bluuebunny/crossref_metadata_2025 huggingface dataset:
Is the counterion condensation point on polyelectrolytes a trigger of structural transition?
Authors: Manning Gerald S.
Published: July 2002

Experimental data on well documented global polymer transitions can be correlated with the linear charge density critical for counterion condensation. Other data exhibit abrupt behavior at the counterion condensation point and may consistently be interpreted in terms of a transition between locally folded and extended structures. We suggest that the underlying cause of these observations is the onset of mechanical instability of locally folded structures at the counterion condensation point.
Nonlinear optical response of three-level systems
Authors: Bavli R. , Heller D. F. , Band Y. B.
Published: July 2002

We investigate the properties of a homogeneously broadened three-level system interacting with two optical fields, Eω1(x,t) and Eω2(x,t), with central frequencies ω1 and ω2 whose amplitudes are assumed to be slowly varying in time compared with dephasing times of the optical transitions. We calculate Raman susceptibilities allowing each transition to be induced by both fields and without employing the rotating wave approximation (RWA); this enables correct calculation of the optical response of the medium when the fields are on-resonance or arbitrarily far off-resonance with the Raman transition. In some frequency regions the RWA causes substantial errors. A distinction is drawn between two different types of RWA: (a) a neglect of off-resonant components of the polarization for direct optical transitions, and (b) neglect of off-resonant components of off-diagonal density matrix elements of order Eω1(x,t)Eω2(x,t)* in the field amplitudes. The first RWA is known to become inaccurate when the pump frequency is significantly off-resonance with the pump transition. We show that the second RWA becomes inaccurate when ‖ω1−ω2‖ is small or comparable to the active Raman mode frequency (even when the field are close to resonance). Calculations based upon our expressions show that when the pump frequency is close to resonance with the optical transition from the ground state, a nonlinear absorption at the Stokes shifted frequency occurs. This absorption can significantly decrease the Raman gain. Also, we find that Rabi splittings in a three-level system can occur even when the pulse durations of the incident fields are short compared with the T1 times of the system. We solve analytically for the optical response of the system to two fields of arbitrary strength and frequency. We analyze this solution and its perturbative expansion in the field amplitudes. We use the Maxwell–Bloch equations developed here to propagate time-dependent pulses.
Computational study of formation dynamics and structure of amorphous ice condensates
Authors: Zhang Q. , Buch V.
Published: July 2002

Condensation dynamics and structure of low temperature ≤100 K amorphous ices is studied using the classical trajectory simulation technique. Specifically, classical equations of motion are solved for a sequence of H2O molecules impinging at random on an initial nucleus of ten water molecules. The sticking probability to the cluster is found to be unity. Using this technique we accumulated three amorphous clusters of 100–300 water molecules. The analysis of the results included collision dynamics and molecular structure. Upon collision with the cluster, the impinging H2O molecule forms typically two hydrogen bonds. The two new bonds are almost always asymmetric, i.e., one bond is via an O atom of the new molecule, and the other via one of its H atoms. We identified two distinct types of collision trajectories—‘‘simple’’ trajectories and ‘‘complex’’ ones. In a simple trajectory, a new molecule is attached to the surface, without disrupting the preexisting hydrogen bond network in the cluster. In a complex trajectory, the preexisting bond network is modified significantly during the collision. Up to ∼15 hydrogen bonds can be changed (formed or destroyed) during such a trajectory; and the spatial extent of the changes can be as large as ∼20 Å.

The complex trajectories comprise 60%∼70% of all the collision trajectories. Thus, the hydrogen bond network evolves continuously during the growth of the ice sample; the net trend being towards the four coordinated molecular configuration around each molecule. The average coordination number of a molecule in the final clusters is 3.5∼3.7, despite the fact that most of the water molecules are on the exposed surface of the cluster. The main features of the local molecular structure in the clusters are (a) ordering of OO and OH distances within hydrogen bonds (b) very significant disorder in O–O–O angles between adjacent hydrogen bonds; the width of the O–O–O angle distribution is ∼40°. The molecular structure seems to be closely related to that of liquid water. The experimental electron diffraction patterns of amorphous clusters are reproduced very well by our models. Comparison was also made with the radial distribution functions derived for bulk amorphous ice from x-ray and neutron diffraction experiments. Reasonable agreement was obtained within the range ≲4 Å, while at larger distances finite size effects become important.
Dynamics and rheology of complex interfaces. I
Authors: Doi Masao , Ohta Takao
Published: July 2002

When a concentrated mixture of two immiscible fluid is sheared, a rather complex interface is formed due to the coagulation, rupture, and deformation of droplets. The dynamics and rheological properties of such system is discussed. A semiphenomenological kinetic equation is proposed which describes the time evolution of droplet size and orientation, and also the macroscopic stress in a flow field. The rheological properties are shown to be quite unusual: for example, the steady-state viscosity is independent of the shear rate, while the normal stress difference is nonzero and proportional to the magnitude of the shear rate.
Scattering of NH3 by ortho- and para-H2: Expansion of the potential and collisional propensity rules
Authors: Rist Claire , Alexander Millard H. , Valiron P.
Published: July 2002

We present the general expansion of the interaction potential between a rigid, but invertible symmetric top and a linear molecule. The development is derived from the average over electronic coordinates of the fundamental electrostatic interaction. Our expansion for the electrostatic potential takes into account explicitly the symmetry under inversion of the position of all the nuclei, which was not included in earlier expansions. A numerical fit of calculated ab initio points for the NH3–H2 system suggests the presence of a similar inversion symmetry for the total interaction potential. We then use our expansion to derive, in the Born and energy sudden limits, propensity rules for collisions of NH3 with ortho- and para-hydrogen. Although at a formal level these propensity rules are equally valid for both hydrogen rotomers, close coupling calculations show that they are considerably weakened for o-H2. This is a manifestation of the additional anisotropy in the interaction between NH3 and a non-spherical collision partner. Notwithstanding, evidence of these propensity rules is clear, in contrast to earlier calculations based on a preliminary version of the NH3–H2 interaction potential used here.
Considerations in constructing a multireference second-order perturbation theory
Authors: Kozlowski Pawel M. , Davidson Ernest R.
Published: July 2002

Several possible definitions for a multireference second-order perturbation theory are suggested. These are tested against some standard test problems from the literature.
Dynamical density functional theory of gas–liquid nucleation
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

We present a consistent dynamical nucleation theory based on density functional theory. By considering the properties of stable droplets in closed volumes, the height and shape of the barrier to nucleation are calculated. Contributions from fluctuations in the center of mass of the nucleating cluster are taken into account. Forward and backward rates for cluster dynamics are obtained, and nucleation rates are then evaluated under steady-state conditions. We test the quantitative effects of several shortcuts to calculating nucleation rates. The predictions of the full theory presented here show very modest changes from those of the simpler nonclassical theory proposed earlier by Oxtoby and co-workers.
Anisotropic rigid rotor potential energy function for H2O–H2
Authors: Phillips Timothy R. , Maluendes Sergio , McLean A. D. , Green Sheldon
Published: July 2002

We have calculated the interaction for H2O–H2 at 722 points on a five-dimensional surface where both molecules are treated as rigid rotators and we have fitted the ab initio points to a 48-term angular expansion of products of spherical harmonics and rotation matrices. The resulting potential energy function shows strong angle dependence with a large contribution from electrostatic interactions. When averaged over H2 orientations, the resulting water-atom-like surface is found to have zero crossing and minimum at similar distances to the corresponding H2O–He surface but to be generally more repulsive at short range and more attractive at long range. The isotropic average of the potential has a zero-crossing radius σ=3.05 Å and a well depth ε=49.5 cm−1 at an intermolecular separation Rm=3.52 Å.
Diffusivity in 2-butoxyethanol/water mixtures of noncritical composition approaching the liquid/liquid coexistence curve
Authors: Schmitz J. , Belkoura L. , Woermann D.
Published: July 2002

Results of dynamic light scattering experiments are reported which demonstrate that the mutual diffusion coefficient of 2-butoxyethanol/water mixtures of noncritical composition exhibits a characteristic temperature and composition dependence: The diffusion coefficients of the two phases of noncritical composition coexisting along the liquid/liquid coexistence curve decrease by an order of magnitude in the temperature range (Tp−Tc)<10 K approaching the lower critical point (Tp, temperature of phase separation; Tc, critical temperature). In the homogeneous 1-fluid phase region of the phase diagram, the D vs (T−Tp) curves of mixtures of noncritical composition are shifted to smaller values of D with decreasing values of ‖y−yc‖ (y, mass fraction of 2-butoxyethanol; yc, critical mass fraction). These findings are discussed on the basis of the model of regular mixtures.
Photon echoes in impulsive optical spectroscopy of phonons
Authors: Leegwater Jan A. , Mukamel Shaul
Published: July 2002

Multidimensional off resonant spectroscopy of crystals using a train of optical pulses can be effectively used to probe nuclear dynamics in solids. We predict a clear photon echo signal even in the absence of inhomogeneous broadening. This technique may be used for studying phonon dynamics in solids, in structurally frozen systems such as glasses, and in systems where the validity of the concept of phonons is not equally well established, such as liquids and gases. With this technique it is also possible to obtain information on the lifetime of phonons. We predict a long time tail of the nonlinear signal proportional to t−n/5, where n is the order of the response function studied.
A refined Monte Carlo study of Mg2+ and Ca2+ hydration
Authors: Bernal-Uruchurtu M. I. , Ortega-Blake I.
Published: July 2002

Monte Carlo simulations of the hydration of Ca2+ and Mg2+ were performed. The interaction potential used includes polarization and nonadditivity, factors that have been reported to be critical for the proper description of hydration. The effect of cell size, amount of configurations needed to attain equilibrium, and size of the statistical sample were studied. We validated the simulation by obtaining agreement with experiments. A microscopic analysis of the hydration shows that Mg2+ and its first shell can be considered as a supermolecule imbedded in the solvent. For Ca2+ this character is less marked and its first shell responds to the solvent more easily. From this view we explain the coordination properties of both ions. We found that the differences in the coordination properties of both ions are due to a great extent to the differences in the water depletion zone after the first hydration shell. An analysis of the energetic contributions of each hydration shell to the total enthalpy of hydration was made as well as an analysis of local densities to understand electrostriction.
A density functional study of borane and alane monoammoniate (BH3NH3,AlH3NH3)
Authors: Leboeuf Martin , Russo Nino , Salahub Dennis R. , Toscano Marirosa
Published: July 2002

Molecular structures, harmonic vibrational frequencies, dissociation energies, and barriers to internal rotation have been determined using a Gaussian density functional method. Both local and nonlocal levels of theory have been employed. The calculated equilibrium geometry and harmonic vibrational frequencies for BH3NH3 compare well with those determined by microwave and infrared experiments. The rotational barrier is found to be 2.09 kcal/mol in very good agreement with the experimental data. The calculated properties for ammonia-alane (AlH3NH3) are compared with those obtained previously from high level correlated methods.
Lattice-sum methods for calculating electrostatic interactions in molecular simulations
Authors: Luty Brock A. , Tironi Ilario G. , van Gunsteren Wilfred F.
Published: July 2002

When simulating a periodic molecular system, lattice sum methods may be used to evaluate the electrostatic interactions without resorting to an unphysical truncation of the potential. In a previous publication, we compared the computational efficiency of the Ewald and Particle-Particle Particle-Mesh (PPPM) lattice-sum methods. Because the PPPM method discretizes the field equations and utilizes the highly efficient fast Fourier transform algorithm, it requires significantly less computational effort than the Ewald method and scales almost linearly with system size. In this paper, we take a more detailed look into the theory behind the lattice-sum methods to clarify the underlying similarities between the Ewald method and the PPPM method. We also investigate the errors introduced by different approximation used in the discretization of the field equations in the PPPM method.
Double layer forces between heterogeneous charged surfaces: The effect of net surface charge
Authors: Miklavcic S. J.
Published: July 2002

In this paper a theoretical analysis is outlined which describes forces between macroscopic heterogeneous surfaces that bear a significant residual charge after, say, the nonuniform adsorption of large, oppositely charged macromolecules. Quite unlike the case of uniform surface charges, extremely strong and long-ranged attractive forces of electrostatic origin can exist between surfaces with regularly spaced charge distributions. However, when a residual surface charge is present, this attractive component is reduced, compared with the case of net uncharged nonuniform surfaces, due to screening of the surface charges by electrolyte which has accumulated in double layers adjacent to each surface. Exact analytical formulas for the free energy expressed in terms of elliptic functions show that to leading order the asymptotic decay of the interaction is independent of average surface charge, while the magnitude is affected by both this charge and amount of bulk electrolyte present. Full numerical calculations of the disjoining pressure show that under conditions of low net surface charge and low electrolyte the influence of nonuniformities can be of some significance. However, because of the different decay rates, this influence is invariably limited to separations smaller than the Debye length. At moderate to high concentrations, the decay rates become similar and although the magnitude of the nonuniform term is diminished there is a regime of surface charge for which it does influence the interaction. The relative strength of the heterogeneous contribution, however, rapidly diminishes as the net surface charge increases. It is concluded that even in this ideal case of regularly spaced charge distributions, the effect of inhomogeneities is not important when there is a significant net surface charge.
Femtosecond time-resolved photoionization and photoelectron spectroscopy studies of ultrafast internal conversion in 1,3,5-hexatriene
Authors: Cyr Douglas R. , Hayden Carl C.
Published: July 2002

Ultrafast photodynamics in a 1,3,5-hexatriene are studied using femtosecond time-resolved photoionization and photoelectron spectroscopy. The trans and cis isomers have distinctly different dynamics following excitation at the S2 origin near 250 nm. An intermediate, presumably the S1 state, is observed for both trans and cis isomers with lifetimes of 270 fs and 730 fs, respectively. Time-delayed photoelectron spectra of cis-hexatriene determine a 300 fs time scale for vibrational energy redistribution within the intermediate S1 state.
A technique for the calculation of mass, energy, and momentum densities at planes in molecular dynamics simulations
Authors: Daivis Peter J. , Travis Karl P. , Todd B. D.
Published: July 2002

We present a new technique for the evaluation of hydrodynamic densities (for example mass, energy or momentum densities) at planes in molecular dynamics simulations. This technique employs an easily computed expression for the density at a plane that is formally exact, unlike other expressions such as histogram approximations. We present simple examples of applications of this procedure to the calculation of mass and momentum densities, and hence the streaming velocity, at planes in a fluid undergoing planar Poiseuille flow, and show how the temperature profile can be obtained by the same procedure.
Resonance enhanced multiphoton ionization spectroscopy of carbonyl sulphide
Authors: Morgan Ross A. , Orr-Ewing Andrew J. , Ascenzi Daniela , Ashfold Michael N. R. , Buma Wybren Jan , Scheper Connie R. , de Lange Cornelis A.
Published: July 2002

Rydberg excited states of the OCS molecule in the energy range 70500–86000 cm−1 have been investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet-cooled sample of the parent molecule. Spectral interpretation has been assisted by companion measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. The present study supports the earlier conclusions of Weinkauf and Boesl [J. Chem. Phys. 98, 4459 (1993)] regarding five Rydberg origins in the 70500–73000 cm−1 energy range, attributable to, respectively, states of 3Π, 1Π, 3Δ, 1Δ and 1Σ+ symmetry arising from the 4pλ←3π orbital promotion. We also identify a further 21 Rydberg origins at higher energies. These partition into clumps with quantum defects ca. 3.5 and 4.5, which we associate with the orbital promotions npλ←3π (n=5,6), and others with near integer quantum defect which are interpretable in terms of excitation to s,d and (possibly) f Rydberg orbitals. We also identify MPI resonances attributable to CO(X 1Σ+) fragments and to S atoms in both their ground (3P) and excited (1D) electronic states. Analysis of the former resonances confirms that the CO(X) fragments resulting from one photon dissociation of OCS at excitation wavelengths ca. 230 nm are formed with a highly inverted, bimodal rotational state population distribution, whilst the latter are consistent with previous reports of the wavelength dependence for forming ground and excited state S atoms in the near uv photolysis of OCS.
A comparative theoretical study of atomic hydrogen adsorption on the (110) faces of Al, Cu, Ni, and NiAl
Authors: Castro G. R. , Drakova D. , Grillo M. E. , Doyen G.
Published: July 2002

The interaction of atomic hydrogen with Al(110), Cu(110), Ni(110), and NiAl(110) is investigated using a spin-unrestricted screened Hartree–Fock method. As Al is a pure sp-metal, Cu and NiAl have completely filled d-bands, and Ni provides unfilled d-states, this allows study of the influence of the d-electrons on the hydrogen adsorption mechanism. The band structures of Cu and NiAl are rather similar and we can address the problem of whether the adsorption is dominated by the local character or the band structure. A detailed analysis of the chemisorption bond is given, based on a density matrix partitioning technique. The adsorption energy is found to increase from 1.8 eV via 2.1 eV to 2.4 eV and 3.0 eV in the order Al, NiAl, Cu, Ni in qualitative agreement with the available experimental data. On NiAl(110), hydrogen adsorbs with approximately the same energy above the Ni and Al rows, although the components of the adsorption energy vary significantly. The reduced adsorption energy on NiAl compared to Cu and Ni is traced back to the increased sp-electron density and the more diffuse character of the d-orbitals in NiAl. Adsorption strength is determined by the competing effects of energy gain due to polarization of metal electrons toward the attractive proton potential and energy loss due to Pauli repulsion. Comparison with the mechanism of formation of gas phase chemical bonds reveals a novel mechanism of the chemisorption bond which is uncommon in the gas phase. The long-range interaction with the image potential, the high polarizability, and the spillover of metal electrons play a decisive role in determining the new mechanism of hydrogen chemisorption on metal and alloy surfaces.
Interfacing relativistic and nonrelativistic methods. I. Normalized elimination of the small component in the modified Dirac equation
Authors: Dyall Kenneth G.
Published: July 2002

The introduction of relativistic terms into the nonrelativistic all-electron Schrödinger equation is achieved by the method of normalized elimination of the small component (ESC) within the matrix representation of the modified Dirac equation. In contrast to the usual method of ESC, the method presented retains the correct relativistic normalization, and permits the construction of a single matrix relating the large and small component coefficient matrices for an entire set of positive energy one-particle states, thus enabling the whole set to be obtained with a single diagonalization. This matrix is used to define a modified set of one- and two-electron integrals which have the same appearance as the nonrelativistic integrals, and to which they reduce in the limit α→0. The normalized method corresponds to a projection of the Dirac–Fock matrix onto the positive energy states. Inclusion of the normalization reduces the discrepancy between the eigenvalues of the ESC approach and the Dirac eigenvalues for a model problem from order α2 to order α4, providing a closer approximation to the original, uneliminated solutions. The transition between the nonrelativistic and relativistic limits is achieved by simply scaling the fine structure constant α.
Photoelectron spectroscopy of liquid water, some alcohols, and pure nonane in free micro jets
Authors: Faubel Manfred , Steiner Björn , Toennies J. Peter
Published: July 2002

The recently developed technique of accessing volatile liquids in a high vacuum environment by using a very thin liquid jet is implemented to carry out the first measurements of photoelectron spectra of pure liquid water, methanol, ethanol, 1-propanol, 1-butanol, and benzyl alcohol as well as of liquid n-nonane. The apparatus, which consists of a commercial hemispherical (10 cm mean radius) electron analyzer and a hollow cathode discharge He I light source is described in detail and the problems of the sampling of the photoelectrons in such an environment are discussed. For water and most of the alcohols up to six different electronic bands could be resolved. The spectra of 1-butanol and n-nonane show two weakly discernable peaks from which the threshold ionization potential could be determined. A deconvolution of the photoelectron spectra is used to extract ionization potentials of individual molecular bands of molecules near the surface of the liquid and shifts of the order of 1 eV compared to the gas phase are observed. A molecular orientation for water molecules at the surface of liquid water is inferred from a comparison of the relative band strengths with the gas phase. Similar effects are also observed for some of the alcohols. The results are discussed in terms of a simple “Born-solvation” model.
Natural bond orbital analysis of steric interactions
Authors: Badenhoop J. K. , Weinhold F.
Published: July 2002

We describe an ab initio procedure for extracting the Pauli exchange antisymmetry (“steric”) contributions to molecular potential energy in the framework of self-consistent-field molecular orbital (SCFMO) theory. This “natural steric analysis” method is based on natural bond orbital (NBO) representation of the SCFMO wave function, which allows the steric exchange energy to be approximated as an energy difference between “preorthogonal” and final NBOs, analogous to the procedure of Sovers et al. [J. Chem. Phys. 49, 2592 (1965)]. We show how the total NBO steric exchange energy can in turn be approximated in terms of pairwise-additive interactions between localized bonding units, comparable to the empirical steric potentials of molecular mechanics models. The accuracy of NBO steric analysis is tested in applications to various rare-gas interactions (expected to be of pure steric exchange type), and excellent agreement is found with the full ab initio potential curves over a wide range of separations. Limitations of the simple pairwise-additive model of steric exchange interactions are noted and characterized.
Normal order and extended Wick theorem for a multiconfiguration reference wave function
Authors: Kutzelnigg Werner , Mukherjee Debashis
Published: July 2002

A generalization of normal ordering and of Wick’s theorem with respect to an arbitrary reference function Φ as some generalized “physical vacuum” is formulated in a different (but essentially equivalent) way than that suggested previously by one of the present authors. Guiding principles are that normal order operators with respect to any reference state must be expressible as linear combinations of those with respect to the genuine vacuum, that the vacuum expectation value of a normal order operator must vanish (with respect to the vacuum to which it is in normal order), and that the well-known formalism for a single Slater determinant as physical vacuum must be contained as a special case. The derivation is largely based on the concepts of “Quantum Chemistry in Fock space,” which means that particle-number-conserving operators (excitation operators) play a central role. Nevertheless, the contraction rules in the frame of a generalized Wick theorem are derived, that hold for non-particle-number-conserving operators as well. The contraction rules are formulated and illustrated in terms of diagrams. The contractions involve the “residual n-particle density matrices” λ, which are the irreducible (non-factorizable) parts of the conventional n-particle density matrices γ, in the sense of a cumulant expansion for the density. A spinfree formulation is presented as well. The expression of the Hamiltonian in normal order with respect to a multiconfiguration reference function leads to a natural definition of a generalized Fock operator. MC-SCF-theory is easily worked out in this context. The paper concludes with a discussion of the excited configurations and the first-order interacting space, that underlies a perturbative coupled cluster type correction to the MCSCF function for an arbitrary reference function, and with general implications of the new formalism, that is related to “internally contracted multireference configuration interaction.” The present generalization of normal ordering is not only valid for arbitrary reference functions, but also if the reference state is an ensemble state.
Ab initio
potential energy surface and infrared spectra of H2-CO and D2-CO van der Waals complexes
Authors: Jankowski Piotr , Szalewicz Krzysztof
Published: July 2002

A new four-dimensional intermolecular potential energy surface for the H2-CO complex with fixed intramolecular distances of H2 and CO is presented. The symmetry-adapted perturbation theory has been used to calculate the interaction energy. A large basis set of spdfg type has been used including bond functions. An analytical fit of the ab initio potential energy surface has the global minimum of −109.272 cm−1 at the intermolecular separation of 7.76 bohr for the linear geometry with the C atom pointing toward the H2 molecule. This potential has been used to calculate rovibrational energy levels of CO-para-H2 and CO-ortho-D2 complexes. The resulting dissociation energies are 23.709 cm−1 and 30.756 cm−1, respectively. The computed levels have been used to generate the infrared spectrum accompanying the fundamental vibrational excitation of CO. The transition energies predicted agree well with those observed by McKellar [Chem. Phys. Lett. 186, 58 (1991)].
Competitive solvation of K+ by benzene and water: Cation-π interactions and π-hydrogen bonds
Authors: Cabarcos Orlando M. , Weinheimer Corey J. , Lisy James M.
Published: July 2002

The competition between ion–molecule and hydrogen bond interactions in K+(benzene)1–5(water)1,2 is examined using infrared spectroscopic and mass spectrometric methods. The cation-π interaction and π-hydrogen bond play an important role in the structure of the mixed cluster ions. Important observations include: the preferential binding of benzene vs water to K+; the “dehydration” of the potassium ion by benzene; and the observation of water acting as a double proton donor with π-hydrogen bonds to two benzenes.
Molecular multipole moments of water molecules in ice Ih
Authors: Batista Enrique R. , Xantheas Sotiris S. , Jónsson Hannes
Published: July 2002

We have used an induction model including dipole, dipole–quadrupole, quadrupole–quadrupole polarizability and first hyperpolarizability as well as fixed octopole and hexadecapole moments to study the electric field in ice. The self-consistent induction calculations gave an average total dipole moment of 3.09 D, a 67% increase over the dipole moment of an isolated water molecule. A previous, more approximate induction model study by Coulson and Eisenberg [Proc. R. Soc. Lond. A 291, 445 (1966)] suggested a significantly smaller average value of 2.6 D. This value has been used extensively in recent years as a reference point in the development of various polarizable interaction potentials for water as well as for assessment of the convergence of water cluster properties to those of bulk. The reason for this difference is not due to approximations made in the computational scheme of Coulson and Eisenberg but rather due to the use of less accurate values for the molecular multipoles in these earlier calculations.
Experimental measurement of the transition moment for the (2,0) band of the CN A 2Π←X 2Σ+ red system at 789.5 nm
Authors: He G. , Tokue I. , Macdonald R. Glen
Published: July 2002

The square of the electronic–vibrational transition moment for the (2,0) band of the cyano radical (CN), A 2Π←X 2Σ+, red system at 789.5 nm has been determined using a direct time-resolved absorption technique. The line strength of the R1(8.5) transition of 12C14N was measured by comparison to the known line strength of the H12C14N(001)←(000) P(8) infrared υ3 fundamental transition near 3.05 μm. The CN radical was created by pulsed-laser photolysis of cyanogen, (CN)2, in a mixture of H2 and (CN)2 in either Ar or He carrier gas at 293 K. Both CN and HCN were monitored by time-resolved absorption spectroscopy during the same photolysis laser pulse. A simple kinetic model was used to relate the two absorption features to each other. The square of the electronic–vibrational transition moment for the A←X(2,0) band was found to be 7.8±1.0×10−38 esu2 cm2 in good agreement with theoretical calculations based on ab initio quantum chemistry methods.
How to mesh up Ewald sums. I. A theoretical and numerical comparison of various particle mesh routines
Authors: Deserno Markus , Holm Christian
Published: July 2002

Standard Ewald sums, which calculate, e.g., the electrostatic energy or the force in periodically closed systems of charged particles, can be efficiently speeded up by the use of the fast Fourier transformation (FFT). In this article we investigate three algorithms for the FFT-accelerated Ewald sum, which have attracted widespread attention, namely, the so-called particle–particle–particle mesh (P3M), particle mesh Ewald (PME), and smooth PME method. We present a unified view of the underlying techniques and the various ingredients which comprise those routines. Additionally, we offer detailed accuracy measurements, which shed some light on the influence of several tuning parameters and also show that the existing methods — although similar in spirit — exhibit remarkable differences in accuracy. We propose a set of combinations of the individual components, mostly relying on the P3M approach, that we regard to be the most flexible. The issue of estimating the errors connected with particle mesh routines is reserved to paper II.
Vibrational spectra of charged defects in a series of α,α′-dimethyl end-capped oligothiophenes induced by chemical doping with iodine
Authors: Casado J. , Hernández V. , Hotta S. , López Navarrete J. T.
Published: July 2002

We present the results of a detailed study carried out on a few α,α′-dimethyl end-capped oligothiophenes which have been in situ doped with dry iodine vapors, at different concentrations. The doping process of these well sized oligomers (from dimer to hexamer) has been monitored by using Fourier transform infrared absorption and Fourier transform raman scattering spectroscopies. These data, when combined with the vibrational full assignment of the oligomers in the neutral state and the electronic absorption bands recorded for both neutral and doped compounds, provide a complete spectroscopic characterization of a full series of positive polaron-type model defects of doped polythiophene. We have also performed density functional theory quantum-chemical calculations in order to analyze the effects of ionization on the geometries and vibrational spectra of these systems.
Kinetic energy density dependent approximations to the exchange energy
Authors: Ernzerhof Matthias , Scuseria Gustavo E.
Published: July 2002

Two nonempirical kinetic energy density dependent approximations are introduced. First, the local τ approximation (LTA) is proposed in which the exchange energy Ex depends only on a kinetic energy density τ. This LTA scheme appears to be complementary to the local spin density (LSD) approximation in the sense that its exchange contribution to the atomization energy ΔEx=Exatoms−Exmolecule is fairly accurate for systems where LSD fails. On the other hand, in cases where LSD works well LTA results for ΔEx are worse. Secondly, the τPBE approximation to Ex is developed which combines some of the advantages of LTA and of the Perdew–Burke–Ernzerhof (PBE) exchange functional. Like the PBE exchange functional, τPBE is free of empirical parameters. Furthermore, it yields improved atomization energies compared to the PBE approximation.
From polypeptide sequences to structures using Monte Carlo simulations and an optimized potential
Authors: Derreumaux Philippe
Published: July 2002

The ab initio prediction of the structure of a polypeptide from its sequence necessarily requires the detection of the lowest energy forms which correspond to the native state of the polypeptide. A potential for modeling the energy hypersurface of polypeptides using a hybrid level of description is optimized for the structures of four training peptides which have been shown experimentally to adopt α, ββ, αβ, and ββα conformations in aqueous solution. This potential is then used in diffusion process-controlled Monte Carlo simulations to predict the native structures of this training set of peptides and a test set of 20 peptides which were not themselves used during the optimization of the potential. Starting from various fully extended conformations, all simulations lead to an ensemble of conformations compatible with experimental results. These conformations include simple motifs such as coil, α helix, β-turn, β-hairpin, βα, and coil-α conformations, but also more complex motifs such as turnlike, ββα, βββ, and α-helical hairpin conformations.
Hyper-parallel tempering Monte Carlo: Application to the Lennard-Jones fluid and the restricted primitive model
Authors: Yan Qiliang , de Pablo Juan J.
Published: July 2002

A new generalized hyper-parallel tempering Monte Carlo simulation method is presented. The method is particularly useful for simulation of many-molecule complex systems, where rough energy landscapes and inherently long characteristic relaxation times can pose formidable obstacles to effective sampling of relevant regions of configuration space. In this paper, we demonstrate the effectiveness of the new method by implementing it in a grand canonical ensemble for the Lennard-Jones fluid and the restricted primitive model. Coexistence curves and critical behavior have been explored by the new method. Our numerical results indicate that the new algorithm can be orders of magnitude more efficient than previously available techniques.
Kinetic and Monte Carlo models of thin film coarsening: Cross over from diffusion-coalescence to Ostwald growth modes
Authors: Lo Alan , Skodje Rex T.
Published: July 2002

Thin films of adsorbates on solid surfaces often exhibit irreversible clustering and island growth phenomena where the mean island size grows larger with a temporal power law dependence, accompanied by a scaling island size distribution function. This coarsening process is typically described within a thermodynamic framework using the Ostwald ripening formalism. However, there are strong indications that the Ostwald formulation is incomplete since it omits critical atomic level phenomena such as island mobility, spatial correlation between kinetic processes, and surface roughening of the islands. We have simulated thin film coarsening on an FCC(100) surface using a large Monte Carlo lattice gas model. Scaling exponents and island distribution functions were extracted from the simulations. From the Monte Carlo, we have computed rate constants for island evaporation–recondensation and island coalescence. Using a high-dimensional set of rate equations, a quasichemical mean field approach is formulated as a high dimensional set of second-order kinetics equations. The power law scaling behavior of the coarsening is reproduced by both the Monte Carlo simulations and the mean field theory. The relative importance of Ostwald theory versus island coalescence is evaluated.
Study of inhomogeneity of supercritical water by small-angle x-ray scattering
Authors: Morita Takeshi , Kusano Kohei , Ochiai Hiroto , Saitow Ken-ichi , Nishikawa Keiko
Published: July 2002

Small-angle x-ray scattering (SAXS) experiments using synchrotron radiation were carried out for supercritical water along isotherms at the temperatures T=660.0, 661.5, 663.0, 677.0, and 687.5 K, from a gaslike density region to a liquidlike one, including an intermediate density region. The high-temperature and high-pressure sample holder for SAXS measurements suitable for supercritical water was redesigned for more precise measurements. The curves illustrating the density dependence of density fluctuations and correlation lengths show a slight shift of the maximum from critical isochore. The deviations become larger with increasing temperature. The results for the density fluctuations and correlation lengths for supercritical water are compared with those for supercritical CO2 and CF3H at T/Tc=1.02 and 1.06. The comparison allows us to draw the conclusion that the behavior in the long-range inhomogeneity of water in the supercritical state is in discord with the ordinary behaviors for other molecular substances. Density fluctuations in water are also compared with those of Ar and Hg calculated thermodynamically by use of the equations of state. The correlation of the symmetry between the contour of density fluctuations and the vapor–liquid coexistence curve is discussed.
Spectroscopy of Mg atoms solvated in helium nanodroplets
Authors: Reho J. , Merker U. , Radcliff M. R. , Lehmann K. K. , Scoles G.
Published: July 2002

We have measured the laser-induced fluorescence excitation spectra of the 3 1P10←3 1S0 transition of Mg atoms solvated in helium nanodroplets. The observed blue shifts and line broadenings mirror the shifts and broadenings observed in studies of Mg atoms solvated in bulk liquid helium. This similarity allows us to conclude that Mg atoms reside in the interior of the helium droplet. The 3 1P10←3 1S0 transition shows a splitting which we attribute to a quadrupolelike deformation of the cavity which forms around the solute atom after excitation. Temporal evolution of the fluorescence from the solvated 3 1P10 Mg yields a longer lifetime (2.39±0.05 ns) than found in vacuum (1.99±0.08 ns). This difference can be accounted for quantitatively by evaluation of the anisotropic distribution of the helium density in the neighborhood of the excited Mg atom. The question of solvation vs surface location for the guest atoms is also discussed in light of the model of Ancilotto et al. [F. Ancilotto, P. B. Lerner, and M. W. Cole, J. Low Temp. Phys. 101, 1123 (1995)], of existing metal atom–helium potential energy functions, and of our own calculations for the MgHe and CaHe ground states. While the Ancilotto model successfully predicts solvation (or lack of it) if the solvation parameter of the guest atom is not too near the threshold of 1.9, the present knowledge of the interatomic potentials is not precise enough to test the model in the neighborhood of the critical value.
Reconciling Gibbs and van der Waals: A new approach to nucleation theory
Authors: Schmelzer Jürn W. P. , Schmelzer Jürn , Gutzow Ivan S.
Published: July 2002

A new approach for the determination of the work of critical cluster formation in the description of nucleation processes is developed. For an illustration, the method is applied here to phase formation processes in solid and liquid solutions. However, it is applicable quite generally and not restricted to this particular important case. The present approach represents a generalization of the classical Gibbs’ method. It is—like the classical method—conceptionally simple and directly applicable to real systems, but avoids its shortcomings. Central to our method is the formulation and application of a new well-founded principle we denote as generalized Ostwald’s rule in nucleation. The method allows the determination of the work of critical cluster formation provided the bulk properties and the macroscopic values of the surface tension (at planar interfaces) for the possible different states of the system under consideration are known. Similarly to the van der Waals–Cahn–Hilliard and density functional calculations in the determination of the work of critical cluster formation, the newly developed method reproduces the results of the classical Gibbs’ nucleation theory (involving the capillarity approximation) for small values of the supersaturation. However, in contrast to the classical and in agreement with van der Waals-type methods, for initial states approaching the spinodal curve, the work of critical cluster formation, determined via the newly developed approach, is shown to tend to zero. As an immediate additional consequence, the method gives a more accurate description of the experimental results on nucleation rates also in the intermediate ranges of the initial supersaturation. Some further implications for the theoretical interpretation of nucleation-growth experiments are discussed as well.
The dynamics of the CO–N2 interaction: Strong Coriolis coupling in CO-paraN2
Authors: Xu Yunjie , Jäger Wolfgang
Published: July 2002

Rotational and ro-vibrational spectra of the paraN2 species of the CO–N2 complex were measured with a pulsed molecular beam Fourier transform microwave spectrometer in the frequency region from 4 to 26 GHz. a-type rotational transitions within the relatively unperturbed upper K=1 levels of CO-paraN2, previously studied in the infrared region by Xu and McKellar [J. Chem. Phys. 104, 2488 (1996)], were observed and analyzed. Two new states, namely the K=0 levels of the first excited van der Waals vibrational N2 bending state and the lower K=1 levels of the ground vibrational state of CO-paraN2, were investigated. Both a-type rotational transitions within these two stacks and b-type ro-vibrational transitions connecting these two stacks were detected. A strong Coriolis interaction between these two new states was studied in detail. Nuclear quadrupole hyperfine splittings due to the presence of two equivalent N14 nuclei were resolved and analyzed to give additional information about the angular anisotropy of the interaction potential energy surface. In addition, rotational and ro-vibrational spectra of the paraN2 species of two minor isotopomers, i.e., C1613O–N2 and C1813O–N2, were measured. Strong Coriolis interaction was also observed and studied for the C1613O-paraN2 isotopomer. In the spectra of C1613O-paraN2, additional splittings due to C13 spin–rotation interaction were detected, besides the N14 nuclear quadrupole hyperfine structure. These data provide further important information about the intermolecular interaction between CO and N2.
The near ultraviolet rotation-vibration spectrum of water
Authors: Zobov N. F. , Belmiloud Djedjiga , Polyansky Oleg L. , Tennyson Jonathan , Shirin S. V. , Carleer M. , Jenouvrier A. , Vandaele A.-C. , Bernath P. F. , Mérienne M. F. , Colin R.
Published: July 2002

A new ab initio linelist of water vibration-rotation transitions extending up to 26 000 cm−1 is presented. This linelist is used to analyze the near-ultraviolet portion of the long pathlength water absorption spectrum of Carleer et al. [J. Chem. Phys. 111, 2444 (1999)]. A total of 299 of the 568 observed transitions between 21 400 and 25 232 cm−1 are assigned. These transitions belong to eight excited vibrational states: (4,2)−1, (7,0)+0, (7,0)−0, (6,0)+2, (6,0)−2, (7,0)−1, (8,0)−0, and (8,0)−0, in local mode notation. Only three of these states have been observed previously. Observed and calculated energy levels are presented for these vibrational states and the band origins are determined.
Behavior of confined telechelic chains under shear
Authors: Huh June , Balazs Anna C.
Published: July 2002

Using Monte Carlo simulations, we investigate the rheological behavior of end-functionalized chains confined between two surfaces. The linear chains contain a reactive group (“sticker”) at each end and thus, have a telechelic architecture. We vary the functionality, f, of these stickers from 0 to 3 and calculate the shear stress for different shear rates. For f>1, the chains form a transient network that effectively blocks the stretching of the chains under shear. As a consequence, the shear stress plateaus to a constant value at high shear rates. These findings are in agreement with recent experimental studies on a comparable system. For f>1, we also find that the limiting shear stress decreases with increasing values of f.
Simplicial minisuperspace I. General discussion
Authors: Hartle James B.
Published: July 2002

The use of the simplicial methods of the Regge calculus to construct a minisuperspace for quantum gravity and approximately evaluate the wave function of the state of minimum excitation is discussed.
The Cauchy problem for stringy gravity
Authors: Choquet-Bruhat Yvonne
Published: July 2002

The Cauchy problem and the propagation of discontinuities for stringy gravity—that is, equations with Gauss–Bonnet terms—in arbitrary dimensions are studied.