bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodetachment and photofragmentation studies of semiconductor cluster anions
Authors: Liu Y. , Zhang Q.-L. , Tittel F. K. , Curl R. F. , Smalley R. E.
Published: July 2002

Silicon, germanium, and gallium arsenide negative cluster ions are produced by laser vaporization followed by free supersonic expansion. Electron affinities (EA) of the corresponding neutral clusters are roughly bracketed by measuring the fluence dependence for photodetachment from anions at discrete probe laser wavelengths (above the photodetachment threshold the dependence on fluence is linear, below it is quadratic). An even/odd alternation is found in the negative ion distribution with gallium arsenide clusters with an odd number of atoms having higher EA’s than their even neighbors. This suggests that the surfaces of the even clusters are extensively restructured in a way which eliminates dangling chemical bonds. For GaxAsy with x+y constant, EA increases with increasing ratio of y to x. The EA of silicon increases smoothly with cluster size extrapolating towards the EA of bulk silicon. Photofragmentation studies show that, like the corresponding positive ions, silicon and germanium negative ions with 11 to 23 atoms fission into mainly 5 to 10 atom negative ions. Si−10 and Ge−10 are the favorite daughters suggesting the existence of a special structure for ten atoms.
Spin-orbit interactions and dynamical Jahn–Teller effect in the 4E states of Mn2+ in cubic and nearly cubic symmetry
Authors: Parrot R. , Naud C. , Gendron F. , Porte C. , Lemercier M.
Published: July 2002

A very detailed analysis of the fine structure of the 4E(G) state of Mn2+ in cubic and nearly cubic clusters is presented to account for the unusual fine structure observed in ZnSe:Mn. First, experimental results are reported which show that the spin-orbit interaction in the 4E(G) state of Mn2+ in ZnSe is about one half of that previously observed in cubic or nearly cubic clusters as MnS4, MnBr4, and MnCl4 and that the state Γ8 is neatly shifted with respect to the Kramers doublets Γ6 and Γ7. Second, a model is elaborated which shows that second-order spin-orbit interactions reduced by a Jahn–Teller coupling to E vibrational modes accounts well for the observed overall splitting of the 4E(G) state in the studied clusters and that fourth-order spin-orbit interactions correctly describe the shifts of the Γ8 state. The validity of the perturbation schemes is discussed and the results are compared to those obtained from a complete diagonalization in the d5 configuration.
Properties of the fractal measure describing the hydrodynamic force distributions for fractal aggregates moving in a quiescent fluid
Authors: Meakin Paul , Deutch John M.
Published: July 2002

Using the Kirkwood–Riseman theory we show that the penetration of the hydrodynamic field for a fractal aggregate (diffusion-limited cluster–cluster aggregate) moving with a constant translational velocity with respect to a fluid can be described in terms of a fractal measure. The spectrum of singularities,  f(α) of strength α defined by Halsey et al. has been estimated by scaling the total force distribution and the distribution of force components parallel and perpendicular to the relative velocity for aggregates of different sizes. The scaling of the moments of the force and force component distributions with the cluster mass has also been investigated. Each moment scales with a different exponent which is related to the spectrum  f (α) and the corresponding infinite hierarchy of fractal dimensionalities Dq.
Electron-impact spectroscopy of acetaldehyde
Authors: Walzl K. N. , Koerting C. F. , Kuppermann A.
Published: July 2002

Acetaldehyde has been studied by the technique of low-energy variable-angle electron energy-loss spectroscopy. With this method the low-lying spin-forbidden transitions have been located via the behavior of the relative differential cross sections, providing the first identification by this technique of such states in acetaldehyde. High-lying states were also investigated and some assignments of dipole symmetry-forbidden/quadrupole symmetry-allowed excitations were made on the basis of characteristic angular behavior, evident for the asymmetric molecule acetaldehyde just as for the symmetric molecules formaldehyde and acetone. Through a comparison of the acetaldehyde results with those for formaldehyde and acetone the trends in the allowed and forbidden transition energies were examined as a function of methyl substitution and found to be relatively linear.
Theoretical studies of vibrationally excited polyatomic molecules using canonical Van Vleck perturbation theory
Authors: Sibert Edwin L.
Published: July 2002

The highly excited vibrational states of polyatomic molecules are investigated using canonical Van Vleck perturbation theory, implemented in a superoperator framework. This approach is used to transform a vibrational Hamiltonian to a new representation which has a form ideally suited to the study of the dynamics of interest. The key advantage is that the solution to the full problem is obtained in the new representation using significantly smaller basis sets than are needed to obtain the solutions in the original representation. The transformations are applied to the Hamiltonian operator itself, not the Hamiltonian matrix; this superoperator approach obviates the need for large basis sets. The tedious and complex algebra, that is required to perform these transformations, is readily implemented with FORTRAN codes. Combining these two features has enabled the investigations of vibrational dynamics in energy regimes and densities of states, unattainable by standard methods. These methods are applied to two model problems and to the study of the highly excited overtones of CHD3 with up to five quanta of excitation in the CH bond.
Electronic states and decay mechanisms of the N2O2+ dication
Authors: Price S. D. , Eland J. H. D. , Fournier P. G. , Fournier J. , Millie P.
Published: July 2002

Energies of the electronic states of the triatomic dication N2O2+ in the Franck–Condon zone of neutral N2O have been determined by a combination of (1) double charge transfer spectroscopy to locate singlet states, (2) photoionization measurements to locate the lowest triplet state, and (3) configuration-interaction calculations to identify the states and to predict the energies of other triplets. It seems likely that two distinct charge separation reactions compete in the relatively slow decay of the N2O2+ ground state.
Lattice models of polymer fluids: Monomers occupying several lattice sites. II. Interaction energies
Authors: Pesci Adriana I. , Freed Karl F.
Published: July 2002

Nearest neighbor nonbonded van der Waals interaction energies are appended to the description of lattice models of flexible polymers in which monomers have specific structures and may cover several lattice sites. A formally exact representation is derived for the free energy of multicomponent system of these structured self and mutually avoiding lattice polymers with nearest neighbor van der Waals interactions. Systematic expansions of the free energy are developed in powers of the inverse of the lattice coordination number and the van der Waals energies using the mean field Flory–Huggins approximation as the zeroth order reference. Diagrammatic rules are given for the evaluation of energies, and the free energy of a binary blend is calculated to second order beyond Flory–Huggins theory for polymers with monomers having differing sizes and shapes. The accompanying paper compares these results with experiment and applies them to analyze heats of mixing of blends, their temperature and composition dependence, and the enthalpic and entropic contributions to the Flory χ parameter.
Solvent-induced interactions: Hydrophobic and hydrophilic phenomena
Authors: Ben-Naim A.
Published: July 2002

The solvent-induced interactions between two kinds of molecules (or groups) are examined from the point of view of the solvation free energies of a pair of interacting molecules (or groups). The first interaction is that between two simple nonpolar solutes (or groups) in water and is known as the hydrophobic interaction. Both theoretical as well as experimental evidence indicate that these interactions are quite weak. The solvent-induced interactions between two functional groups that can form hydrogen bonds, referred to as intramolecular hydrophilic interactions, are found to be much stronger than the corresponding hydrophobic interactions. It is therefore argued that intramolecular hydrophilic interactions are probably more significant in biochemical processes than hydrophobic interactions.
High-resolution, slit jet infrared spectroscopy of hydrocarbons: Quantum state specific mode mixing in CH stretch-excited propyne
Authors: McIlroy Andrew , Nesbitt David J.
Published: July 2002

A direct absorption, difference frequency, infrared laser spectrometer with 10−4 cm−1 resolution combined with slit supersonic jet optical pathlengths is presented as a tool for the study of mode–mode vibrational coupling in laser-excited hydrocarbons. These weak mode–mode couplings are evidenced in our frequency domain studies by virtue of transitions to isolated upper J states that are split into multiplets under sub-Doppler resolution. Instrument performance is demonstrated by investigating vibrational coupling in the 3000–3300 cm−1 C–H stretch fundamental region of 12C3 propyne, as well as the 12C213C propynes observed in natural isotopic abundance. No appreciable state mixing is observed in ν1=1←0 and ν6=1←0 spectra at T=4 K. However, near-resonant two-state mixing of ν2 and ν5+ν8+3ν10 in the ν2=1←0 transition of 12C3 propyne is detected and deperturbed to provide an anharmonic coupling matrix element of 0.096 41(38) cm−1. This matrix element is independent of J′ and thus arises from purely anharmonic, non-Coriolis-mediated couplings. The implications of anharmonic coupling matrix elements of this magnitude in overtone vibrational dynamics are discussed.
Structural and electronic transitions in polyaniline: A Fourier transform infrared spectroscopic study
Authors: Sariciftci N. S. , Kuzmany H. , Neugebauer H. , Neckel A.
Published: July 2002

Fourier transform infrared (FTIR) spectroscopic studies are carried out on different, potentiometric well defined oxidation states of polyaniline in aqueous acidic and organic electrolytes. During the oxidation process ring structures transform from benzenoid into quinoid states. The fully reduced state of polyaniline shows differences in the anion contents in acidic and organic electrolytes. The 400 mV vs saturated calomel electrode (SCE) oxidized state has the maximum number of the intercalated anions in aqueous acidic media in accordance with supporting potentiometric titration experiments. This conducting form of polyaniline shows similar FTIR spectra in organic as well as in acidic media. For the oxidized state at 800 mV vs SCE, a deintercalation of anions in aqueous acidic, or further intercalation in organic electrolyte is observed. Beyond 800 mV vs SCE, polyaniline shows degradation processes in aqueous acidic media which are found to proceed via formation of benzoquinone-like structures and finally result in a complete dissolution of the polymer.
Manifestation of bifurcations and diabolic points in molecular energy spectra
Authors: Sadovskii D. A. , Zhilinskii B. I. , Champion J. P. , Pierre G.
Published: July 2002

The general scheme of the qualitative analysis of finite-particle quantum problems is discussed. Theoretical methods for the qualitative analysis of energy spectra in quantum problems are applied to the study of the rovibrational energy levels of spherical top molecules. The existence of modifications in the rotational cluster structure and of redistributions of the energy levels between different branches under the rotational excitation is shown for the ν2 /ν4 dyad of the 12 CH4 and 28 SiH4 molecules. An interpretation of the qualitative features of energy spectra in terms of bifurcations and formations of conical intersection points (diabolic points) on the energy surfaces of the corresponding classical problem is proposed.
The thermal conductivity of ethane in the critical region
Authors: Mostert R. , van den Berg H. R. , van der Gulik P. S. , Sengers J. V.
Published: July 2002

New experimental data are presented for the thermal conductivity of ethane. The thermal conductivity has been measured with a parallel-plate method as a function of temperature along eleven different isochores so as to obtain detailed information on the enhancement of the thermal conductivity in the critical region. The experimental results appear to be consistent with a recently proposed theoretical equation that accounts for both the asymptotic and nonasymptotic critical behavior of the thermal conductivity of fluids.
Adsorption of CO on Rh(100) studied by infrared reflection–absorption spectroscopy
Authors: Leung L.-W. H. , He J.-W. , Goodman D. W.
Published: July 2002

The interaction of CO with a Rh(100) surface at 90 and 300 K has been investigated with infrared reflection–absorption spectroscopy (IRAS). Absorption bands due to the C–O stretch are found in both the linear and bridging regions at all coverages for both adsorption temperatures. For adsorption at 300 K, an ordered CO layer is formed at ∼0.4 monolayers (ML) as evidenced by a sharp, highly symmetrical, linear-CO band. At θCO>0.45 ML, the reduction in intermolecular separation and increasing intermolecular repulsive forces in the CO adlayer are evidenced by the increased broadness of both the linear- and the bridged-CO bands. CO adsorption at 90 K yields a nonuniform adlayer dominated by island formation at θCO<0.5 as indicated by peak splitting of the linear-CO band and the presence of a broad bridged-CO band. At θCO∼0.5, a highly ordered CO adlayer is suggested by the appearance of a highly symmetrical linear-CO band as well as the formation of a sharp c(2×2) LEED pattern. The presence of multiple components in the linear-CO band along with the drastic variations in the relative absorption intensities for both CO bands at coverages between 0.5 and 0.7 ML imply that the CO adlayer undergoes a phase transition within this coverage regime. These IRAS data are in agreement with the observed transition of a sharp c(2×2) pattern at θCO=0.5 to an ordered LEED pattern with a p(4×4)R45° coincidence structure at θCO=0.7. From a series of temperature-dependent IRAS measurement, it has been determined that the difference in binding energy of linear and bridged sites at θCO=0.2–0.5 ranges from ∼0.1 to ∼0.4 kcal/mol with the bridged-bound CO being the more stable species.
An analysis of the hydrogen bond in ice
Authors: White Julia C. , Davidson Ernest R.
Published: July 2002

In this study, the total self-consistent field binding energy of a small ice cluster is estimated as the sum of two- and three-body interaction terms. The energy of each term is analyzed using the Morokuma method. Counterpoise calculations were performed to estimate the basis set superposition errors. Additional calculations were done to determine the correlation energy contribution to the total binding energy. Results show that the nearest-neighbor two-body components contribute most to the total binding energy, but also that the contiguous three-body terms must be included to reproduce the total binding energy of the system.
Melting line of Yukawa system by computer simulation
Authors: Meijer Evert Jan , Frenkel Daan
Published: July 2002

We located the melting line of the Yukawa system by determining the free energy of both fluid and solid phases by computer simulations. At the high densities the fluid freezes into a body-centered-cubic (bcc) solid, whereas for low densities it freezes into a face-centered-cubic (fcc) solid. For both the bcc-fluid and the fcc-fluid part of the melting line two coexistence points were determined. We observed that the relative root-mean-square displacement in the solid (Lindeman ratio) varies considerably along the melting line. At the bcc-fluid part of the melting line the Lindeman ratio equals 0.19, whereas at the fcc-solid part of the melting line it is smaller than 0.16.
A generalized Langevin equation approach to reorientational dynamics in nematic liquid crystals
Authors: Gertner Bradley J. , Lindenberg Katja
Published: July 2002

A new model of reorientational dynamics in nematic liquid crystals, based on a linear generalized Langevin equation (GLE) representation of the dynamics of a probe molecule, is developed. Derived in the limit of high order, the linearized angular motion of a probe molecule under the influence of director fluctuations is analyzed when the time scale of rotational relaxation is comparable to that of the cooperative modes of the liquid crystal solvent. This model allows negative total solvent contributions (director fluctuations plus a negative cross term) to the spectral density J1(ω) relative to the rotational diffusion contribution, a result predicted experimentally by least-squares data fits. This result cannot be justified in terms of existing theories that assume a separation of time scales between the probe molecule motion and relaxation of the cooperative modes of the solvent. Results from the GLE-based model (and the standard model) are compared to measured spectral densities of a highly ordered spin probe dissolved in a nematic liquid crystal [W. H. Dickerson, R. R. Vold, and R. L. Vold, J. Phys. Chem. 87, 166 (1983)]. Because the frequencies involved are low, the model predictions are very similar and excellent numerical agreement is found with both models. However, because the total solvent contribution is observed to involve inhibition of relaxation relative to the rotational diffusion, the standard model must be rejected on the basis of being physically unreasonable. The GLE model, on the other hand, is on firm physical ground and completely reasonable. The observed negative total solvent contribution to the spectral density can be explained in terms of a coupling of cooperative solvent modes with the molecular reorientation of the probe molecule that interferes with relaxation.
Orientation and absolute coverage of furan and 2,5-dihydrofuran on Ag(110) determined by near edge x-ray absorption fine structure and x-ray photoelectron spectroscopy
Authors: Solomon J. L. , Madix R. J. , Stöhr J.
Published: July 2002

The orientation, absolute coverage and core-electron binding energies of furan (C4H4O) and 2,5-dihydrofuran (C4H6O) on Ag(110) have been measured using near edge x-ray absorption fine structure and x-ray photoelectron spectroscopy. Both furan and 2,5-dihydrofuran (2,5-DHF) are tilted 22±7° from the plane of the surface for both submonolayer and monolayer coverages. The saturation monolayer coverages on the Ag(110) surface of 0.45±0.07 ML for furan and 0.41±0.06 ML for 2,5-DHF are consistent with expectations based on van der Waals radii. The C(1s) binding energies for a monolayer of furan on Ag(110) are 284.8 and 286.2 eV, while the C(1s) binding energies for 2,5-DHF are 285.1 and 286.6 eV. The O(1s) binding energies are 532.8 and 533.8 eV for furan and 2,5-DHF monolayers, respectively. The onset of excitations from C(1s) levels to the continuum lies well above the Fermi level due to the weak bonding interactions with the surface.
Dissociation of small methanol clusters after excitation of the O–H stretch vibration at 2.7 μ
Authors: Huisken Friedrich , Kulcke Axel , Laush Curtis , Lisy James M.
Published: July 2002

Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters upon excitation of the O–H stretch vibration at 2.7 μ. The tunable infrared radiation has been obtained from a Nd:YAG laser pumped optical parametric oscillator. Pure methanol dimer spectra, without contamination from larger clusters, have been measured using the scattering selection technique or working at reduced temperatures. The dimer spectrum features two absorption peaks at 3574.4 and 3684.1 cm−1. The lower frequency peak is redshifted by 106.6 cm−1 from the monomer value and is assigned to the excitation of the proton donor in the hydrogen bonded complex. The proton acceptor peak is blueshifted by only 3.1 cm−1. Methanol trimer dissociation is observed at 3462 cm−1, whereas larger clusters absorb at still lower frequencies. Absolute photodissociation cross sections were measured for both methanol dimer bands. The integrated dissociation cross sections were 5.59(25)×10−21 cm2 /molecule and 7.1(2.2)×10−22 cm2 /molecule for the proton donor and acceptor bands, respectively. The integrated dissociation cross sections are directly related to the O–H line strengths in the dimer. The increased line strength, for the proton donor band relative to the acceptor band by a factor of ∼8, is in accord with previous observations associated with hydrogen bond formation.
Coupled-cluster method truncated at quadruples
Authors: Oliphant Nevin , Adamowicz Ludwik
Published: July 2002

The coupled-cluster (CC) equations including single, double, triple, and quadruple excitation amplitudes (CCSDTQ) are derived diagramatically, and the complete set of CCSDTQ equations are presented. These equations have been programmed and an iterative reduced linear equation method is used to solve these equations. The potential curves for the dissociation of a model system with a single bond (Li2 and LiH) is calculated using CC doubles (CCD), singles and doubles (CCSD), singles, doubles, and triples (CCSDT), and CCSDTQ. These calculations demonstrate the magnitude of the CC contributions arising from single, double, triple, and quadruple excitation amplitudes to the stretching of a chemical bond.
Estimation of the chemical potential of chain molecules by simulation
Authors: de Pablo Juan J. , Laso Manuel , Suter Ulrich W.
Published: July 2002

A novel method is presented for the calculation of the chemical potential of molecules with articulated structure by molecular simulation. The method, based on a biased sampling of phase space, is applied here to systems of linear alkanes represented by methylene pseudoatoms connected by rigid bonds at fixed angles. The results of our biased-sampling scheme are compared to those of other available methods (when they can provide results) for calculation of the chemical potential of dense systems.
Vibrational dynamics of aniline (N2)1 clusters in their first excited singlet state
Authors: Hineman M. F. , Kim S. K. , Bernstein E. R. , Kelley D. F.
Published: July 2002

The first excited singlet state S1 vibrational dynamics of aniline(N2)1 clusters are studied and compared to previous results on aniline(CH4)1 and aniline(Ar)1. Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) rates fall between the two extremes of the CH4 (fast IVR, slow VP) and Ar (slow IVR, fast VP) cluster results as is predicted by a serial IVR/VP model using Fermi’s golden rule to describe IVR processes and a restricted Rice–Ramsperger–Kassel–Marcus (RRKM) theory to describe unimolecular VP rates. The density of states is the most important factor determining the rates. Two product states, 00 and 10b1, of bare aniline and one intermediate state ∼(00) in the overall IVR/VP process are observed and time resolved measurements are obtained for the 000 and ∼(000) transitions. The results are modeled with the serial mechanism described above.
Application of the complex coordinate method to the conical resonances of Jahn–Teller spectra
Authors: Atabek O. , Lefebvre R.
Published: July 2002

Various versions of the complex coordinate method are applied to the characterization (positions and widths) of the Slonczewski resonances associated with the conical intersections of a E⊗ε Jahn–Teller system in the linear approximation and strong coupling regime. The procedure is based on the direct integration of the coupled differential equations either in a diabatic or in an adiabatic formulation, with imposition of appropriate boundary conditions. Complex rotation of the radial coordinate changes the real eigenenergies of this bound system into complex ones with imaginary parts giving rate constants. Various checks on the significance of these energies are made. Padé approximants are used to relate them to the results of Slonczewski obtained under the assumption of zero restoring force.
A theoretical treatment of the predissociation of the individual rovibronic levels of OH/OD(A 2Σ+)
Authors: Yarkony David R.
Published: July 2002

The electronic structure aspects of the radiationless decay of the A 2Σ+ state of OH/OD are considered using large scale configuration interaction (CI) (590 000–750 000 configuration state functions) wave functions and the full microscopic Breit–Pauli interaction. The A 2Σ+∼(4Σ−,2Σ−,4Π) spin–orbit induced perturbations are reported for 1.25≤R≤10 a0. These perturbations are used to determine the total decay rates (radiative plus radiationless decay) within a Fermi golden rule approximation. Excellent agreement with the available experimental data is found.
Franck–Condon analysis of the S→T1 absorption and phosphorescence spectra of biphenyl and bridged derivatives
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

The equilibrium geometry and the vibrational force field of the ground and the lowest triplet electronic states of biphenyl and three bridged derivatives−biphenylene, fluorene and phenanthrene−are computed by using an updated version of the QCFF/PI (Quantum Chemical Force Field/π electron) Hamiltonian. The displacement parameters between T1 and S0 are obtained and used to model the S0→T1 absorption and the phosphorescence spectra. The calculated Franck–Condon envelopes are found to be in excellent agreement with the vibrational structure of the observed spectra. The common features of the phosphorescence spectra of biphenyl and fluorene are related to the same orbital nature of the lowest triplet state. The observed asymmetry between the phosphorescence and singlet–triplet absorption spectra of biphenyl is reproduced when the twisted equilibrium geometry of S0 is considered. It is shown that evidence of the nonplanarity of the ground state of biphenyl is manifested by the lower intensity of the band observed in the phosphorescence at 747 cm−1 with respect to the intensity of the same band in fluorene. The increased vibrational activity calculated in the lower frequency region for biphenylene and phenanthrene agrees with the observed spectra and reflects the different orbital nature of the lowest triplet state of the two strongly perturbed bridged derivatives with respect to biphenyl and fluorene. From the analysis of the computed vibrational frequencies, it is suggested that the false origin of the symmetry forbidden phosphorescence of biphenylene is due to the lowest out-of-plane mode of au symmetry.
A molecular theory of solvation dynamics
Authors: Raineri Fernando O. , Resat Haluk , Perng Baw-Ching , Hirata Fumio , Friedman Harold L.
Published: July 2002

The dynamic solvation time correlation function 𝒵(t) is, within linear response, formulated in terms of the intermolecular solute–solvent interactions, without recourse to the intrinsically macroscopic concept of a cavity carved out of a dielectric medium. For interaction site models (ISM) of both the solute and the solvent, the theory relates the fluctuating polarization charge density of the solvent to the fluctuating vertical energy gap that controls 𝒵(t). The theory replaces the factual (or bare) solute charge distribution by a surrogate expressed in terms of the solute–solvent site–site direct correlation functions. Calculations for solute ions in water and in acetonitrile lead to 𝒵(t) and the second moment of the associated spectral density in good agreement with molecular dynamics simulation results in the literature. We also use the theory to calculate 𝒵(t) for model solutes in which the ‘‘sudden’’ change of the charge distribution involves multipoles of higher order. The response is qualitatively similar in the various cases studied here.
Liquid theory for the instantaneous normal modes of a liquid
Authors: Wan Yi , Stratt Richard M.
Published: July 2002

At short enough times, the dynamics of a liquid can be resolved rigorously into independent simple harmonic motions called instantaneous normal modes. The spectrum of such modes is easily accessible via computer simulation, but, despite the existence of theories for other kinds of liquid modes, it has been difficult to come up with analytical approaches of power sufficient to explain the universal appearance of instantaneous normal-mode spectra—though Wu and Loring were recently able to arrive at a theory by exploiting the analogy between this problem and the master equation. In this paper we propose a hierarchy of liquid-theoretical treatments that do show the analogy between instantaneous normal modes and other collective excitations in liquids, but are nonetheless capable of leading to accurate predictions of instantaneous normal-mode spectra. The crucial ideas are that the theoretical treatment must respect the fact these modes conserve momentum and must also recognize the strongly local character of intermolecular force constants. We discuss two theories in detail—a renormalized mean-field theory, which turns out to be identical to the Wu–Loring theory, and a higher-order theory—and apply both to a simple atomic liquid. Both theories successfully predict the results of computer simulations, including the fact that the spectrum depends much more on density than on temperature in the normal liquid range. The higher-order theory, though, gives a slightly more accurate prediction of the fraction of imaginary modes.
Folding kinetics of proteinlike heteropolymers
Authors: Socci Nicholas D. , Onuchic José Nelson
Published: July 2002

Using a simple three-dimensional lattice copolymer model and Monte Carlo dynamics, we study the collapse and folding of proteinlike heteropolymers. The polymers are 27 monomers long and consist of two monomer types. Although these chains are too long for exhaustive enumeration of all conformations, it is possible to enumerate all the maximally compact conformations, which are 3 ×3×3 cubes. This allows us to select sequences that have a unique global minimum. We then explore the kinetics of collapse and folding and examine what features determine the various rates. The folding time has a plateau over a broad range of temperatures and diverges at both high and low temperatures. The folding time depends on sequence and is related to the amount of energetic frustration in the native state. The collapse times of the chains are sequence independent and are a few orders of magnitude faster than the folding times, indicating a two-phase folding process. Below a certain temperature the chains exhibit glasslike behavior, characterized by a slowing down of time scales and loss of self-averaging behavior. We explicitly define the glass transition temperature (Tg), and by comparing it to the folding temperature (Tf), we find two classes of sequences: good folders with Tf≳Tg and non-folders with Tf<Tg.
Rydberg spectra of ArH. Bound–bound interactions, predissociation, and radiative lifetimes of the Rydberg states
Authors: Theodorakopoulos G. , Petsalakis I. D.
Published: July 2002

Bound–bound 2Σ+–2Σ+ and 2Σ+–2Π interactions and their effect on the rotational-vibrational levels of Rydberg states of ArH/ArD have been investigated. Strong interactions have been obtained for the B 2Π–C 2Σ+, D 2Σ+–E 2Π, and E 2Π–1 2Δ pairs of states in accord with recent experimental analyses of Rydberg spectra of ArD. Predissociation lifetimes of A 2Σ+, C 2Σ+, and D 2Σ+ as well as transition energies and radiative lifetimes for Rydberg states up to 4f have been also calculated.
Spectral line shape. I. Kinetic equation for arbitrary frequency detunings
Authors: Tvorogov S. D. , Rodimova O. B.
Published: July 2002

A theoretical formulation of the problem of the spectral line shape is presented using the superoperator technique. Conventional approximations, such as the long wave approximation, the binary approximation and the classical motion of molecular centers of mass, are employed. The kinetic equation for the line profile from resonance to the far wings is derived. Its main feature is that the terms prevailing at small or large frequency detunings are separated in the equation itself.
Ozone generation through photolysis of an oxygen matrix at 11 K: Fourier transform infrared spectroscopy identification of the O...O3 complex and isotopic studies
Authors: Schriver-Mazzuoli L. , de Saxcé A. , Lugez C. , Camy-Peyret C. , Schriver A.
Published: July 2002

This article describes the Fourier transform infrared spectra of ozone obtained by photolysis of an oxygen matrix at λ≥245 nm. A complex between ozone and atomic oxygen was identified. Results with isotopic oxygen are consistent with a structure in which the oxygen atom is bonded to the central oxygen atom of ozone. Complex formation does not change the photochemistry of matrix isolated ozone. Investigation of the ozone formation was also performed based on the dissociation of normal and scrambled ozone isolated in oxygen matrix. Formation was found to be of a first order reaction limited by the initial number of reactive species. Results are compared with those in gas phase for which recently, molecular oxygen in highly excited vibrationally states has received attention as a possible source of stratospheric O3 via an autocatalytic process.
Analysis of the zero-point energy problem in classical trajectory simulations
Authors: Guo Yin , Thompson Donald L. , Sewell Thomas D.
Published: July 2002

We examine methods for dealing with the flow of zero-point energy in classical trajectory simulations and identify some of the problems associated with their use. Fundamental issues which must be considered, both in assessing the extent of the zero-point energy problem and in the development of useful remedies, are discussed.
Tunneling currents in electron transfer reaction in proteins. II. Calculation of electronic superexchange matrix element and tunneling currents using nonorthogonal basis sets
Authors: Stuchebrukhov Alexei A.
Published: July 2002

In this paper we further develop the concept of interatomic tunneling currents [A.A. Stuchebrukhov, J. Chem. Phys. 104, 8424 (1996)] for the description of long-range electron tunneling in proteins. Here we discuss a formulation of the theory for the case when nonorthogonality of the atomic basis set of the medium propagating electron is explicitly taken into account. This method provides an effective computational scheme for an exact, i.e., nonperturbative, evaluation (in one-electron approximation) of the superexchange electron tunneling matrix element, and allows one to determine which regions in the protein matrix are important for the tunneling process. The theory is applied for calculation of tunneling currents and the electronic matrix element in His126-Ru-modified blue copper protein azurin from a recent experimental work of Gray and co-workers. Analysis of interatomic currents reveals a nontrivial structure of the tunneling flow between donor and acceptor in the intervening protein medium in this system.
Efficient continuum model for simulating polymer blends and copolymers
Authors: Grest Gary S. , Lacasse Martin-D. , Kremer Kurt , Gupta Anshu M.
Published: July 2002

An efficient continuum model for simulating polymer blends and copolymers is presented. In this model, the interactions are short-range and purely repulsive, thus allowing for excellent computational performances. The driving force for phase separation is a difference in the repulsive interaction strength between like and unlike mers. The model consists essentially of a molecular-dynamics algorithm, supplemented by an appropriate Monte Carlo exchange process. To demonstrate the effectiveness of the model we study two systems, a symmetric binary blend of polymers and a symmetric diblock copolymer system. For the binary blend, we determine the phase diagram and find, as predicted by theory, that the critical interaction parameter scales with the inverse of the chain length of the polymers. For the diblock copolymer system, we study both the one-phase region and the microphase separated lamellar region. For the latter, we show that constant-pressure algorithms are more appropriate since, contrary to recent lattice simulations, the lamellar spacing can self-adjust in such an ensemble.
Rate coefficients for the endothermic reactions C+(2P)+H2(D2)→CH+(CD+)+H(D) as functions of temperature from 400–1300 K
Authors: Hierl Peter M. , Morris Robert A. , Viggiano A. A.
Published: July 2002

We have measured the bimolecular rate coefficients for the reactions of C+(2P) with H2 and D2 as functions of temperature from 400 to 1300 K using a high temperature flowing afterglow apparatus. The temperature dependences of these rate coefficients are accurately fit by the Arrhenius equation, with activation energies equal within experimental uncertainty to the reaction endothermicities. Internal energy dependences have been deduced by combining the present data with previous drift tube and ion beam measurements. We found that reactant rotational energy and translational energy are equally effective in surmounting the energy barrier to reaction, and that vibrational excitation of the neutral reactant to the v=1 state enhances the rate coefficients by a factor of ∼1000 for the reaction with H2 and by ∼6000 for the reaction with D2 at temperatures of 800 and 500 K, respectively. This vibrational enhancement is larger than the enhancement that would be produced if the same amount of energy were put into translational and/or rotational modes of the reactants. In addition, rate coefficients have been derived for the three-body association reaction of C+(2P) with H2 in a helium buffer over the temperature range 300–600 K.
Statistical-mechanically exact simulation of polymer conformation in an external field
Authors: Krishnaswami Soundararajan , Ramkrishna Doraiswami , Caruthers James M.
Published: July 2002

A mathematically exact method is presented for sampling conformations of polymer molecules in an external field with fixed energy or energy range in accord with the formulation of statistical mechanics for a microcanonical ensemble. As a consequence, conformations of negligible Boltzmann weight can be selectively eliminated from simulations for efficient calculation of macroscopic polymer properties. The method is applicable for conformations that are described by a stochastic differential equation along the contour length in the field-free situation. It is based on the concept of a stochastic bridge process for which a new stochastic differential equation is derived that has stipulations at both ends of the process. This idea is exploited on a pair of stochastic differential equations in the conformation vector X and an augmented variable Z which represents the running Boltzmann weight in the given field, transforming to a new pair of equations for which the terminal Boltzmann weight can be arbitrarily stipulated. The stochastic equation for the bridge involves solving the Fokker–Planck equation for the original stochastic pair. We demonstrate the method on the conformation of a “Brownian” polymer in a quadratic external field of varying strength. The stochastic differential equations for the bridge process in this case can be derived analytically. Sample conformations are displayed that satisfy exactly energy constraints either at fixed values or within a stipulated range. It is shown that polymer properties can be computed more efficiently and accurately with the bridge process simulations than by unconstrained process simulations. The bridge process approach presented here must be distinguished from other approaches such as umbrella sampling methods because of the former’s ability to sample conformations exactly with stipulated energy constraints.
An adaptive numerical integrator for molecular integrals
Authors: Krack Matthias , Köster Andreas M.
Published: July 2002

We present a new numerical integrator for molecular integrals that generates automatically an adaptive molecular grid. The tolerance of the numerical integration is the only input parameter of the integrator besides the atomic coordinates and the atomic numbers. The adaptive numerical integrator was successfully implemented in our new density-functional theory method (DFT method) ALLCHEM using the self consistent field (SCF) procedure. The accuracy of the numerical integration is superior to pruned fixed grids for a given number of grid points. The adaptive grid generator allows a very efficient optimization of the grid for an individual molecular system. It is shown that the grid accuracy increases monotonically, if the given tolerance of the numerical integration is decreased. In this way it is possible to obtain results of high numerical precision. For a given tolerance of the numerical integration the adaptive grid generator automatically adjusts the grid to the basis set and the molecular structure. This feature increases the reliability of the numerical integration considerably and the number of expensive reference grid calculations can be reduced. The time for the grid generation is small compared to the total computation time for a SCF calculation.
A microscopic model of gemini surfactants: Self-assemblies in water and at air–water interface
Authors: Maiti Prabal K. , Chowdhury Debashish
Published: July 2002

We report the results of large scale Monte Carlo simulations of novel microscopic models of gemini surfactants to elucidate (i) their spontaneous aggregation in bulk water and (ii) their spatial organization in a system where water is separated from the air above it by a sharp well-defined interface. We study the variation of the critical micellar concentration (CMC) with the variation of the (a) length of the spacer, (b) length of the hydrophobic tail, and (c) the bending rigidity of the hydrocarbon chains forming the spacer and the tail; some of the trends of variation are counterintuitive but are in excellent agreement with the available experimental results. Our simulations elucidate the effects of the geometrical shape, size, and density of the surfactant molecules, the ionic nature of the heads, and the hydrophobicity/hydrophilicity of the spacer not only on the shapes of the micellar aggregates and the magnitude of the CMC, but also on their conformations close to the air–water interface.
Thermodynamics and the global optimization of Lennard-Jones clusters
Authors: Doye Jonathan P. K. , Wales David J. , Miller Mark A.
Published: July 2002

Theoretical design of global optimization algorithms can profitably utilize recent statistical mechanical treatments of potential energy surfaces (PES’s). Here we analyze the basin-hopping algorithm to explain its success in locating the global minima of Lennard-Jones (LJ) clusters, even those such as LJ38 for which the PES has a multiple-funnel topography, where trapping in local minima with different morphologies is expected. We find that a key factor in overcoming trapping is the transformation applied to the PES which broadens the thermodynamic transitions. The global minimum then has a significant probability of occupation at temperatures where the free energy barriers between funnels are surmountable.
Basis sets and active space in multiconfigurational self-consistent field calculations of nuclear magnetic resonance spin–spin coupling constants
Authors: Guilleme J. , San Fabián J.
Published: July 2002

The dependence of spin–spin nuclear magnetic resonance (NMR) coupling constants on the basis set and electron correlation has been investigated in methane using Hartree–Fock and multiconfigurational self-consistent field wave functions (HF-SCF and MCSCF). The effect of the size, contraction, and tight s functions of the basis sets is analyzed. Some suggestions about the contraction scheme are indicated. MCSCF wave functions with different numbers of active orbitals and different numbers of excited electrons were used. An approximation to determine spin–spin coupling constants at a high level of electron correlation from three calculations with a smaller level of correlation and reduced computational cost is investigated. The best calculated JCH1 and JHH2 couplings are 120.63 and −13.23 Hz, respectively, which are 0.24 and 1.24 Hz smaller than those experimentally obtained for the equilibrium geometry. The remaining error in these coupling constants can be attributed mainly to correlation and not to basis set effects.
Structure of ice multilayers on metals
Authors: Witek H. , Buch V.
Published: July 2002

Simulations are presented of model ice adsorbate layers on metals, in the coverage range of 2–4 bilayers. The issues investigated include the decay mechanism of ferroelectricity imposed by bonding of the first bilayer to the metal, and the influence of first bilayer flattening (due to bonding to the metal) on the adsorbate structure. A new kind of defect in the ice structure—an internal dangling OH bond—is reported, and shown to provide a mechanism for decay of ferroelectricity over the range of 2–3 bilayers only. Flattening of the bottom bilayer results in “sandwich-like” adsorbate structures, and disappearance of most of dangling-OH bonds from the upper surface.
A theory for self-diffusion in liquids
Authors: Vergeles Maxim , Szamel Grzegorz
Published: July 2002

We propose an alternative approach to self-diffusion in an atomic liquid. Our starting point is an oscillatory motion of a tagged particle in its first solvation shell (cage). Only after cage relaxation is taken into account is the tagged particle able to diffuse. This approach is suitable for describing liquids where the concept of binary collisions breaks down and the self-diffusion coefficient is small. Our predictions quantitatively agree with the results of MD simulations in a broad range of densities and temperatures up to the freezing transition.
Nucleation in periodically driven electrochemical systems
Authors: Smelyanskiy V. N. , Dykman M. I. , Rabitz H. , Vugmeister B. E. , Bernasek S. L. , Bocarsly A. B.
Published: July 2002

We calculate both the exponent and the prefactor in the nucleation rate of a periodically driven system. Nucleation dynamics is described by the Fokker–Planck equation for the probability distribution of the nuclei over their size. This distribution is found using the concept of the most probable (optimal) nucleation path. The results apply in a broad range of driving force amplitudes, from weak to moderately strong forces where the nucleation rate is changed exponentially strongly, and also in the broad range of the driving frequencies, from low-frequency driving, where the system follows the force adiabatically, to high-frequency nonadiabatic driving. For strong driving forces, the time dependence of the nucleation rate changes from strongly nonsinusoidal to a weak with the increasing frequency of driving. The response of the nucleation rate to the driving force is described in terms of logarithmic susceptibility (LS), which can be obtained from the optimal nucleation path in the absence of the driving. LS is a smooth function of frequency, and therefore even a driving force with comparatively high frequency can change the modulation rate exponentially strongly. LS and the Faraday current are calculated for simple models of electrochemical systems, where the ac driving is produced by modulation of the electrode potential. We also suggest how to find LS from measurements of the average nucleation rate.