bluuebunny/crossref_metadata_2025 huggingface dataset:
A computer simulation study of the liquid–vapor coexistence curve of water
Authors: Guissani Yves , Guillot Bertrand
Published: July 2002

The liquid–vapor coexistence curve of a model water (the extended simple point charge model, SPCE) is evaluated by molecular dynamics simulation in the (N,V,E) ensemble. It is shown that the simulated system (N=256 water molecules) is too small to present a spinodal decomposition and, hence, can be described by a classical equation of state whose the critical parameters (Tc=651.7 K, ρc=0.326 g/cm3, and Pc=189 bar) are found to be very close to that of real water (Tc=647.13 K, ρc=0.322 g/cm3, and Pc=220.55 bar). The critical parameters for SPCE water in the thermodynamic limit are deduced from the simulation data employing Wegner type expansions for the order parameter and the coexistence curve diameter; here also the values of the critical parameters (Tc=640 K, ρc=0.29 g/cm3, and Pc=160 bar) are close to that of real water. The temperature dependence of the dielectric constant for water and steam at orthobaric densities is next evaluated between ambient and Tc; the agreement with the experimental data is quite remarkable (e.g., εSPCE=81.0 at 300 K and εSPCE=6. at Tc instead of 78.0 and 5.3, respectively, in real water). The modifications experienced by water’s architecture with the temperature are deduced from the evaluation of the atom–atom correlation functions. It is shown that a structural change occurs in the temperature range 423–473 K. This important reorganization is characterized by a shift of the second shell of neighbors from 4.5 to 5.5 A and the loss of almost all angular correlations beyond the first solvation shell. Moreover, it is observed that the average number of hydrogen bonds per molecule nHB scales with the density all along the saturation curve. In the same way the values of nHB for orthobaric densities seems to follow a law analogous to the law of rectilinear diameter for orthobaric densities.
Effective core potential-configuration interaction study of electronic structure and geometry of small neutral and cationic Agn clusters: Predictions and interpretation of measured properties
Authors: Bonačić-Koutecký V. , Češpiva L. , Fantucci P. , Koutecký J.
Published: July 2002

The ground state geometries of small neutral Agn(n=2–9) and cationic Agn+ (n=2–9) clusters have been determined in the framework of the Hartree–Fock and complete active space self-consistent field (CASSCF) procedure employing a relativistic effective core potential (RECP) accounting for core–valence correlation effects (CVC). Similarities and differences between topologies of neutral and charged structures have been found. Large scale configuration interaction (CI) calculations for 5s electrons only have been carried out for determining stabilities and ionization potentials (IP). A comparison between predicted and measured observables allows tentative geometrical assignments. Structural and electronic properties of small Ia and Ib clusters are compared.
Molecular conformation and potentials for out-of-plane ring vibrations in the S and S1 electronic states of 1,4-benzodioxan from supersonic-jet fluorescence and high-resolution absorption measurements
Authors: Gordon Robert D. , Hollas J. Michael
Published: July 2002

In a study of the 281 nm S1–S0 electronic spectrum of 1,4-benzodioxan, the high-resolution vapor absorption spectrum, the fluorescence excitation spectrum in a supersonic jet, and the dispersed single-vibronic-level fluorescence spectra for excitation into several different S1 levels have been measured. Four out-of-plane ring vibrations have been characterized in each electronic state, and it is shown that the partially saturated six-membered ring is rigidly nonplanar with a twisted C2 conformation. The twisting vibration is nearly harmonic and the barrier to planarity is high in both electronic states; however, there is some lowering of the barrier upon excitation. There is strong Duschinsky mixing in S1 between two of the out-of-plane ring bending modes, one of which is quite anharmonic with a flat-bottomed potential.
Transport theory of dense, strongly inhomogeneous fluids
Authors: Pozhar Liudmila A. , Gubbins Keith E.
Published: July 2002

The generalized Enskog-like kinetic equation (GEKE) derived recently for inhomogeneous fluids [L. A. Pozhar and K. E. Gubbins, J. Chem. Phys. 94, 1367 (1991)] has been solved using the 13-moments approximation method to obtain linearized Navier–Stokes equations and the associated zero-frequency transport coefficients. Simplified transport coefficient expressions have been obtained for several special cases (simplified geometries, homogeneous fluid). For these cases it is shown that the main contributions to the transport coefficients can be related to those for dense homogeneous fluids calculated at ‘‘smoothed’’ number densities and pair correlation functions. The smoothing procedure has been derived rigorously and shown to be an intrinsic feature of the GEKE approach. These results have been established for an arbitrary dense inhomogeneous fluid with intermolecular interactions represented by a sum of hard-core repulsive and soft attractive potentials in an arbitrary external potential field and/or near structured solid surfaces of arbitrary geometries.
Ab initio and semiempirical methods for molecular dynamics simulations based on general Hartree–Fock theory
Authors: Hammes-Schiffer Sharon , Andersen Hans C.
Published: July 2002

We present two new methods for molecular dynamics simulations based on general Hartree–Fock (GHF) theory. The first method involves approximating ab initio STO-3G matrix elements with fitting functions to enable faster computation of the energy and forces for molecular dynamics simulations. The implementation of this method includes a frozen-core approximation. The second method involves developing semiempirical potentials by reparametrizing the fitting functions obtained in the first method to fit experimental data. This second method enables us to reproduce experimental quantities with only the computational effort of an STO-3G calculation. We successfully applied both of these methods in conjunction with the Car–Parrinello ab initio molecular dynamics method to the geometry optimization of lithium clusters, cationic and neutral, of up to five atoms.
The computation of a potential of mean force: Choice of the biasing potential in the umbrella sampling technique
Authors: Beutler Thomas C. , van Gunsteren Wilfred F.
Published: July 2002

Generally, the conformational equilibrium of flexible molecules changes upon solvation. The equilibrium distribution of conformers is determined by the potential of mean force for the relevant degrees of freedom. However, the determination of the potential of mean force by straightforward computer simulation is often impracticable owing to the presence of large energy barriers which prevent efficient sampling. A frequently chosen technique to overcome the problems associated with such barriers is the umbrella sampling technique. Using molecular dynamics simulation of 1,2-dichloroethane solvated in water as a model case, we have investigated the influence of the choice of the biasing potential in the umbrella sampling technique on the computation of the dihedral angle potential of mean force. The efficiencies of one- and multiple-window techniques are compared. For short simulation periods (25 ps) the latter technique performs better than the former. For longer simulation periods the performance difference is obscured by fluctuations caused by insufficient sampling of other slow processes occurring in the system, induced by the anisotropy of the periodic boundary conditions.
Transition states and folding dynamics of proteins and heteropolymers
Authors: Chan Hue Sun , Dill Ken A.
Published: July 2002

We study the folding dynamics of short two-dimensional self-avoiding lattice model proteins and copolymers with specific HP sequences (H, hydrophobic, P, polar) that fold to unique native structures. HH contacts are favorable. The conformational landscape is found by exhaustive enumeration using two different move sets, and time evolution is modeled by a transition matrix with the Metropolis criterion. The kinetic sequence of folding events depends strongly on both the monomer sequence and on the move set. But certain features are common to all sequences with both move sets. Beginning with open conformations, chains fold through multiple paths, are slowed by kinetic traps, and accumulate in compact nonnative conformations. Most chains must surmount energy barriers, which we identify as the transition state. Strikingly, the transition state involves a chain expansion, due to the breaking of HH contacts of compact denatured conformations. Mutations can significantly affect the energy landscape and folding dynamics, but in ways that appear too subtle to predict just from knowledge of native structures. Consistent with other models, we find that sequences with a large energy gap between the native and the next higher energy level can be stable and fold rapidly.
Evaporation of small fragments during the scattering of argon clusters at thermal kinetic energies from a graphite surface
Authors: Vach H. , De Martino A. , Benslimane M. , Châtelet M. , Pradère F.
Published: July 2002

We present a thermokinetic model together with new experimental results for the scattering of large argon clusters off a graphite surface. Both angular and time-of-flight distributions are shown for a large range of surface temperatures, incidence angles, and incident cluster sizes. A quantitative comparison between the proposed thermokinetic model and our measurements allows one to interpret most of the experimental results as due to thermal evaporation of very small fragments from their parent clusters gliding along the surface. The coefficient of tangential velocity conservation cF and the local temperature Tlocal of the evaporating fragments have been determined quantitatively. Although the investigated parameters were varied over a large range, Tlocal remains essentially constant around (140 ± 20) K. The coefficient cF turns out to be approximately (0.80 ± 0.05) independent of surface temperature and incident cluster size for all incidence angles larger than 40°. It increases, however, rapidly to 1.4 when the incidence angle is reduced to 20° suggesting a substantial transfer from normal to tangential kinetic energy. For high enough surface temperatures, incident cluster sizes, and incident angles, both experimental time-of-flight (TOF) and angular distributions show the appearance of a grazing exit angle component attributed to large cluster fragments leaving the surface before total evaporation.
Analytical derivatives for molecular solutes. II. Hartree–Fock energy first and second derivatives with respect to nuclear coordinates
Authors: Cammi R. , Tomasi J.
Published: July 2002

We present the theory of the analytical first and second derivatives of the Hartree–Fock energy for a molecular solute with respect to the nuclear coordinates, within the framework of the polarizable continuum model. The formulation refers to a cavity with an accurately modelled molecular shape.
Elastic constants of ice III by Brillouin spectroscopy
Authors: Tulk C. A. , Gagnon R. E. , Kiefte H. , Clouter M. J.
Published: July 2002

Brillouin spectroscopy has been used to determine the adiabatic elastic constants of ice III at −20 °C and 2.2 kbar. A new technique was used to grow large single crystals. The bulk adiabatic moduli were found to be C11=153.7, C12=99.5, C13=65.1, C33=115.5, C44=44.6, C66=56.8 kbar. An averaging method was used in conjunction with these constants to determine longitudinal and transverse velocities and elastic moduli for isotropic polycrystalline aggregates and compared to previous values. This work is now being extended to ice II, IV, V, and VI.
On the vibrational assignment of fullerene C60
Authors: Schettino Vincenzo , Remigio Salvi Pier , Bini Roberto , Cardini Gianni
Published: July 2002

A recent density functional perturbation theory calculation of the vibrational frequencies of C60 is compared with the infrared spectrum of the crystal. The vibrational assignment of C60 is completed with the help of the calculation plus the available infrared, Raman, and inelastic neutron scattering spectra.
Chain stiffness and bond correlations in polymer brushes
Authors: Wijmans Christopher M. , Leermakers Frans A. M. , Fleer Gerard J.
Published: July 2002

We have incorporated chain stiffness and correlations between neighboring bonds into a self-consistent field (SCF) lattice model for end-attached polymer layers (commonly known as ‘‘brushes’’). An increase in the chain stiffness leads to an increasing brush height. This increase is directly related to the change of the length of a Kuhn segment in the polymer chain. Introducing correlations between neighboring bonds gives a higher density of the brush, corresponding to a decrease of the brush height. For not too stiff chains these two effects virtually compensate each other. Hence, the volume fraction profile of ‘‘real’’ grafted chains is nearly identical to that of a polymer brush consisting of freely jointed chains.
Primitive models of chemical association. I. Theory and simulation for dimerization
Authors: Kalyuzhnyi Yu. V. , Stell G. , Llano-Restrepo M. L. , Chapman W. G. , Holovko M. F.
Published: July 2002

The structure and thermodynamic properties of a model of associating particles that dimerize into fused-sphere dumbbells are investigated by MC simulation and by integral-equation theory. The model particles, introduced by Cummings and Stell, associate as a result of shielded attractive shells. The integral equation theories are of two types. The first is an extension of Wertheim’s associative Percus–Yevick (APY) equation to the case of the shielded sticky shell model, which is the limiting case of the shielded attractive shell model that can be handled analytically. The second is the extended mean spherical approximation (EMSA) of Zhou and Stell applied to the shielded sticky shell model. In the case of partially associated systems, the EMSA requires as input the equilibrium association constant, which is obtained here using an exact relation between monomer density and a cavity correlation function, together with an equation of state due to Boublik. The structure obtained from the EMSA is in good agreement with the predictions of the MC simulation over a substantial density range that includes liquid-state densities, while the thermodynamic input from Boublik’s equation is in excellent agreement with the simulation results for all densities. Predictions of the APY approximation are also in good agreement with the simulation results as long as the density of the system is relatively low or, at high density, when the hard-core volume of a dimer is not substantially less than that of the two free monomers from which it is formed. There is an intermediate density range in which neither integral-equation theory gives correlation functions of high quantitative accuracy.
The Ω=1 van der Waals and Ω=0+ double well potentials of Xe 6s[3/2]1+Kr 1S determined from tunable vacuum ultraviolet laser spectroscopy
Authors: Pibel Charles D. , Yamanouchi Kaoru , Miyawaki Jun , Tsuchiya Soji , Rajaram Bhavani , Field Robert W.
Published: July 2002

The laser induced fluorescence spectrum of jet-cooled XeKr has been measured in the vicinity of the Xe 6s[3/2]01–1S0 atomic transition at 68 045.663 cm−1. The spectrum consists of two band systems, corresponding to transitions to the Ω=0+,1 electronic states from v″=0 of the ground electronic state. By using the observed band positions and intensities, we have constructed model potentials for both excited electronic states. The Ω=0+ state has a double minimum potential [inner well, re′ = 3.09(3) Å, De′ = 624(3) cm−1; outer well, re′ = 5.1(2) Å, De′ = 101(1) cm−1] while the Ω=1 state potential has only a shallow van der Waals potential [re′ = 5.24(4) Å, De′ = 52.2(7) cm−1]. The double minimum potential for the Ω=0+ state and the difference between the potentials for the Ω=0+ and Ω=1 states are understood in terms of the dominance of two different types of bonding interactions over different ranges of the internuclear distance. At long range, the interaction is dominated by weak dispersion and overlap repulsion between the closed shell Kr atom and the excited Xe atom, giving rise to shallow minima at r≊5 Å in both states. At short range, the XeKr interaction is better described by a XeKr+ ion-core with an excited 6sσ Rydberg electron.

The Ω=0+ state is associated with the strongly bound 2Σ+1/2 XeKr+ ion-core, while the Ω=1 state corresponds to the weakly bound 2Π3/2 XeKr+ ion-core. The dual nature of the bonding which gives rise to the double minimum potential in the Ω=0+ state is similar to the bonding seen in excited states of HgAr and HgNe [Duval et al., J. Chem. Phys. 85, 6324 (1986); Okunishi et al., ibid. 98, 2675 (1993); Onda et al., ibid. 101, 7290 (1994); Onda and Yamanouchi, ibid. (submitted)] or the long range s–s, short range d–d bonding seen in the ground state of Cr2 [Casey and Leopold, J. Chem. Phys. 97, 816 (1993)], but is different from some double minima states seen in other diatomics, such as H2 (E,F 1Σ+g) [Davidson, J. Chem. Phys. 35, 1189 (1960); Kolos and Wolniewicz, ibid. 50, 3228 (1968)], Na2 (4 1Σ+g) [Tsai et al., J. Chem. Phys. 101, 25 (1994)], and Cl2 (1 1Σ+u) [Yamanouchi et al., Chem. Phys. Lett. 156, 301 (1989); Tsuchizawa et al., J. Chem. Phys. 93, 111 (1990)] which arise from curve crossings between ionic and covalent diabatic states.
Orthogonally spin-adapted single-reference coupled-cluster formalism: Linear response calculation of static properties
Authors: Kondo A. E. , Piecuch P. , Paldus J.
Published: July 2002

An orthogonally spin-adapted coupled-cluster (CC) linear response (LR) method, to calculate first-order static properties of closed-shell systems, is formulated and implemented. Cluster operators are truncated at single and double excitations (CCSD-LR approach) and the explicit first-order CCSD-LR equations, in terms of molecular integrals and zeroth- and first-order cluster amplitudes, are given. Various aspects of the computer implementation of the CCSD-LR approach are discussed. In particular, the efficiency of the CCSD-LR approach is compared with corresponding finite field calculations. Advantages of using the LR approach are illustrated by calculating electrostatic multipole moments for CH2 and HF at equilibrium and displaced geometries.
Computer simulation of liquid/liquid interfaces. II. Surface tension-area dependence of a bilayer and monolayer
Authors: Feller Scott E. , Zhang Yuhong , Pastor Richard W.
Published: July 2002

A constant normal pressure-surface tension algorithm for molecular dynamics simulation, developed in the preceding paper, was used to laterally expand and compress the surface area of a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer. Then, from simulations carried out at constant normal pressure and surface area, values of the surface tension and other thermodynamic variables such as the internal energy and system volume were determined at four different values of the surface area per lipid, 60.0, 65.1, 68.1, and 72.1 Å2. The surface tension shows dramatic variations with area, going from 6 to 60 dyn/cm at areas per molecule of 65.1 and 68.1 Å2, respectively. An approximate thermodynamic analysis indicates that an area of 68.1 Å2/lipid is the closest of the four to the free energy minimum for this system, in agreement with experimental measurements. The effect of surface area changes on the calculated deuterium order parameters, which can be compared with those obtained from nuclear magnetic resonance experiments, is found to be quite large. Additionally, simulations of lipid monolayers were performed at the same surface areas and, though the dependence of the surface tension with area shows qualitative agreement with experiment, the simulation results are more sensitive to area changes than is observed experimentally. The variation in surface tension with area is much greater for the bilayer than the monolayer, suggesting that monolayers are a good model of bilayers only in a narrow range of surface areas.
Computer simulation studies of anomalous diffusion in gels: Structural properties and probe-size dependence
Authors: Netz Paulo A. , Dorfmüller Thomas
Published: July 2002

We have carried out simulations of particle diffusion through polyacrylamide gel networks. The model structures were built on a diamond lattice, in a simulation box with periodic boundary conditions. The method of structure generation consists of a random distribution of knots on the lattice and interconnection between randomly chosen pairs of knots. The structures generated by this procedure approximate the topology of real polymer gels. Parameters that control the distance between knots and the degree of stretching of the chain permit us to simulate a polyacrylamide system in which the concentration of species as well as the degree of crosslinking can be compared to realistic gels as prepared by the available experimental procedures. These structures were geometrically characterized by the analysis of the pore size distribution and excluded volume. The structures thus generated are used as model networks for Monte Carlo studies of the diffusion of hard spheres in the restricted geometry. Modeling the deviations from the normal diffusion behavior as a purely geometrical phenomenon, these simulations can give us insights into the factors which lead to anomalous diffusion in gel-like systems. In these simulations a sphere of variable radius is allowed to perform an off-lattice random walk through the space between the generated structures, which are assumed to be rigid. It will be shown that the study of the influence of tracer size on diffusion is complementary to the study of the influence of obstacle concentration on the diffusion of monodispersed particles. While the latter method can give us information about the fractal nature of the internetwork space the former method provides information about the fractal nature of the network and the connectivity of the internetwork space.
On the adequacy of pairwise additive potentials for rare gas–halogen systems: The effect of anisotropy of interactions between atoms
Authors: Naumkin F. Yu. , Knowles P. J.
Published: July 2002

A simple model is presented for the potential energy functions of rare gas dihalides RgX2, which uses empirical potentials for diatomic fragments and takes properly into account anisotropic interactions between atoms, resulting in diabatic potentials which correlate with the ground state X2 molecule and Rg atom. Specific results are obtained for potential energy surfaces of ArX2 (X =F, Cl, Br, I) complexes and compared to those from several widely used models based on pairwise additive isotropic interactions. All these earlier models are found to underestimate the binding in the linear geometry, predicting a complete absence of a linear bound state; this feature is especially significant for ArF2 in which the anisotropic model predicts the linear configuration to be more stable. The new anisotropic model leads to Ar–X2 dissociation energies in good agreement with experiments.
Laser-induced thermal desorption of aniline from silica surfaces
Authors: Voumard Pierre , Zenobi Renato
Published: July 2002

A complete study on the energy partitioning upon laser-induced thermal desorption of aniline from silica surfaces was undertaken. The measurements include characterization of the aniline–quartz adsorption system using temperature-programmed desorption, the extrapolation of quasiequilibrium desorption temperatures to the regime of laser heating rates on the order of 109–1010 K/s by computational means, measurement of the kinetic energy distributions of desorbing aniline using a pump–probe method, and the determination of internal energies with resonance-enhanced multiphoton ionization spectroscopy. The measurements are compared to calculations of the surface temperature rise and the resulting desorption rates, based on a finite-difference mathematical description of pulsed laser heating. While the surface temperature of laser-heated silica reaches about 600–700 K at the time of desorption, the translational temperature of laser-desorbed aniline was measured to be Tkin=420±60 K, Tvib was 360±60 K, and Trot was 350±100 K. These results are discussed using different models for laser-induced thermal desorption from surfaces.
A novel method to calculate eigenfunctions and eigenvalues in a given energy range
Authors: Takatsuka Kazuo , Hashimoto Naoyuki
Published: July 2002

A new method to calculate eigenfunctions and eigenvalues in a given energy range is proposed, which can therefore be applied to highly excited states of electronic and/or vibrational states of a molecule. The spectral components of a wave packet that lie outside the energy range are projected out through the time evolution; that is, the packet is screened onto the energy range. If the range includes only a single root, the corresponding eigenfunction is screened first, and the eigenvalue follows as its expectation value. For a case where there is more than a single root, several methods can be figured out. One typical and effective procedure is to construct local basis functions in terms of the aforementioned energy screened wave packets to represent the Hamiltonian in them and to diagonalize it. The concept to construct a local basis was originally developed by Neuhauser [J. Chem. Phys. 93, 2611 (1990)]. The present method performs it in a more efficient and theoretically satisfactory way.
The influence of temperature on pairwise hydrophobic interactions of methane-like particles: A molecular dynamics study of free energy
Authors: Lüdemann Susanna , Schreiber Hellfried , Abseher Roger , Steinhauser Othmar
Published: July 2002

The association of a pair of hydrophobic solutes in water has been investigated by free energy molecular dynamics simulations of a system containing 516 water molecules. Convergence of the calculations is guaranteed by the comparison of data obtained with two independent free energy sampling techniques, which have been optimized for our system. Coulomb interactions have been treated with the Ewald method. Using this computationally expensive approach many of the previously reported discrepancies in the temperature, pressure and interaction parameter dependence of hydrophobic association are clarified. A temperature effect on both the free energy of association and the equilibrium between contact and solvent-separated species is observed. Raising temperature favors association. The most pronounced temperature dependence occurs in the interval between 300 and 350K.
Continuous slit-jet infrared spectrum of the CO–N2 complex
Authors: Xu Yunjie , McKellar A. R. W.
Published: July 2002

The weakly bound complex CO–N2 has been studied in the 4.7 μm infrared region of the CO stretching vibration using a continuous slit-jet supersonic expansion and a tunable diode laser spectrometer. A total of 152 lines were observed and assigned to four connected subbands with K=0←1, 0←0, 1←0, and 2←1, and to one unconnected subband with K=1←1. Analysis of these bands yielded K-state origins, rotational parameters, and centrifugal distortion parameters. The effective intermolecular separation for the complex in its ground state was found to be 4.025 Å, and predictions of rotational frequencies were made to aid in the search for CO–N2 microwave transitions. The spectra observed were surprisingly simple and well behaved, to the extent that they could virtually be ascribed to a (fictitious) complex of CO with a rare gas atom having a mass of 28 a.m.u. This simplicity may be explained by postulating that the N2 undergoes relatively free internal rotation in the complex. All but one of the observed bands involve levels which correlate with the rotationless J=0 state of ortho-N2. Further spectroscopic work in the infrared and microwave regions should be combined with theoretical studies in order to learn more about the orientational structure and intermolecular potential of this atmospherically relevant system.
An accurate multireference configuration interaction calculation of the potential energy surface for the F+H2→HF+H reaction
Authors: Stark Klaus , Werner Hans-Joachim
Published: July 2002

A three dimensional potential energy surface for the F+H2→HF+H reaction has been computed using the internally contracted multireference configuration interaction (MRCI) method with complete active space self-consistent field (CASSCF) reference functions and a very large basis set. Calibration calculations have been performed using the triple-zeta plus polarization basis set employed in previous nine-electron full CI (FCI) calculations of Knowles, Stark, and Werner [Chem. Phys. Lett. 185, 555 (1991)]. While all variational MRCI wave functions yield considerably larger barrier heights than the FCI, excellent agreement with the FCI barrier height and the exothermicity was obtained when the Davidson correction was applied (MRCI+Q). The convergence of the barrier height and exothermicity, spectroscopic constants of the HF and H2 fragments, and the electron affinity of the fluorine atom with respect to the basis set has been carefully tested. Using the largest basis sets, which included 5d, 4f, 3g, and 2h functions on fluorine, a linear barrier height of 1.84 kcal/mol and an exothermicity of 31.77 kcal/mol (exp. 31.73 kcal/mol) was obtained. The true saddle point has a bent structure and the barrier height is predicted to be (1.45±0.25) kcal/mol. About 700 points on the three-dimensional potential energy surface have been computed using a slightly smaller basis set, which yield F–HH barrier heights of 1.92 kcal/mol (linear), 1.54 kcal/mol (bent), and an exothermicity of 31.3 kcal/mol. The barrier height for the H+FH→HF+H exchange reaction is predicted to be 41.2 kcal/mol. In the entrance channel cuts through the three potentials correlating with F(2P3/2,1/2)+H2(1Σ+g) have been computed, and the effect of spin–orbit coupling is investigated. It is found that the spin–orbit coupling increases the barrier height relative to the asymptotic F(2P3/2)+H2(1Σ+g) ground state by about 0.35 kcal/mol, leading to final estimates for the effective collinear and bent barriers of (2.18±0.25) kcal/mol and (1.80±0.25) kcal/mol, respectively. An accurate global analytical fit of the potential (without the effect of spin–orbit coupling) has been obtained using the method of Aguado and Paniagua. Our new ab initio potential is compared to various potentials used so far in dynamics calculations for the F+H2 reaction.
Orthogonally spin-adapted single-reference coupled-cluster formalism: Linear response calculation of higher-order static properties
Authors: Kondo A. E. , Piecuch P. , Paldus J.
Published: July 2002

An algorithm for the calculation of nth-order static properties of closed-shell systems is formulated and implemented, up to and including fourth order, using an orthogonally spin-adapted coupled-cluster (CC) linear response (LR) method. Cluster operators are truncated at the single and double excitation level (CCSD-LR approach) and previously published [A. E. Kondo et al., J. Chem. Phys. 102, 6511 (1995)] first-order CCSD-LR equations, in terms of molecular integrals and zeroth- and first-order cluster amplitudes, are generalized to an arbitrary order. Static properties, including electrostatic dipole and quadrupole moments, polarizabilities and hyperpolarizabilities, are calculated for the lowest singlet state of CH2 and for the ground state of HF at both equilibrium and displaced geometries. The advantages of the LR formalism vs the finite field approach, particularly when evaluating higher-order properties for highly stretched geometries, are pointed out.
New operators for electronic density calculation. I. Derivations and formal analysis
Authors: Rassolov Vitaly A. , Chipman Daniel M.
Published: July 2002

The electronic charge and spin density at any point in space are reexpressed in terms of the expectation values of any member of a general class of global operators. For practical use with approximate wave functions, two particular choices of operator are made that should provide advantages for the difficult case of density evaluation at a nucleus. Formal properties of these operators are derived and discussed in detail. Certain serious difficulties known for the behavior of the global operator previously introduced by Hiller, Sucher, and Feinberg are ameliorated in the new formulations.
A quantum chemistry study of benzene dimer
Authors: Jaffe Richard L. , Smith Grant D.
Published: July 2002

We have performed a detailed quantum chemistry study of the gas-phase benzene dimer. Large atomic orbital basis sets with multiple polarization functions were used. The effects of basis set size, electron correlation, and basis set superposition error were investigated for the low-energy planar sandwich (D6h and C6v), parallel displaced (C2h), and T-shaped (C2v) dimer structures. Our studies indicate that the C2h-symmetry parallel displaced geometry is the lowest-energy structure for the benzene dimer. The T-shaped structure was found to be a low-energy saddle point for interconversion between parallel displaced structures, while the planar sandwich structure was found to be a saddle point on a higher-energy interconversion path between parallel displaced structures. Detailed analysis of the low-energy (T-shaped saddle point) path revealed the presence of a shallow minimum corresponding to a tilt angle between phenyl ring planes of about 45°. Much of the behavior of the benzene dimer observed through molecular jet spectroscopy can be explained by considering a population of the low-energy parallel displaced structure and structures along the low-energy interconversion path, including the tilt- and T-shaped structures.
pH and salt concentration dependence of the microstructure of poly(N-isopropylacrylamide-co-acrylic acid) gels
Authors: Shibayama Mitsuhiro , Ikkai Fumiyoshi , Inamoto Satoshi , Nomura Shunji , Han Charles C.
Published: July 2002

The structure of environment-sensitive gels consisting of poly(N-isopropylacrylamide-co-acrylic acid) (NIPA/AAc) hydrogels was investigated by small-angle neutron scattering (SANS). Effects of the variation of ionic circumstance were observed both on the volume phase transition and on the structure factors: (1) The volume phase transition temperature, Tc, increases with increasing pH from 4 to 9 and then decreases with further increasing pH. (2) the volume gap between the shrunken and swollen gels at Tc is highest around pH 7 to 9. (3) Though the structure factor, I(q), has a maximum above the Tc of the corresponding NIPA homopolymer gels, the peak intensity decreases with increasing pH, where q is the magnitude of the scattering vector. When NaCl is added, a similar scattering maximum appears for T≳Tc. However, strong ionic screening leads to a lower swelling power and a decrease in Tc, resulting in precipitation at a lower temperature than for the gel at a moderate pH. The roles of pH and salt are discussed on the basis of Donnan membrane equilibrium.
A multiphoton polarization study of the Rydberg states of OCS in the 70 500–74 500 cm−1 energy region
Authors: Berger J. Ph. , Baker J. , Couris S.
Published: July 2002

The polarization dependence of the (2+1) and (3+1) resonance enhanced multiphoton ionization of OCS have been investigated in the 70 500–74 500 cm−1 energy region. This region contains a complex system of bands arising from the excitation of the 4p Rydberg states. The symmetry of most of the observed bands have been unambiguously determined based on the intensity changes of the two and three photon resonant spectra using both linearly and circularly polarized light. These results generally confirm the assignments suggested in a previous study. New bands have also been observed and some new assignments are proposed. The vibrational frequencies ν1, ν2, and ν3 of the Rydberg states in that energy region are determined.
Statistical mechanics of a correlated energy landscape model for protein folding funnels
Authors: Plotkin Steven S. , Wang Jin , Wolynes Peter G.
Published: July 2002

In heteropolymers, energetic correlations exist due to polymeric constraints and the locality of interactions. Pair correlations in conjunction with the a priori specification of the existence of a particularly low energy state provide a method of introducing the aspect of minimal frustration to the energy landscapes of random heteropolymers. The resulting funneled landscape exhibits both a phase transition from a molten globule to a folded state, and the heteropolymeric glass transition in the globular state. We model the folding transition in the self-averaging regime, which together with a simple theory of collapse allows us to depict folding as a double-well free energy surface in terms of suitable reaction coordinates. Observed trends in barrier positions and heights with protein sequence length and thermodynamic conditions are discussed within the context of the model. We also discuss the new physics which arises from the introduction of explicitly cooperative many-body interactions, as might arise from sidechain packing and nonadditive hydrophobic forces.
Simulation study of the role and structure of monatomic ions multiple hydration shells
Authors: Degrève Léo , de Pauli Vânia Maria , Duarte Marcio Antonio
Published: July 2002

The hydration structures of five monatomic cations and two monatomic anions are studied by Monte Carlo NpT simulations conducted on infinite dilute aqueous electrolyte solutions. The complete first and second hydration shells are defined by the successive minima of the radial distribution functions’ ion–water oxygen atoms. The first shell structure is determined essentially by the electrostatic charge of the ion and by the short-range ion–water molecule interactions so that it is always constituted by six molecules located at the vertices of regular octahedra unless if the distance between the charge of the ion and the opposite charge in the water molecule is large enough to allow the presence of more than six water molecules. This is the case of the K+ which presents eight molecules in its first shell. The pairs of hydrogen atoms are, in the first shell, preferentially perpendicular. The structure of the second shell is defined by the bulk tetrahedral structure induced by the water molecules so that they are similar for cations and anions. The resulting second shell densities are very close to the pure water density. Both complete hydration shells are maintained stable by means of a compromise between the strong attraction that the ion carry out on the water molecules and by the resulting repulsion between these molecules which are forced to get too close to one another.
Femtosecond Stokes shift in styryl dyes: Solvation or intramolecular relaxation?
Authors: Kovalenko S. A. , Ernsting N. P. , Ruthmann J.
Published: July 2002

Transient absorption and gain spectra of the styryl dye LDS-750 in solution have been studied by the pump/supercontinuum probe (PSCP) technique with excitation at 530 nm. The pump/probe intensity correlation width was 70 fs, providing a time resolution of 40 fs. Spectra were detected in the range 400–800 nm with 1.5 nm resolution. Before 70 fs, prominent spectral structure is observed due to resonant Raman scattering from a 1500 cm−1 active mode of the chromophore. At later time, the gain spectrum undergoes an ultrafast redshift and change of shape, with time constants of ∼200 and ∼600 fs for acetonitrile and chloroform solutions, respectively. At high pumping energy (1.2 μJ), the final emitting state is reached by internal conversion from higher electronic states without a further essential Stokes shift. The emitting state is assigned to an excited isomeric form of the molecule. At low pumping energy (0.3 μJ), the first excited electronic state isomerizes in an ultrafast process followed by a slower process, the dynamics of which is controlled by the solvent. The geometrical and electronic nature of these processes and their coupling to the solvent needs further clarification.
Derivation of rotation-vibration Hamiltonians that satisfy the Casimir condition
Authors: Mardis Kristy L. , Sibert III Edwin L.
Published: July 2002

A general prescription for deriving rotation-vibration Hamiltonians that satisfy the Casimir condition is presented. This condition, achieved using the Eckart constraints, is that there is no vibrational angular momentum in the molecular equilibrium configuration. The Eckart condition, while useful for studying rotation-vibration interactions, is difficult to apply for coordinates other than rectilinear normal coordinates. The present derivation allows one to derive Hamiltonians in curvilinear coordinates, yet still take advantage of the most relevant property of the Eckart frame, this being that Coriolis coupling is minimized in the limit of small vibrations.
Laser spectroscopy of C3: Stimulated emission and absorption spectra of the Ã 1Πu–X̃ 1Σg+ transition
Authors: Tokaryk D. W. , Chomiak D. E.
Published: July 2002

Several bands of the Ã 1Πu–X̃ 1Σg+ transition of C3 have been observed between 24868 and 25140 cm−1 in a liquid-nitrogen cooled discharge through methane and helium. The spectra were taken with a frequency-doubled cw Ti:sapphire ring laser. Bands were observed either in stimulated emission or in absorption. The stimulated emission spectra include the bands 020–000 1Πu(−)−1Σg+, 020–020 1Πu(−)− 1Σg+, 020–020 1Πu(−)−1Δg, and 020–020 1Φu −1Δg. Analysis shows that entanglement of the 020–000 1Πu(−)−1Σg+ band with the previously unanalyzed 020–020 1Φu−1Δg band has led to flaws in past rotational analyses of the Ã 020 1Πu(−) level. The absorption spectra include two bands for which the positions of the Q branch heads were reported previously [Can. J. Phys. 45, 4103 (1967)]. We confirm that the first band is the 002–100 1Πu−1Σg+ band, as suggested by Balfour et al. [J. Chem. Phys. 101, 10 343 (1994)], and show that the second is the 102–200 1Πu−1Σg+ band.
Optimization of electrostatic binding free energy
Authors: Lee Lee-Peng , Tidor Bruce
Published: July 2002

An analytic result is derived that defines the charge distribution of the tightest-binding ligand given a receptor charge distribution and spherical geometries. Using the framework of continuum electrostatics, the optimal distribution is expressed as a set of multipoles determined by minimizing the electrostatic free energy of binding. Results for two simple receptor systems are presented to illustrate applications of the theory.
Phase diagrams of scalemic mixtures: A Monte Carlo simulation study
Authors: Vlot Margot J. , van Miltenburg J. Cornelis , Oonk Harry A. J. , van der Eerden Jan P.
Published: July 2002

In this paper, a simplified model was used to describe the interactions between the enantiomers in a scalemic mixture. Monte Carlo simulations were performed to determine several thermodynamic properties as a function of temperature and mole fraction of solid, liquid, and gas phase. Phase diagrams were constructed using a macroscopic thermodynamic program, PROPHASE. The model consists of spherical D and L molecules interacting via modified Lennard–Jones potentials (σDD=σLL, εDD=εLL, εDL=eεDD, and σDL=sσDD.) The two heterochiral interaction parameters, e and s, were found to be sufficient to produce all types of phase diagrams that have been found for these systems experimentally. Conglomerates were found when the heterochiral interaction strength was smaller than the homochiral value, e<1. A different heterochiral interaction distance, s≠1, led to racemic compounds, with an ordered distribution of D and L molecules. The CsCl-structured compound was found to be stable for short DL interactions, s<1 (e=1), with an enantiotropic transition to a solid solution for s=0.96. Longer heterochiral distances, s>1, result in the formation of layered fcc compounds. The liquid regions in the phase diagram become larger for s≠1, caused by a strong decrease of the melting point for s<1 and s>1, in combination with only a small effect on the boiling point for s<1, and even an increase of the boiling point for s>1. Segregation into two different solid solutions, one with low mole fraction and the other one close to x=0.25, was obtained for these mixtures as well.
Equilibrium thermodynamics of homopolymers and clusters: Molecular dynamics and Monte Carlo simulations of systems with square-well interactions
Authors: Zhou Yaoqi , Karplus Martin , Wichert John M. , Hall Carol K.
Published: July 2002

The thermodynamics of homopolymers and clusters with square-well interactions of up to 64 particles are studied with constant-temperature discontinuous molecular dynamics (DMD) simulations; for comparison Monte Carlo (MC) simulations are also reported. Homopolymers composed of more than five beads are found to exhibit two or more equilibrium transitions. In the long chain limit, these multiple transitions correspond to gas-to-liquid, liquid-to-solid, and solid-to-solid transitions. In particular, the liquid-to-solid-like disorder-to-order transition for isolated 32mers and 64mers is strongly first order (bimodal energy distribution) at the reduced square-well diameter λ=1.5. As λ decreases from 1.5 to 1.3, the bimodal distribution becomes unimodal. The use of Lindemann’s rule for solids indicates that the structure formed right below the liquid-to-solid transition temperature has a solid core but a liquid surface. Comparing the homopolymer results with those for square-well clusters indicates that the bonding constraint in homopolymers increases the temperatures of transitions but decreases their strength. The solid structure of an isolated 64mer is nearly identical to that of a cluster of 64 beads. Possible approaches to the experimental observation of the solid-state for an isolated chain are discussed.
Relationship between polymer chain conformation and phase boundaries in a supercritical fluid
Authors: Luna-Bárcenas Gabriel , Meredith J. Carson , Sanchez Isaac C. , Johnston Keith P. , Gromov Dmitry G. , de Pablo Juan J.
Published: July 2002

We investigate the solvent density driven changes in polymer conformation and phase behavior that occur in a supercritical fluid, with a particular emphasis on conditions near the lower critical solution temperature (LCST) phase boundary. Using continuous space Monte Carlo simulations, the mean square end-to-end distance (R) and radius of gyration (Rg) are calculated for a single chain with 20 Lennard-Jones segments in a monomeric solvent over a broad range of densities and temperatures. The chains collapse as temperature increases at constant pressure, or as density decreases at constant temperature. A minimum in R and Rg occurs at a temperature slightly above the coil-to-globule transition temperature (C-GTT), where the chain adopts a quasi-ideal conformation, defined by the balance of binary attractive and repulsive interactions. Expanded ensemble simulations of finite-concentration polymer–solvent mixtures reveal that the LCST phase boundary correlates well with the single chain C-GTT. At temperatures well above the LCST, the chain expands again suggesting an upper critical solution temperature (UCST) phase boundary above the LCST.