bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo study of phase separation in aqueous protein solutions
Authors: Lomakin Aleksey , Asherie Neer , Benedek George B.
Published: July 2002

The binary liquid phase separation of aqueous solutions of γ-crystallins is utilized to gain insight into the microscopic interactions between these proteins. The interactions are modeled by a square-well potential with reduced range λ and depth ε. A comparison is made between the experimentally determined phase diagram and the results of a modified Monte Carlo procedure which combines simulations with analytic techniques. The simplicity and economy of the procedure make it practical to investigate the effect on the phase diagram of an essentially continuous variation of λ in the domain 1.05≤λ≤2.40. The coexistence curves are calculated and are in good agreement with the information available from previous standard Monte Carlo simulations conducted at a few specific values of λ. Analysis of the experimental data for the critical volume fractions of the γ-crystallins permits the determination of the actual range of interaction appropriate for these proteins. A comparison of the experimental and calculated widths of the coexistence curves suggests a significant contribution from anisotropy in the real interaction potential of the γ-crystallins. The dependence of the critical volume fraction φc and the reduced critical energy ε̂c upon the reduced range λ is also analyzed in the context of two ‘‘limiting’’ cases; mean field theory (λ→∞) and the Baxter adhesive sphere model (λ→1). Mean field theory fails to describe both the value of φc and the width of the coexistence curve of the γ-crystallins. This is consistent with the observation that mean field is no longer applicable when λ≤1.65. In the opposite case, λ→1, the critical parameters are obtained for ranges much shorter than those investigated in the literature. This allows a reliable determination of the critical volume fraction in the adhesive sphere limit, φc(λ=1)=0.266±0.009.
Configurational averaging of dipolar interactions in magnetically diluted spin networks
Authors: Fel’dman Edward B. , Lacelle Serge
Published: July 2002

Some analytical results, concerning the disorder average of magnetic dipolar interactions in dilute spin networks and valid in the thermodynamic limit, are derived within a mean field approximation. In particular, it is shown that the quenched substitutional disorder average of the free induction decay of the nuclear magnetization in such a homonuclear spin system yields, 〈G(t)〉c ∼ exp[−ρ(γ2ℏt)D/3], where ρ is the concentration of magnetic nuclei embedded in a D-dimensional space and γ the nuclear magnetogyric ratio. Furthermore, the relative fluctuations about the disorder average, {[〈G2(t)〉c−〈G(t)〉2c]1/2/〈G(t)〉 ic}, scale as [ρ1/2(γ2ℏt)D/6/N1/2], when ρ(γ2ℏt)D/3≪1, and as N−1/2 exp[ρ(γ2ℏt)D/3] [1+ρ−3(γ2ℏt)−D]1/2 when ρ(γ2ℏt)D/3≫1; N is the number of spins in the lattice. The distinction between the role of the arithmetic mean and the most probable value of the distribution of dipolar coupling constants in the configurational averages of the free induction decay and its relative fluctuations is pointed out. Implications for nuclear magnetic resonance (NMR) experiments in magnetically diluted solids are also examined.
Proton dynamics in supercooled water by molecular dynamics simulations and quasielastic neutron scattering
Authors: Di Cola Daniela , Deriu Antonio , Sampoli Marco , Torcini Alessandro
Published: July 2002

A detailed study of the single-particle dynamics of liquid water in normal and supercooled regime has been carried out by comparing molecular dynamics (MD) simulation results with now available high resolution quasielastic neutron scattering (QENS) data. Simulation runs have been performed at 264, 280, 292, and 305 K, using the extended simple point charge model, well suited for reproducing single-particle properties of H2O. The microscopic dynamics has been probed over a wide range of times and distances. The MD results indicate that a substantial coupling between translational and rotational dynamics exists already at about 1 ps. The decay of the translational dynamic correlations has been phenomenologically analyzed in terms of three exponential components, and the agreement between the parameters thus obtained from experimental and simulation derived datasets is quite satisfactory. Both QENS and MD data can not be described with sufficient accuracy by simple diffusion models over the entire range of examined wave vectors.
Electronic states of ketene
Authors: Szalay Péter G. , Császár Attila G. , Nemes László
Published: July 2002

State of the art ab initio techniques of molecular electronic structure theory have been employed to investigate different structural aspects of the electronic states of ketene. Vertical excitation energies of more than 40 singlet and triplet states have been determined from equation-of-motion coupled cluster singles and doubles (EOM-CCSD) and configuration interaction singles (CIS) calculations employing extended basis sets. Most importantly, all singlet and triplet electronic states below 70 000 cm−1, close to the first ionization potential of ketene, 77 500 cm−1 have been determined. Only four pure excited valence states have been identified: the 1 3A1, 2 3A1, 1 3A2, and 1 1A2 states. Vertical ionization energies have also been determined at the EOM-CCSD level. They are found to be in nice accord with the available experimental results. All lower-lying members of the Rydberg series of ketene terminating at 77 500 cm−1 are assigned based on theoretical data, symmetry considerations, and use of the Rydberg formula and the available experimental results. High-quality estimates are given for the possible occurrence of fluorescence emission from the first singlet state of ketene.
On the origin of the dip in the KrF laser gain spectrum
Authors: Shaw M. J. , Divall E. J. , Hirst G. J. , Hooker C. J. , Lister J. M. D. , Ross I. N. , Kvaran A. , Osvay K.
Published: July 2002

High-resolution spectra of KrF (B–X) amplified spontaneous emission from various discharge-pumped and electron-beam-pumped KrF lasers have been analyzed. An underlying structured absorption spectrum has been discovered with a well-resolved peak at 248.91 nm. The absorption coefficient of this peak was found to vary in exact proportion to the peak laser gain coefficient but was independent of laser gas purity. We suggest that the absorption arises internally within the KrF molecule and is due to transitions from the B state to a higher-lying Rydberg state. This hypothesis was tested by simulating the absorption spectrum from KrF*(B) to a weakly repulsive state dissociating to Kr*(3P1)+F(2P3/2). A good agreement was obtained between simulated and experimental absorption spectra.
Time resolved four- and six-wave mixing in liquids. I. Theory
Authors: Steffen Thomas , Fourkas John T. , Duppen Koos
Published: July 2002

Low-frequency intermolecular dynamics in liquids is studied by ultrafast four- and six-wave mixing. The theory of these nonlinear optical processes is given for electronically nonresonant optical interactions up to fifth order in the electric field. The Born–Oppenheimer approximation is used to separate the motional part of the response functions from coordinate independent electronic hyperpolarizabilities. A large variety of experiments, involving far-infrared absorption, ordinary Rayleigh–Raman or hyper Rayleigh–Raman scattering is covered by this theory. The response in nonresonant six-wave mixing comprises four dynamically different processes. It is shown that one of the terms contains information on the time scale(s) of intermolecular dynamics, that is not available from lower-order nonresonant experiments. For instance, homogenous and inhomogeneous contributions to line broadening can be distinguished. The optical response of harmonic nuclear motion is calculated for nonlinear coordinate dependence of the polarizabilities. Results for level-dependent and level-independent damping of the motion are compared. It is shown that level-dependent damping destroys the interference between different quantum mechanical pathways, yielding an extra contribution to the fifth-order response that has not been discussed before. When two or more nuclear modes determine the optical response, their relative contributions to the four- and six-wave mixing signals are in general different. These contributions are determined by the coordinate dependence of the electronic polarizability, which is usually not fully known. Model calculations are presented for the dynamic parameters of liquid CS2. The theory of this paper will be employed in Part II, to analyze experimental results on femtosecond four- and six-wave mixing.
Electron-impact ionization cross sections of atmospheric molecules
Authors: Kim Y.-K. , Hwang W. , Weinberger N. M. , Ali M. A. , Rudd M. E.
Published: July 2002

A theoretical model for electron-impact total ionization cross sections, which has been found to be reliable for a wide range of molecules, is applied to molecules of interest to atmospheric science. The new theory, the binary-encounter-Bethe (BEB) model, combines the binary-encounter theory and the Bethe theory for electron-impact ionization, and uses simple theoretical data for the ground state of the target molecule, which are readily available from molecular structure codes. Total ionization cross sections of 11 molecules, CS, CS2, COS, CH4, H2S, NH3, NO2, N2O, O3, S2, and SO2, are presented for incident electron energies from threshold to 1 keV with an average accuracy of 15% or better at the cross section peak. We also found that the use of vertical ionization potentials (IPs) rather than adiabatic IPs for the lowest IPs significantly improves BEB cross sections between the threshold and cross section peak for molecules whose adiabatic and vertical IPs are different by ~1 eV or more (CH4 and NH3). The BEB cross sections are presented in a compact analytic form with a small number of constants, making the cross sections suitable for modeling applications.
Anharmonic and harmonic intermolecular vibrational modes of the DNA base pairs
Authors: Špirko Vladimı́r , Šponer Jiřı́ , Hobza Pavel
Published: July 2002

Intermolecular vibrational modes of the H-bonded adenine…thymine Watson–Crick (AT) base pair were studied for the first time using multidimensional nonharmonic treatment. Relying on a Born–Oppenheimer–like separation of the fast and slow vibrational motions, the complete multidimensional vibrational problem is reduced to a six-dimensional subproblem in which all rearrangements between the pair fragments (i.e., adenine and thymine) can be described. Following the Hougen–Bunker–Johns approach and using appropriate vibrational coordinates, a nonrigid reference is defined which covers all motions on the low-lying part of the intermolecular potential surface and which facilitates the derivation of a suitable model Hamiltonian. The potential energy surface is determined at the ab initio Hartree–Fock level with minimal basis set (HF/MINI-1) and an analytic potential energy function is obtained by fitting to the ab initio data. This function is used to calculate vibrational energy levels and effective geometries within the framework of the model Hamiltonian, disregarding the role of the kinematic and potential (in-plane)–(out-of-plane) interactions. The calculations are in reasonable agreement with the normal coordinate analysis (harmonic treatment) thus indicating physical correctness of this standard approach for an approximate description of the lowest vibrational states of the AT base pair. In addition, to get a deeper insight, harmonic vibrational frequencies of the AT pair and 28 other base pairs are evaluated at the same and higher levels of theory [ab initio Hartree–Fock level with split-valence basis set (HF/6-31G**)]. The HF/MINI-1 and HF/6-31G** intermolecular harmonic vibrational frequencies differ by less than 30%. For all the base pairs, the buckle and propeller vibrational modes [for definition and nomenclature see R. E. Dickerson et al., EMBO J. 8, 1 (1989)] are the lowest ones, all being in the narrow interval (from 4 to 30 cm−1 in the harmonic approximation). Although most of the evaluated harmonic frequencies are qualitatively correct approximations to the genuine vibrational frequencies, in some cases due to a strong nonharmonic behavior of the buckle motion, they become physically meaningless. To get physically correct results in such cases, apparently, the standard harmonic oscillator approach should be replaced by a more adequate approach, for instance, by the approach we used in the case of the adenine…thymine pair.
On the calculation of transport properties of polymer melts from nonequilibrium molecular dynamics
Authors: Xu Zhengfang , Khare Rajesh , de Pablo Juan J. , Kim Sangtae
Published: July 2002

This note presents an analysis of the viscosity of linear and branched polymer melts at low shear rates and compares the results of several different nonequilibrium molecular dynamics methods. The new, more precise results presented here indicate that earlier calculations of the zero shear viscosity of linear polymers are consistent with Green–Kubo estimates. For branched chains, however, these results suggest that earlier extrapolation of viscosities to zero shear rate overpredicted the effect of branching.
A new N-body potential and basis set for adiabatic and non-adiabatic variational energy calculations
Authors: Kinghorn Donald B. , Adamowicz Ludwik
Published: July 2002

A new functional form for multi-body expansions of potential energy surfaces and basis functions for correlated adiabatic and fully non-adiabatic variational energy calculations is presented. N-body explicitly correlated Gaussians with pre-multiplying factors consisting of products of powers of internal distance coordinates are utilized in a dual role to analytically represent isotropic potentials and energy eigen-functions in the same internal coordinate system. Practical aspects of this new methodology are presented. The ideas and methods are prototyped and illustrated with two simple diatomic examples; the Morse potential and an accurate H2 potential for which essentially exact results are obtained for vibrational energy levels.
On the size and shape of self-assembled micelles
Authors: Nelson Peter H. , Rutledge Gregory C. , Hatton T. Alan
Published: July 2002

Equilibrium size and shape distributions of self-assembled micelles are investigated using lattice Monte Carlo simulation techniques. The micellar size distributions are shown to include a Gaussian peak of spherical micelles, in combination with an exponential tail of cylindrical micelles.
Numerical study of the phase behavior of rodlike colloids with attractive interactions
Authors: Bolhuis Peter G. , Stroobants Alain , Frenkel Daan , Lekkerkerker Henk N. W.
Published: July 2002

We examine the influence of attractive interactions on the phase behavior of rodlike colloids. We model the rodlike particles by spherocylinders, for which the phase diagram, in the absence of attraction, is known for arbitrary aspect ratio. We consider the case that the attraction is due to depletion forces caused by the addition of nonadsorbing polymer. The range of this attraction is determined by the size of the polymer. If the radius of gyration of the polymer is small compared to the diameter of the rods, we can model the polymer-induced attraction by a suitable generalization of the square-well model for spherical particles. However, for longer ranged attractions, pairwise additive attractions lead to phase behavior that is very different from what is found when the nonadditivity of depletion forces is taken into account. In our simulations, we find evidence for demixing transitions in the isotropic, nematic and solid phases. We compare our simulation results with predictions based on the perturbation theory of Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. A crucial input in this theory is the so-called free-volume fraction of the hard spherocylinder reference system. In the work of Lekkerkerker and Stroobants, this quantity is estimated using scaled-particle theory. We test the validity of this approach by comparing it to numerical results for the free-volume fraction.
The short-time intramolecular dynamics of solutes in liquids. II. Vibrational population relaxation
Authors: Goodyear Grant , Stratt Richard M.
Published: July 2002

Events such as the vibrational relaxation of a solute are often well described by writing an effective equation of motion—a generalized Langevin equation—which expresses the surrounding medium’s influence on the intramolecular dynamics in terms of a friction and a fluctuating force acting on the solute. These quantities, though, can be obtained from the instantaneous normal modes (INMs) of the system when the relaxation takes place in a fluid, suggesting that we should be able to analyze in some detail the solvent motions driving the relaxation, at least for short times. In this paper we show that this promise can indeed be realized for the specific case of a vibrating diatomic molecule dissolved in an atomic solvent. Despite the relatively long times typical of vibrational population relaxation, it turns out that understanding the behavior of the vibrational friction at the short times appropriate to INMs (a few hundred femtoseconds) often suffices to predict T1 times. We use this observation to probe the dependence of these relaxation rates on thermodynamic conditions and to look at the molecular mechanisms underlying the process. We find that raising the temperature at any given density or raising the density at any given temperature will invariably increase the rate of energy relaxation. However, since these two trends may be in conflict in a typical constant-pressure laboratory experiment, we also find that it is possible to make sense of the “anomalous” inverted temperature dependence recently seen experimentally. We find, as well, that the INM theory—which has no explicit collisions built into it—predicts exactly the same density dependence as the venerable independent-binary-collision (IBC) theory (an intriguing result in view of recent claims that experimental observations of this kind of dependence provide support for the IBC theory). The actual mechanisms behind vibrational population relaxation are revealed by looking in detail at the vibrational friction “influence spectrum”—the spectrum of INMs weighted by how efficiently each mode acts to promote the relaxation. Through suitable projections we show that the average influence spectrum is dominated by longitudinal motion of the solvent atoms in the first solvation shell. We go further, however, and examine the nature of the instantaneous relaxation promoted by individual liquid configurations. The number of instantaneous modes that contribute significantly fluctuates strongly from configuration to configuration, and the number of solvent atoms strongly coupled to the solute has a certain amount of variation as well, but invariably each significant mode ends up promoting the relaxation by moving just one or two significant solvent atoms—a feature we explore in a companion paper.
Structural study of supercritical water. I. Nuclear magnetic resonance spectroscopy
Authors: Matubayasi Nobuyuki , Wakai Chihiro , Nakahara Masaru
Published: July 2002

The proton chemical shift of water is measured at temperatures up to 400°C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60g/cm3. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is more than one for a water molecule in the supercritical densities. The density and temperature dependence of the chemical shift at supercritical temperatures are analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities.
Semiclassical molecular dynamics simulations of ultrafast photodissociation dynamics associated with the Chappuis band of ozone
Authors: Batista Victor S. , Miller William H.
Published: July 2002

In this paper we investigate the nonadiabatic ultrafast dynamics of interconversion between the two lower lying excited states of A″1 symmetry (1 A″1 and 2 A″1) of ozone following photoexcitation of the molecule in the gas phase with visible light. Our algorithm involves a semiclassical initial value representation method which is able to describe electronically nonadiabatic processes within the time dependent picture through the quantization of the classical electron–nuclear model Hamiltonian of Meyer and Miller [J. Chem. Phys. 70, 7 (1979)]. We explore the capabilities of these techniques as applied to studying the dynamics of a realistic reaction of photodissociation on full ab initio electronic potential energy surfaces. Our semiclassical results provide an intuitive understanding of the most fundamental dynamical features involved in the process of predissociation, such as decay and recurrence events, as well as an interpretation of experimental studies of the Chappuis band of ozone in the frequency domain.
Valence shell structures in the distributions of the Laplacian of the electron density and the one-electron potential for diatomic molecules
Authors: Chan Wai-To , Hamilton I. P.
Published: July 2002

The valence shell charge concentration, VSCC, in an atom is defined by Bader as the outermost spherical region for which ∇2ρ(r)<0. We compare and contrast an alternate definition of the VSCC as the outermost spherical region for which ∇2ρ(r)/2ρ(r)<0. The quantity ∇2ρ(r)/2ρ(r), termed the one-electron potential (OEP), is implicit in the (exact) one-electron Schrödinger equation. In a homonuclear diatomic bound by shared interaction, the VSCCs of the atoms are merged. Provided that the merged VSCCs persist, the diatomic is enclosed by an outermost zero-valued surface encompassing the VSCC of the molecule. The outermost ∇2ρ(r)=0 surface is termed the reactive surface while the outermost OEP=0 surface is termed the molecular envelope. In cases where the VSCCs are not revealed in the atoms, the reactive surfaces and molecular envelopes are incomplete or absent in the diatomics. We show that in many diatomics the molecular envelope is present although the reactive surface is missing. In an ionic diatomic bound by closed-shell interaction, the VSCC of the cation can disappear. Otherwise, fragments of the cationic VSCC, termed ligand-opposed charge-concentration (LOCC), may remain opposite to the anion. In general, the LOCC is absent when the VSCC is not revealed in the atom. We show that the LOCC is more widespread in the distribution of OEP than ∇2ρ(r).
Numerical pattern recognition analysis of acetylene dispersed fluorescence spectra
Authors: O’Brien Jonathan P. , Jacobson Matthew P. , Sokol Jennifer J. , Coy Stephen L. , Field Robert W.
Published: July 2002

Polyad quantum numbers have been assigned to 134 vibrational levels of the X̃1Σg+ state of acetylene with internal energies from 3,000 to 15,000 cm−1. These polyad assignments have been made possible by two advances: (1) the recording of new, rigorously calibrated acetylene Ã1Au→X̃1Σg+ dispersed fluorescence spectra, and (2) the development of a numerical pattern recognition technique which identifies groups of transitions in the spectra that terminate on eigenstates with the same polyad quantum numbers. This pattern recognition technique is based on the Extended Cross-Correlation, which has been reported previously in this Journal [J. Chem. Phys. 107, 8349, 8357 (1997)], and requires neither a priori knowledge of the number of polyads in the spectra nor the pattern of spectral lines that is associated with each polyad. No evidence for the breakdown of the polyad quantum numbers is found, at the 7 cm−1 resolution of our spectra, at internal energies up to at least 15,000 cm−1. The ability to assign polyad quantum numbers to spectral features with up to 15,000 cm−1 of internal energy provides a panoramic perspective on the trends in the short-time (∼1 ps) dynamics of acetylene at high internal energy.
Computer simulations of bilayer membranes: Self-assembly and interfacial tension
Authors: Goetz Rüdiger , Lipowsky Reinhard
Published: July 2002

Binary Lennard-Jones fluids consisting of “solvent” and “surfactant” molecules are studied as simplified model systems for amphiphilic molecules in solution. Using Monte Carlo and molecular dynamics simulations, we observe the self-assembly of the surfactant molecules into bilayer membranes. These bilayers are fluid since the surfactants exhibit rapid lateral diffusion. We also measure the interfacial tension and the compressibility modulus of these bilayers. We show that they exhibit a tensionless state and characterize the corresponding stress profile. In this way, we bridge the gap between previous theoretical studies which were based (i) on discrete models with atomic resolution and (ii) on continuum models in which the bilayer membrane is treated as a smooth surface.
HF dimer: Empirically refined analytical potential energy and dipole hypersurfaces from ab initio calculations
Authors: Klopper Wim , Quack Martin , Suhm Martin A.
Published: July 2002

The interaction between two HF molecules as a function of all six internal coordinates is reported as calculated for 3284 selected points in configuration space at counterpoise-corrected explicitly correlated levels and fitted to an analytical expression, which is described in detail. The unweighted rms deviation for all 3284 points is 21 cm−1. Empirical refinements are applied through mixing and scaling of the ab initio data, guided by the comparison of multidimensional nuclear quantum energy levels with experimental data. The resulting semiempirical pair potentials (labeled SC-2.9 and SO-3) contain 67 and 61 freely adjusted parameters and are combined with a four parameter monomer potential of generalized Pöschl–Teller type. Various minimum energy paths and cuts are investigated. Major improvements over earlier HF dimer potentials are demonstrated via multidimensional solutions of the nuclear Schrödinger equation. Comparison with other high level ab initio calculations and with various experimental data reveals very good overall consistency. The new potential suggests strong Coriolis coupling in the librational degrees of freedom. Best estimates of stationary points, of the dimer dissociation energy (De=19.1±0.2 kJ/mol), of the electronic barrier to hydrogen bond exchange (4.2±0.2 kJ/mol), of the electronic barrier to linearity (3.9±0.2 kJ/mol), and of the electronic barrier to hydrogen exchange (175±10 kJ/mol) are inferred. Based on accurate electric dipole functions for the monomer and distortion contributions calculated with a large basis at SCF level, a simple analytical six-dimensional electric dipole hypersurface is presented.
Liquid crystalline phase behavior in systems of hard-sphere chains
Authors: Williamson Dave C. , Jackson George
Published: July 2002

A study of the liquid crystalline phase transitions in a system of hard-sphere chains is presented. The chains comprise m=7 tangentially bonded hard-sphere segments in a linear conformation (LHSC). The isothermal–isobaric Monte Carlo simulation technique is used to obtain the equation of state of the system both by compressing the isotropic (I) liquid and by expanding the solid (K). As well as the usual isotropic and solid phases, nematic and smectic-A liquid crystalline states are seen. A large degree of hysteresis is found in the neighborhood of the I–N transition. The results for the rigid LHSC system were compared with existing data for the corresponding semiflexible hard-sphere chains (FHSC): the flexibility has a large destabilizing effect on the nematic phase and consequently it postpones the I–N transition. The results of the simulations are also compared with rescaled Onsager theories for the I–N transition. It is rather surprising to find that the Parsons approach, which has been so successful for other hard-core models such as spherocylinders and ellipsoids, gives very poor results. The related approach of Vega and Lago gives a good description of the I–N phase transition. The procedure of Vega and Lago, as with all two-body resummations of the Onsager theory, only gives a qualitative description of the nematic order.
Ab initio/RRKM approach toward the understanding of ethylene photodissociation
Authors: Chang A. H. H. , Mebel A. M. , Yang X.-M. , Lin S. H. , Lee Y. T.
Published: July 2002

The optimized structures and harmonic frequencies for the transition states and intermediates on the ground state potential energy surfaces of ethylenes, including C2H4, C2D4, D2CCH2, and cis- and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G(d,p) level. The coupled cluster method, CCSD(T)/6-311+G(3df,2p), was employed to calculate the corresponding energies with the zero-point energy corrections by the B3LYP/6-311G(d,p) approach. Ethylidene was found to be an intermediate in the 1,2-H2 elimination channel. The barrier for the 1,1-H2 elimination was computed to be the lowest (4.10–4.16 eV), while the 1,2-H2 elimination and H loss channels have barriers of a similar height (4.70–4.80 eV). The rate constant for each elementary step of ethylene photodissociation at 193 and 157 nm was calculated according to the RRKM theory based on the ab initio surfaces. The rate equations were subsequently solved, and thus the concentration of each species was obtained as a function of time. The concentrations at t→∞ were taken for calculating branching ratios or yields. In accord with previous experimental findings, the calculated branching ratio for the 1,1-H2 elimination process is higher than that for the 1,2-H2 elimination, and the atomic elimination channel is predicted to be favored at increasing excitation energy when competing with the molecular elimination. The significant discrepancies between theoretical and experimental results in the magnitude of the yields and their dependence on the wavelength for the molecular elimination channels suggest the dynamics of either 1,2-H2, or 1,1-H2 elimination, or both channels may be nonstatistical in nature.
On the semiclassical description of quantum coherence in thermal rate constants
Authors: Sun Xiong , Wang Haobin , Miller William H.
Published: July 2002

An earlier paper of ours [J. Chem. Phys. 108, 9726 (1998)] used an approximate (linearized) version of the semiclassical initial value representation (SC-IVR) to calculate reactive flux correlation functions for a model of unimolecular isomerization, namely a 1-d double well potential coupled to an infinite set of harmonic oscillators, obtaining excellent agreement with accurate quantum results for this system. Here we analyze this linearized approximation (LA) further, however, and show that it is not capable of describing quantum interference/coherence effects in the longer time recrossing behavior of the isomerization dynamics. (The recrossing effects seen in our earlier work were due to classical mechanics.) To accentuate quantum effects in the recrossing dynamics, the present article considers the double well potential without the harmonic bath, using both the LA and the full SC-IVR. The results of the calculations show that the flux correlation functions given by the LA agrees well with the exact quantum correlation function for times up to ≈ℏβ, meaning that it describes quantum effects in the direct or transition state theory like dynamics accurately. For the longer time recrossing dynamics, however, it agrees with the classical rather than the quantum correlation function, i.e., it does not describe quantum coherence effects on this time scale. The full SC-IVR calculations, however, are in reasonably good agreement with the quantum correlation function for these longer times.
Radial distribution functions for a multicomponent system of sticky hard spheres
Authors: Santos Andrés , Yuste Santos Bravo , López de Haro Mariano
Published: July 2002

A method to obtain (approximate) analytical expressions for the radial distribution functions and structure factors in a multicomponent system of sticky hard spheres is introduced. In its simplest implementation, the method yields the Percus–Yevick approximation. In the next order, only contact values of the cavity functions and the isothermal compressibility are required. Some tentative strategies to determine the input values are discussed. Illustrative examples following these strategies, in which the radial distribution functions and structure factors are computed, are also presented.
Distribution of solute at solid–liquid interface during solidification of melt
Authors: Fukui Keisuke , Maeda Kouji
Published: July 2002

A model for predicting a distribution coefficient (ki) of solute at the solid–liquid (S–L) interface, when the solid layer is growing, is proposed. The interfacial distribution coefficient is expressed as a function of two gradients of the liquid concentration and equilibrium concentration at the S–L interface. The model is applied to the solidification of a simple eutectic binary liquid of lauric acid and myristic acid in an enclosed rectangular box in which a vertical wall is cooled. The impurity-concentration profile in solid is predicted from the direct numerical computations.
Ab initio and regularized force fields of haloethanes: CH3CH2Cl, CH3CHCl2, CH3CF2Cl, and CH3CFCl2
Authors: Kuramshina G. M. , Weinhold F. , Pentin Yu. A.
Published: July 2002

We present a systematic application of Tikhonov’s regularization method to joint treatment of ab initio and experimental vibrational data for a series of mono-, di-, and tri-substituted haloethanes, based on uniform MP2/6-31G* level of theory. This leads to MP2/6-31G*-“regularized” force field parameters that exactly reproduce experimental frequencies and isotopomer shifts (within prescribed error levels) and are “closest” (in Euclidean norm) to the ab initio force field. We correct a number of previous experimental spectral assignments and investigate the limits of transferability (e.g., of methyl-group force field parameters from one species to another) and other common simplifying approximations (e.g., MVFF modified valence force field treatment). Our results demonstrate how one can systematically combine experimental and ab initio information to create a data base of freon-type force field parameters having greater accuracy and reliability than could be obtained from theory or experiment alone.
Accurately solving the electronic Schrödinger equation of atoms and molecules using explicitly correlated (r12-)MR-CI. II. Ground-state energies of first-row atoms and positive atomic ions
Authors: Gdanitz Robert J.
Published: July 2002

The recently proposed (explicitly correlated) r12-MR-CI and r12-MR-ACPF (averaged coupled-pair functional) methods are applied to the computation of the clamped-nuclei nonrelativistic ground-state energies of the first-row atoms and their positive ions. For the neutral atoms we obtain accuracies of −0.05 (He and Li), −0.013 (Be), +0.12 (B), −0.1 (C and N), +0.3 (O) and +0.6 (F and Ne) mEh. Our energies of B–F are by far the best available. In all cases, the energy eigenvalues of the Schrödinger equation are calculated to better than chemical accuracy (1 kcal/mol). Since our method is completely general, this, for the first time, implies the possibility of performing quantum chemical calculations of general many-electron systems where the error of the computed energy is not any more very large compared to the desired accuracy.
Ionic charging free energies: Spherical versus periodic boundary conditions
Authors: Darden Tom , Pearlman David , Pedersen Lee G.
Published: July 2002

Ionic charging free energies calculated by Ewald summation differ substantially from those calculated in spherical cluster calculations, with or without the inclusion of a Born correction in the latter. Using Gauss’ law, we derive an electrostatic potential for ions in spherical clusters that involves contributions only from the interior solvent. This “interior” potential agrees with the “P-summation” approach proposed by Hummer et al. [J. Phys. Chem. B 101, 3017 (1997)], and leads to charging free energies which agree, within simulation error, with those given by Ewald summation with finite-size corrections. The difference in charging free energies between this approach and the conventional cluster free energies including the Born correction is traced to the surface potential of water.
Virial expansion and liquid–vapor critical points of high dimension classical fluids
Authors: Mon K. K. , Percus J. K.
Published: July 2002

In this paper, we study the liquid–vapor transition of a classical fluid with hard sphere repulsion and short range attraction at criticality in the high dimension limit. We are motivated by physical arguments that relate the presence of percolating clusters to the critical point of the liquid–vapor transition. The volume fraction at the percolation threshold is also expected to decrease for increasing dimension. Together, these imply that the critical density may decrease with dimensionality and for sufficiently high dimension, the truncated low order virial expansion should suffice in describing the critical point. We evaluate analytically the exact second and third virial coefficients for high dimension and explicitly show the modifications of the hard sphere coefficients by the attractive part of the potentials. Then, the truncated virial expansion at criticality is used to demonstrate that the critical volume fraction indeed decreases with dimensionality. This will allow us to propose that the truncated virial expansion provides an accurate description of fluid criticality in the high dimension limit.
Photodissociation spectroscopy and dynamics of the methylthio radical (CH3S)
Authors: Bise Ryan T. , Choi Hyeon , Pedersen Henrik B. , Mordaunt David H. , Neumark Daniel M.
Published: July 2002

The photodissociation spectroscopy and dynamics of the CH3S and CD3S radicals have been investigated using fast radical beam photofragment spectroscopy of the Ã 2A1←X̃ 2E electronic band (T0≅26 400 cm−1) and an unstructured band near 45 600 cm−1. At all energies, only one major channel, CH3(X̃ 2A2″)+S(3Pj), was observed. Photofragment yield spectra for the Ã 2A1←X̃ 2E electronic band show resolved vibrational progressions extending well beyond those seen in laser-induced fluorescence studies of this band. Photofragment translational energy distributions yield the S(3Pj) fine-structure distribution for each vibrational level of the CH3 product. Photofragment angular distributions were found to be highly anisotropic (β=−0.2 to −1.0±0.1) with increasing anisotropy at higher photon energies. The results yield a refined heat of formation for CH3S (1.346±0.018 eV) as well as the mechanism by which the Ã 2A1 state is predissociated. Results at 45 600 cm−1 imply that dissociation occurs on the repulsive B̃ 2A2 state.
Resonant photodetachment via shape and Feshbach resonances: p-benzoquinone anions as a model system
Authors: Schiedt J. , Weinkauf R.
Published: July 2002

For p-benzoquinone anions, the photodetachment spectrum at 0.15–0.65 eV above detachment threshold shows sharp and broad resonances, which we assign to enhanced photodetachment via resonantly excited anion states. The experiment is performed at cold and mass-selected anions to exclude contributions of fragment anions and internally excited molecules. The most prominent, intense and broad spectral feature at 20 200 cm−1 is assigned to an allowed transition from the 2B2g anion ground state to the 2Au shape resonance, which corresponds to a πLUMO*→π* electron promotion. By linewidth we determine an ultrashort lifetime of 25 fs in qualitative agreement with a one-electron autodetachment process. In contrast to this, for the narrow resonances lifetimes between 0.2 and 1.2 ps are determined, in agreement with a slower autodetachment by a two-electron process from Feshbach states. Because of their low photoexcitation cross section they are assigned to dipole and symmetry forbidden n→πLUMO* transitions which can both be only optically active in some vibrations by Herzberg–Teller coupling to the nearby 2Au anion state. The photodetachment photoelectron spectra recorded with wavelengths resonant to some of these excited anion states show that the intensities of the neutral ground state vibrations are mostly determined by the autodetachment process via the excited anion resonance. This shows that the resonant photodetachment is by far the predominant process and not excitation into continuum. The vibrational origin of the anion to neutral transition is situated at 1.860 eV±5 meV and gives directly the electron affinity of p-benzoquinone. The Feshbach and shape states form a dense electronic state ladder, which can enhance fast radiationless relaxation processes, making p-benzoquinone a very efficient electron acceptor in gas phase and in solution.
The “ionic” to “molecular” transitions in AlCl3 and FeCl3 as predicted by an ionic interaction model
Authors: Hutchinson Francis , Walters Malcolm K. , Rowley Adrian J. , Madden Paul A.
Published: July 2002

A polarizable ionic interaction model for AlCl3 and FeCl3, constructed from potentials similar to those used for a variety of ionic halides by simply scaling the cation radius, is found to give a semi-quantitative account of the structural and dynamical properties of the liquid and solid phases of AlCl3 and FeCl3. This indicates that the “covalent” interactions responsible for the remarkable, quasi-molecular nature of these two liquids are contained within the ionic model.
Atom–bond electronegativity equalization method. II. Lone-pair electron model
Authors: Wang Chang-Sheng , Yang Zhong-Zhi
Published: July 2002

Based on the principle of electronegativity equalization and density functional theory, a lone pair electron model was developed through partitioning the molecular electron density into atomic electron densities, chemical bond electron densities, and lone pair electrons’ densities. The expressions of total molecular energy and the “effective electronegativity” of an atom or a chemical bond or a lone pair electron are obtained. Then the lone pair electron electronegativity χlp is explicitly defined and the corresponding parameters are calibrated through regression and least-squares optimization procedure. The atom–bond electronegativity equalization method plus lone pair electron model (ABEEM+lp) is then proposed for the direct calculation of the charge distribution and charge polarization in large molecules. In this paper we give the results of the charge distributions in some large molecules obtained by ABEEM+lp. Through comparing the ABEEM+lp charge distributions and ab initio ones, it can be concluded that the lone pair electron model and the parameters are reasonable. In the following paper we will discuss the results of charge polarization in molecules obtained by ABEEM+lp. Through comparing ABEEM+lp charge polarization and ab initio ones, it will be seen that considering lone pair electron explicitly is not only reasonable but also necessary.
Re-examination of atomization energies for the Gaussian-2 set of molecules
Authors: Feller David , Peterson Kirk A.
Published: July 2002

Atomization energies were computed for 73 molecules, many of them chosen from the GAUSSIAN-2 and G2/97 test sets. A composite theoretical approach was adopted which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory with a quasiperturbative treatment of triple excitations and three corrections: (1) a coupled cluster core/valence correction; (2) a configuration interaction scalar relativistic correction; and (3) an atomic spin-orbital correction. A fourth correction, corresponding to more extensive correlation recovery via coupled cluster theory with an approximate treatment of quadruple excitations, was examined in a limited number of cases. For the molecules and basis sets considered in this study, failure to consider any of these contributions to the atomization energy can introduce errors on the order of 1–2 kcal/mol. Although some cancellation of error is common, it is by no means universal and cannot be relied upon for high accuracy. With the largest available basis sets (including, in some cases, up through aug-cc-pV6Z), the mean absolute deviation with respect to experiment was found to lie in the 0.7–0.8 kcal/mol range, neglecting the effects of higher order excitations. Worst case errors were 2–3 kcal/mol. Several complete basis set extrapolations were tested with regard to their effectiveness at improving agreement with experiment, but the statistical difference among the various approaches was small.
Influence of cholesterol on dynamics of dimyristoylphosphatidylcholine bilayers as studied by deuterium NMR relaxation
Authors: Trouard Theodore P. , Nevzorov Alexander A. , Alam Todd M. , Job Constantin , Zajicek Jaroslav , Brown Michael F.
Published: July 2002

Investigation of the deuterium (2H) nuclear magnetic resonance (NMR) relaxation rates of lipid bilayers containing cholesterol can yield new insights regarding its role in membrane function and dynamics. Spin-lattice (R1Z) and quadrupolar order (R1Q) H2 NMR relaxation rates were measured at 46.1 and 76.8 MHz for macroscopically oriented bilayers of 1,2-diperdeuteriomyristoyl-sn-glycero-3-phosphocholine (DMPC-d54) containing cholesterol (1/1 molar ratio) in the liquid-ordered phase at 40 °C. The data for various segmental positions along the DMPC-d54 acyl chain were simultaneously fitted to a composite membrane deformation model, including fast segmental motions which preaverage the coupling tensor along the lipid acyl chain, slow molecular reorientations, and small-amplitude collective fluctuations. In contrast to pure DMPC-d54 in the liquid-crystalline (Lα) phase, for the DMPC-d54:cholesterol (1/1) system a linear square-law functional dependence of the relaxation rates on the order parameter (quadrupolar splitting) does not appear evident. Moreover, for acyl segments closer to the top of the chain, the angular anisotropy of the H2 R1Z and R1Q relaxation rates is more pronounced than toward the chain terminus. The residual (preaveraged) coupling tensor has its greatest effective asymmetry parameter near the polar groups, decreasing for the groups closest to the end of the chain. The results suggest that axial rotations of the phospholipid molecules occur at a somewhat higher rate than in pure bilayers, as a consequence of the higher ordering and reduction of chain entanglement. On the other hand, the rigid cholesterol molecule appears to undergo somewhat slower axial rotation, possibly due to its noncylindrical shape. Collective motions are found to be less predominant in the case of DMPC-d54:cholesterol than for pure DMPC-d54, which may indicate an increased dynamical rigidity of lipid bilayers containing cholesterol versus pure lipid systems.