential energy surface, computed from the electronic ab initio code, in the VSCF part. No fitting of an analytic potential function is involved. A key element in the approach is the approximation that only interactions between pairs of normal modes are important, while interactions of triples or more can be neglected. This assumption was found to hold well in applications. The new algorithm was applied to the fundamental vibrational excitations of H2O, Cl−(H2O), and (H2O)2, using the Möller–Plesset method for the electronic structure. The vibrational frequencies found are in very good accord with experiments. Estimates suggest that this electronic ab initio/VSCF approach should be feasible, with reasonable computational resources, for all-mode calculations of vibrational energies and wave functions for systems of up to 10–15 atoms. The new method can be also very useful for testing the accuracy of electronic structure codes by comparing with experimental vibrational spectroscopy.
Ewald summation for systems with slab geometry
Authors: Yeh In-Chul , Berkowitz Max L.
Published: July 2002

We propose a modification in the three-dimensional Ewald summation technique for calculations of long-range Coulombic forces for systems with a slab geometry that are periodic in two dimensions and have a finite length in the third dimension. The proposed method adds a correction term to the standard Ewald summation formula. To test the current method, molecular dynamics simulations on water between Pt(111) walls have been carried out. For a more direct test, the calculation of the pair forces between two point charges has been also performed. An excellent agreement with the results from simulations using the rigorous two dimensional Ewald summation technique were obtained. We observed that a significant reduction in computing time can be achieved when the proposed modification is used.