bluuebunny/crossref_metadata_2025 huggingface dataset:
The radiative lifetime of A 1Πu C2
Authors: ONeil S. V. , Rosmus P. , Werner H.-J.
Published: July 2002

This note reports ab initio potential curves of A 1Πu and X 1Σ+g states of C2, and the A–X radiative lifetimes. The calculations are based upon configuration interaction wave functions formed from single and double electron excitations from state-averaged MC-SCF reference functions expanded in a Gaussian basis. The lifetimes for (J′=1,v′) A 1Πu are calculated to be about 25% longer than those found in previous ab initio works, but about 25% shorter than the most recent LIF measurements of Bauer, Becker, Bielefeld, and Meuser. Also reported are the quadrupole moments of the A 1Πu and X 1Σ+g states as a function of internuclear distance and the quadrupole lifetimes of several (v′=0,J′) levels in the X state.
A multicenter numerical integration scheme for polyatomic molecules
Authors: Becke A. D.
Published: July 2002

We propose a simple scheme for decomposition of molecular functions into single-center components. The problem of three-dimensional integration in molecular systems thus reduces to a sum of one-center, atomic-like integrations which are treated using standard numerical techniques in spherical polar coordinates. The resulting method is tested on representative diatomic and polyatomic systems for which we obtain five- or six-figure accuracy using a few thousand integration points per atom.
Pressure dependence of the elastic constants of ice Ih to 2.8 kbar by Brillouin spectroscopy
Authors: Gagnon R. E. , Kiefte H. , Clouter M. J. , Whalley Edward
Published: July 2002

The adiabatic elastic constants of single crystals of ice Ih from the Mendenhall Glacier, and the longitudinal and transverse speeds of sound of polycrystalline ice aggregates, have been measured in the range 0–2.8 kbar at −35.5 °C by Brillouin spectroscopy. The ice was held in a 10 kbar optical cell that allowed the sample to be rotated under loads of up to 10 kbar about one axis from outside the cell. The elastic constants C11, C12, C13, C33, and C44, and the bulk modulus are quadratic in the pressure range 0–2.8 kbar.
The Onsager-type theory for nematic ordering of finite-length hard ellipsoids
Authors: Lee Sin-Doo
Published: July 2002

The Onsager-type theory for nematic ordering described in an earlier paper [J. Chem. Phys. 87, 4932 (1987)] is applied to a liquid-density fluid of hard ellipsoids of revolution, and an accurate calculation that agrees very well with the main features of computer simulations of this system is presented. For larger geometrical anisotropies, the isotropic-nematic transition properties are in better agreement with simulation results.
Dynamic elements of mixed-mode oscillations and chaos in a peroxidase–oxidase model network
Authors: Aguda Baltazar D. , Larter Raima , Clarke Bruce L.
Published: July 2002

Three dynamic elements, DE-1, DE-2, and DE-3, are identified for the four-species Olsen model of the peroxidase–oxidase reaction. DE-1 is the damped Lotka oscillator which tends to generate smaller amplitude oscillations. DE-2 is a switch responsible for the transitions between a small and a larger amplitude oscillation. DE-3 is the reversible flux of O2 which dominates the dynamics of the full network while the concentrations of the intermediates are low. DE-1 is identified with the peroxidase catalytic cycle and DE-2 with a reaction involving compound III in the realistic mechanism. Numerical simulations on the route to chaos are presented for varying parameters affecting the three dynamic elements. Pictures of the evolution of the strange attractor that accompanies chaotic oscillations are given. As shown in the Poincaré sections of the attractor and Poincaré next-return maps, the layers of this attractor develop as even- and odd-period fixed points undergo cascades of period doubling.
NH2 electron affinity
Authors: Wickham-Jones C. Tom , Ervin Kent M. , Ellison G. Barney , Lineberger W. Carl
Published: July 2002

The 363.8 nm (3.408 eV) photoelectron spectrum of the NH2 (X̃ 2B1)+e−←NH−2(X̃ 1A1) transition of the amide anion is reported. The electron affinity of amidogen is found to be EA(NH2) =0.771 ±0.005 eV. P, Q, and R rotational branches are observed in the spectrum; a simple model which accounts for the band structure is presented.
Bending overtones and barrier height of ã 1A1 CH2 by flash photolysis stimulated emission pumping
Authors: Xie Wei , Harkin Carmel , Dai Hai-Lung
Published: July 2002

The J≤7 and Ka≤3 rotational levels of the bending vibrational overtones, (0,2,0), (0,3,0), and (0,4,0), and the (1,1,0) combination band of ã 1A1CH2 were characterized in detail by stimulated emission spectroscopy. The methylene was generated from ketene photolysis. Spectroscopic analyses using a rigid rotor Hamiltonian gave the vibrational term values and rotational constants. The latter were found to be strongly J and Ka dependent suggesting the inadequacy of the rigid rotor Hamiltonian for calculating the rotational energy to high accuracy. Effects of Coriolis, Fermi, and singlet–triplet perturbations were all observed in the spectra and discussed. Abnormally intense ΔKa=3 transitions were observed in the ã←b̃, 2162 and 2163 bands. From fitting the experimentally measured bending vibrational term values by a model calculation based on the WKB theory, the barrier height to linearity in the ã state was determined to be 9870 cm−1.
Floppy structure of the benzene dimer: Ab initio calculation on the structure and dipole moment
Authors: Hobza P. , Selzle H. L. , Schlag E. W.
Published: July 2002

The structure of the benzene dimer has aroused considerable interest due to recent experimental measurements and hence extensive theoretical calculation is topical. Nine structures of the benzene dimer were investigated using the second-order Mo/ller–Plesset theory. The calculations were performed with smaller (MIDI-1+s+p) and larger (6-31+G*) basis sets. The T-shape structure was found to be the most stable but with a very shallow minimum; the wagging motion around the lowest hydrogen in the range of ±10° is practically nonhindered. These results together are consistent with the structure found experimentally. The final binding energy for the T structure (distance of molecular centers equal to 5.0 A) is −2.7±0.4 kcal/mol, which is more than the value derived from experiments. The calculated dipole moment is in excellent agreement with experiment.
Diffusion coefficients and effective viscosity of suspensions of sticky hard spheres with hydrodynamic interactions
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study the short-time and long-time diffusion coefficients and the high-frequency and zero-frequency effective viscosity of a dilute suspension of sticky hard spheres with hydrodynamic interactions. Due to the singular nature of the hydrodynamic interactions near touching the transport coefficients are strongly affected by the short-range attractive potential. This suggests that the transport coefficients may be used as a test of the interaction potential.
Refractivity virial coefficients of gaseous CH4, C2H4, C2H6, CO2, SF6, H2, N2, He, and Ar
Authors: Achtermann H. J. , Magnus G. , Bose T. K.
Published: July 2002

Accurate values of the second and third refractivity virial coefficients BR and CR of gaseous CH4, C2H4, C2H6, CO2, SF6, H2, N2, He (only BR is given), and Ar have been measured using the recently improved differential-interferometric technique. This device basically consists of two coupled grating interferometers. During the overflow experiment of the gas one interferometer, with two similar cells in series, measures differentially the higher-order effects of density, and the second interferometer, with two similar cells in parallel, measures at the same time the absolute value of the refractive index. The second interferometer is also used for an accurate control of the decompression during the overflow experiment. It is thus possible to extend the overflow experiments to high values of pressure. The overflow experiments were carried out up to pressures of 35 MPa. The density range extends up to two times the critical density. The third refractivity virial coefficient CR becomes significant for all the gases examined, except for He, when the measurements approach approximately the value 0.3 of the reduced density ρ/ρc. The fourth refractivity virial coefficient DR becomes significant for C2H4, C2H6, and SF6 when ρ/ρc approach approximately the value of one. The agreement between the classically calculated values of BR, based on the dipole induced dipole model, and our experimental BR values is very good for CH4, C2H4, C2H6, SF6, and Ar, but poor for CO2, H2, N2, and He. For He however, the quantum mechanical calculation of BR obtained by including both the long-range and the short-range effects on the polarizability is in remarkably good agreement with our experimental BR value. Accurate values of the first refractivity virial coefficients AR were determined independently by making absolute measurements of the refractive index as a function of pressure.
Approximating full configuration interaction with selected configuration interaction and perturbation theory
Authors: Harrison Robert J.
Published: July 2002

Selected configuration interaction (CI) calculations and second order perturbation theory are combined to systematically approach the full-CI limit. The resulting algorithm has negligible requirement for memory or disk space, being limited only by available cpu time. Comparison is made to existing full-CI benchmarks (DZ and DZP water, the oxygen atom and its anion, ammonia and the magnesium atom). In all cases the full-CI result is recovered to better than 0.1 kcal/mol.
Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H-atom production in the dissociative recombination of some protonated ions
Authors: Adams N. G. , Herd C. R. , Geoghegan M. , Smith D. , Canosa A. , Gomet J. C. , Rowe B. R. , Queffelec J. L. , Morlais M.
Published: July 2002

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H-atom contributions, fH, to the product distributions for the dissociative recombination of a series of protonated ions (N2H+, HCO+, HCO+2, N2OH+, OCSH+, H2CN+, H3O+, H3S+, NH+4, and CH+5 ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H-atom number density using vuv absorption spectroscopy at the Lα (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO2→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of fH varying from ∼0.2 (for OCSH+ ) to 1.2 (for CH+5 ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO+2/OCSH+ and H3O+/H3S+ showed that the fH values were very different. Possible reasons for these differences are discussed. Comparison is also made with the available theory.
Rate constants for the reactions H+O2→OH+O and D+O2→OD+O over the temperature range 1085–2278 K by the laser photolysis–shock tube technique
Authors: Shin Kuan S. , Michael J. V.
Published: July 2002

Rate constants for the reactions (1) H+O2→OH+O and (2) D+O2→OD+O have been measured over the temperature ranges 1103–2055 K and 1085–2278 K, respectively. The experimental method that has been used is the laser-photolysis–shock-tube technique. This technique utilizes atomic resonance absorption spectrophotometry (ARAS) to monitor H- or D-atom depletion in the presence of a large excess of reactant, O2. The results can be well represented by the Arrhenius expressions k1(T)=(1.15±0.16)×10−10  exp(−6917±193 K/T) cm3 molecule−1 s−1, and k2(T)=(1.09±0.20)×10−10  exp(−6937±247 K/T) cm3 molecule−1 s−1. Over the experimental temperature range, the present results show that the isotope effect is unity within experimental uncertainty. The Arrhenius equations, k−1(T)=(8.75±1.24) ×10−12 exp(1121±193 K/T) cm3 molecule−1 s−1 and k−2 (T)=(9.73±1.79)×10−12 exp(526±247 K/T) cm3 molecule−1 s−1, for the rate constants of the reverse reactions were calculated from the experimentally measured forward rate constants and expressions for the equilibrium constants that have been derived from the JANAF thermochemical database. The theoretical implications of the present results are also discussed.
Solid–fluid equilibria for hard dumbbells via Monte Carlo simulation
Authors: Vega C. , Paras E. P. A. , Monson P. A.
Published: July 2002

We present the results of a study of solid–fluid phase equilibria in systems of hard dumbbells for three values of the dumbbell bond length. Monte Carlo simulations were used to calculate the equation of state and Helmholtz free energy. Four orientationally ordered solid phases have been considered—the α-N2 structure and three different base centered monoclinic structures formed by the stacking of hexagonally packed layers that allow the dumbbells to achieve their maximum packing density. In addition, a face-centered-cubic (fcc) plastic crystal was studied for the system with the lowest bond length. The three base centered monoclinic structures have thermodynamic properties which are indistinguishable at the level of accuracy in our calculations. For longer bond lengths, the stable solid structure tends to be orientationally ordered base centered monoclinic. However, we also consider the stability of an aperiodic crystal for the case of dumbbells formed from tangent spheres. At lower bond lengths, the system freezes into a fcc plastic crystal which becomes unstable with respect to a base centered monoclinic structure at higher pressure. The transition between these solid phases is apparently first order. The behavior for the lowest bond length considered resembles that of nitrogen at high temperatures. Our results suggest that the α-N2 is not a stable crystal structure for hard dumbbell solids at any bond length, but does appear as a metastable phase in some cases.
Conventional and unconventional Turing patterns
Authors: Dufiet V. , Boissonade J.
Published: July 2002

The formation of two-dimensional Turing patterns in nonequilibrium chemical systems is studied by numerical simulations with an activator-substrate depletion model. The relative stabilities of the different hexagonal patterns and the striped patterns are discussed, in particular in the vicinity of the transition, and compared with the present state theory. The generic instabilities of the striped patterns are evidenced. In particular, we report the formation of stable zigzag patterns which share features both of one-dimensional and two-dimensional patterns. We also provide examples of temporal evolution in a weakly confined system.
Photoelectron spectroscopy of group IV heavy metal dimers: Sn−2, Pb−2, and SnPb−
Authors: Ho Joe , Polak Mark L. , Lineberger W. C.
Published: July 2002

Negative ion photoelectron spectra of Sn−2, SnPb−, and Pb−2 are presented for electron binding energies up to 3.35 eV. Each spectrum exhibits multiple electronic bands, most of which contain resolved vibrational structure. Franck-Condon analyses yield spectroscopic parameters (re, ωe, and Te) for the anion ground states and the neutral excited states. Adiabatic electron affinities are determined to be: EA(Sn2)=1.962±0.010 eV, EA(Pb2)=1.366±0.010 eV, and EA(SnPb)=1.569±0.008 eV. The anion dissociation energies D0(Sn−2) and D0(Pb−2) are derived from the electron affinities and the neutral dissociation energies. For SnPb−, the dissociation energy difference D0(SnPb−)−D0(SnPb) is precisely measured. Based on the present data, previous experiments and ab initio calculations, we assign most of the observed bands to the corresponding neutral low-lying electronic states.
Microscopic theory of anchoring transitions at the surfaces of pure liquid crystals and their mixtures. II. The effect of surface adsorption
Authors: Teixeira P. I. C. , Sluckin T. J.
Published: July 2002

We have extended our theory of anchoring transitions to take into account adsorption at the interface between a nematic–non-nematic binary mixture and a solid aligning substrate. This was achieved by decoupling adsorption and anchoring and treating the former in the context of the Cahn theory of wetting, while the latter was studied using our previous anchoring theory. For ease of calculation, we mapped the relevant portion of the phase diagram of the mixture onto that of a (isotropic) regular solution. The anchoring angle has been calculated over the whole concentration range, in the partial wetting regime, and in the near saturation region, in the complete wetting regime. For realistic values of the molecular parameters, anchoring transitions have been found which exhibit a degree of temperature independence (‘universality’) akin to that observed experimentally. Ways of developing a fully consistent theory of anchoring transitions are also briefly discussed.
Density-functional thermochemistry. III. The role of exact exchange
Authors: Becke Axel D.
Published: July 2002

Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
Guided-ion beam measurements of the X++H2O(D2O) (X=Ar,N2) collision systems
Authors: Dressler Rainer A. , Salter Richard H. , Murad Edmond
Published: July 2002

Guided-ion beam cross section and product kinetic energy measurements of charge-transfer and atom-abstraction reactions of the Ar++H2O(D2O) and N2++H2O(D2O) collision systems are presented for collision energies ranging between 0.2 and 20 eV c.m. Both charge-transfer systems exhibit large hyperthermal cross sections exceeding 10 Å2 and are characterized by a long-range interaction, evidenced by the small amount of angular scattering observed in product-ion time-of-flight (TOF) measurements. Weak forward-scattered signals due to orbiting collisions are detected and are interpreted to stem from a dipole orientation that maximizes the long-range attractive forces. The charge-transfer product ion velocity distributions are well described by an osculating complex model. The charge-transfer systems exhibit weak isotope effects that are related to competition with the atom-abstraction channels. The atom-abstraction cross sections represent ∼10% of the total cross section and the observed distinct isotope effects and TOF distributions are consistent with a pairwise-energy model.
‘‘Ab initio’’ liquid water
Authors: Laasonen K. , Sprik M. , Parrinello M. , Car R.
Published: July 2002

An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn–Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step. For an accurate description of the hydrogen bonding in the liquid, it was necessary to extend the exchange functional with a term that depends on the gradient of the electron density. A further important technical detail is that supersoft pseudopotentials were used to treat the valence orbitals of the oxygen atoms in a plane wave expansion. The structural and dynamical properties of the liquid were found to be in good agreement with experiment. The ab initio molecular dynamics also yields information on the electronic structure. The electronic feature of special interest is the lowest unoccupied molecular orbital (LUMO) of the liquid which is the state occupied by a thermalized excess electron in the conductive state. The main result of calculating the liquid LUMO is that it is a delocalized state distributed over interstitial space between the molecules with a significant admixture of the σ* orbitals of the individual water molecules.
N(4S), N(2D), and N(2P) yields in predissociation of excited singlet states of N2
Authors: Walter C. W. , Cosby P. C. , Helm H.
Published: July 2002

Predissociation in several singlet valence and Rydberg states of molecular nitrogen has been investigated using photofragment spectroscopy. We report here measurements of the yields of the atomic fragment products N(4S0), N(2D0), and N(2P0) from predissociation of specific rotational levels in the b′ 1Σu+ (v=9–13), c′ 1Σu+ (v=3,4), c 1Πu (v=3,4), and o 1Πu (v=3) states of N2. These states are prepared by laser excitation from the metastable a″ 1Σg+ (v=0) level in a fast (3 keV) molecular beam. Correlated atomic fragments from single molecular dissociation events are monitored using a position- and time-sensitive detector to obtain a complete and sensitive scheme for all possible N2 dissociation products. Dissociation of the N2 states is found to occur to N(2D0)+N(4S0), and N(2P0)+N(4S0) products; production of N(4S0)+N(4S0) is found not to occur from any of the states investigated here. Branching of the dissociation products between the two active limits is found to be strongly correlated with the energy of the predissociating level: levels lying <112 200 cm−1 above N2 X 1Σg+ (v=0) predissociate primarily to the N(2D0)+N(4S0) limit, while states above this energy primarily predissociate to N(2P0)+N(4S0). This behavior is discussed in terms of the potential energy structure of the triplet states of nitrogen that lead to predissociation.
Pseudospectral Mo/ller–Plesset perturbation theory through third order
Authors: Martinez Todd J. , Carter Emily A.
Published: July 2002

We present a formulation and implementation of Mo/ller–Plesset perturbation theory in a pseudospectral framework. At the second-order level, the pseudospectral formulation is a formally a factor of N/n faster than conventional approaches, while the third order is formally faster by a factor of n, where N is the number of atomic orbitals and n is the number of occupied orbitals. The accuracy of the resulting energies is probed for a number of test cases. Practical timings are presented and show conclusively that the pseudospectral formulation is faster than conventional ones.
Natural energy decomposition analysis: An energy partitioning procedure for molecular interactions with application to weak hydrogen bonding, strong ionic, and moderate donor–acceptor interactions
Authors: Glendening Eric D. , Streitwieser Andrew
Published: July 2002

We present a procedure for partitioning the Hartree–Fock self-consistent-field (SCF) interaction energy into electrostatic, charge transfer, and deformation components. The natural bond orbital (NBO) approach of Weinhold and co-workers is employed to construct intermediate supermolecule and fragment wave functions that satisfy the Pauli exclusion principle, thereby avoiding the principal deficiency of the popular Kitaura–Morokuma energy decomposition scheme. The function counterpoise method of Boys and Bernardi enters the procedure naturally, providing an estimate of basis set superposition error (BSSE). We find that the energy components exhibit little basis set dependence when BSSE is small. Applications are presented for several representative molecular and ion complexes: the weak hydrogen bond of the water dimer, the strong ionic interaction of the alkali metal hydrides, and the moderate donor–acceptor interactions of BH3NH3 and BH3CO. Electrostatic interaction dominates the long-range region of the potential energy surface and charge transfer is strongly stabilizing for fragments within van der Waals contact. The repulsive interaction in the short range region of the potential arises from deformation as the fragment wave functions distort to avoid significant interpenetration.
Separation-shifted scaling, a new scaling method for Lennard-Jones interactions in thermodynamic integration
Authors: Zacharias M. , Straatsma T. P. , McCammon J. A.
Published: July 2002

A new method of simultaneously scaling and shifting the Lennard-Jones (LJ) potential in molecular dynamics (MD) and thermodynamic integration (TI) simulations is presented. The approach allows the smooth creation or annihilation of atoms or molecules in an ensemble of solvent molecules during a molecular simulation. By scaling and shifting the LJ potential in the direction of the interatomic distance between particles, the method eliminates the problem of the creation or annihilation of a large repulsive LJ potential at the initial or final state of a TI. The optimal degree of shifting and scaling the LJ potential as a function of a control variable λ was studied for the annihilation and creation of neon in aqueous solution. The method was further tested on the calculation of the free energy of aqueous solvation of a small molecule, ethanol. In contrast to linear scaling of the LJ potential during TI, the calculated free energies using the new separation-shifted scaling approach are reasonably well converged after 200–500 ps of simulation and show smaller hysteresis comparing forward and reverse TI.
Finite representation of an infinite bulk system: Solvent boundary potential for computer simulations
Authors: Beglov Dmitrii , Roux Benoît
Published: July 2002

An approach is developed to obtain statistical properties similar to those of an infinite bulk system from computer simulations of a finite cluster. A rigorous theoretical formulation is given for the solvent boundary potential which takes the influence of the surrounding bulk into account. The solvent boundary potential is the configuration-dependent solvation free energy of an effective cluster composed of an arbitrary solute and a finite number of explicit solvent molecules embedded inside a hard sphere of variable radius; the hard sphere does not act directly on the solute or the explicit solvent molecules, and its radius varies according to the instantaneous configurations. The formulation follows from an exact separation of the multidimensional configurational Boltzmann integral in terms of the solvent molecules nearest to the solute and the remaining bulk solvent molecules. An approximation to the solvent boundary potential is constructed for simulations of bulk water at constant pressure, including the influence of van der Waals and electrostatic interactions. The approximation is illustrated with calculations of the solvation free energy of a water molecule and of sodium and potassium ions. The influence of bulk solvent on the conformational equilibrium of molecular solutes is illustrated by performing umbrella sampling calculations of n-butane and alanine dipeptide in water. The boundary potential is tested to examine the dependence of the results on the number of water molecules included explicitly in the simulations. It is observed that bulk-like results are obtained, even when only the waters in the first hydration shell are included explicitly.
Constant pressure molecular dynamics algorithms
Authors: Martyna Glenn J. , Tobias Douglas J. , Klein Michael L.
Published: July 2002

Modularly invariant equations of motion are derived that generate the isothermal–isobaric ensemble as their phase space averages. Isotropic volume fluctuations and fully flexible simulation cells as well as a hybrid scheme that naturally combines the two motions are considered. The resulting methods are tested on two problems, a particle in a one-dimensional periodic potential and a spherical model of C60 in the solid/fluid phase.
Theoretical study of hydrogen bonding and proton transfer in the ground and lowest excited singlet states of tropolone
Authors: Vener M. V. , Scheiner Steve , Sokolov N. D.
Published: July 2002

Theoretical models of hydrogen bonding and proton transfer in the ground (S0) and lowest excited ππ* singlet (S1) states of tropolone are developed in terms of the localized OH...O fragment model and ab initio three-dimensional potential energy surfaces (PESs). The PESs for proton transfer in the S0 and S1 states are calculated using ab initio SCF and CIS methods, respectively, with a 6–31G basis set which includes polarization functions on the atoms involved in the internal H bond. The Schrödinger equation for nuclear vibrations is solved numerically using adiabatic separation of the variables. The calculated values for the S0 state (geometry, relaxed barrier height, vibrational frequencies, tunnel splittings and H/D isotope effects) agree fairly well with available experimental and theoretical data. The calculated data for the S1 state reproduce the principal experimental trends, established for S1←S0 excitation in tropolone, but are less successful with other features of the dynamics of the excited state, e.g., the comparatively large value of vibrationless level tunnel splitting and its irregular increase with O...O excitation in S1. In order to overcome these discrepancies, a model 2-D PES is constructed by fitting an analytical approximation of the CIS calculation to the experimental vibrationless level tunnel splitting and O...O stretch frequency of tropolone–OH. It is found that the specifics of the proton transfer in the S1 state are determined by a relatively low barrier (only one doublet of the OH stretch lies under the barrier peak). Bending vibrations play a minor role in modulation of the proton transfer barrier, so correct description of tunnel splitting of the proton stretch levels in both electronic states can be obtained in terms of the two-dimensional stretching model, which includes O...O and O–H stretching vibration coordinates only.
Automation of the implementation of spin-adapted open-shell coupled-cluster theories relying on the unitary group formalism
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

A new implementation of the orthogonally spin-adapted open-shell (OS) coupled-cluster (CC) formalism that is based on the unitary group approach to many-electron correlation problem is described. Although the emphasis is on the so-called state specific single-reference but multiconfigurational OS CC approach, the developed algorithms as well as the actual codes are also amenable to multireference CC applications of the state-universal type. A special attention is given to simple OS doublets and OS singlet and triplet cases, the former being applicable to the ground states of radicals and the latter to the excited states of closed shell systems. The encoding of the underlying formalism is fully automated and is based on a convenient decomposition of the Hamiltonian into the effective zero-, one-, and two-orbital contributions as well as on the general strategy that focuses on the excitation operator driven evaluation of individual absolute, linear, quadratic, etc., coupled cluster coefficients, rather than on the standard molecular (spin) orbital driven algorithms. In this way unnecessary duplications are avoided and efficient codes are developed both for the general formula generation and final executable modules. A thorough testing of this procedure on a number of model cases is described and several illustrative applications at the ab initio level are provided.
Molecular dynamics of heat flow in nematic liquid crystals
Authors: Sarman Sten
Published: July 2002

We have devised a Gaussian constraint algorithm that makes the angular velocity of the director of a liquid crystal, consisting of uniaxial molecules, a constant of motion. By setting the angular velocity equal to zero, a director based coordinate system becomes an inertial frame. This also prevents the director reorientation from interfering with the tails of the time correlation functions. The constraint algorithm consequently makes it possible to correctly evaluate phase functions, time correlation functions, and transport coefficients relative to a director based coordinate system. We have applied the constraint algorithm combined with both equilibrium and nonequilibrium molecular dynamics methods to calculate the thermal conductivity of two nematic liquid crystals consisting of prolate and oblate soft ellipsoid fluids, respectively. In the prolate fluid, the thermal conductivity parallel to the director λ∥ ∥ is greater than the thermal conductivity perpendicular to the director λ⊥⊥. In the oblate fluid, the reverse is true λ⊥⊥≳λ∥ ∥. The constraint algorithm has also been used to calculate the torque exerted by the temperature gradient on the molecules. The prolate ellipsoids are twisted toward the perpendicular orientation relative to the temperature gradient. The oblate ellipsoids are twisted toward the parallel orientation. This phenomenom can be explained by postulating a quadratic coupling between the symmetric traceless order tensor and the temperature gradient. One should also note that in both systems, the molecules orient in such a way that the entropy production is minimized.
Photoelectron spectroscopy of size-selected transition metal clusters: Fe−n, n=3–24
Authors: Wang Lai-Sheng , Cheng Han-Song , Fan Jiawen
Published: July 2002

A higher resolution magnetic bottle photoelectron spectrometer for the study of the electronic structure of size-selected metal clusters is presented. The initial study on Fe−n (n=3–24) is reported at a photon energy of 3.49 eV. The photoelectron spectra of these clusters exhibit sharp features throughout the size range. The spectra for Fe−3–8 show large size dependence with many resolved features. The spectra for Fe−9–15 exhibit some similarity with each other, all with a rather sharp feature near the threshold. An abrupt spectral change occurs at Fe−16, then again at Fe−19 and Fe−23. These photoelectron spectral changes coincide remarkably with changes of the cluster reactivity with H2. Extended Hückel molecular orbital (EHMO) calculations are performed for all the clusters to aid the spectral interpretations. The calculations yield surprisingly good agreement with the experiment for clusters beyond Fe9 when body-centered cubic (bcc) structures are assumed for Fe9–15 and a similarly close-packed structure with a bcc Fe15 core for the larger clusters. The EHMO calculations allow a systematic interpretation of the sharp photoelectron spectral features in Fe−9–15 and reproduced the abrupt spectral change taking place from Fe−15 to Fe−16. Most importantly, the reactivity changes of the clusters with H2 are successfully explained based on the detailed electronic structures of the clusters, as revealed from the photoelectron spectroscopy (PES) spectra and the theoretical calculations. The calculations also correctly predict the existence of magnetism in these clusters and yield reasonable values for the cluster magnetic moments.
A quantitative study of the scaling properties of the Hartree–Fock method
Authors: Strout Douglas L. , Scuseria Gustavo E.
Published: July 2002

Although it is usually stated that the Hartree–Fock method formally scales as N4, where N is the number of basis functions employed in the calculation, it is also well known that mathematical bounds computed with the Schwarz inequality can be used to screen and eliminate four-center two-electron integrals smaller than a certain threshold. In this work, quantitative data is presented to illustrate the effects of this integral screening on the scaling properties of the Hartree–Fock (HF) method. Calculations are performed on a range of carbon–hydrogen model systems, two-dimensional graphitic sheets, and three-dimensional diamond pieces, to determine the effective scaling exponent α of the computational expense. The data obtained in this paper for calculations including over 250 carbon atoms and 1500 basis functions shows two significant trends: (1) in the asymptotic limit of large molecules, α is found to be approximately 2.2–2.3, and (2) for molecules of modest size, α is still very much less than 4. Therefore, integral screening is quantitatively shown to substantially reduce the Hartree–Fock scaling from its formal value of N4.
Renner–Teller effect on the highly excited bending levels of ã 1A1 CH2
Authors: Hartland Gregory V. , Qin Dong , Dai Hai-Lung
Published: July 2002

The effect of Renner–Teller coupling between the ã 1A1 and b̃ 1B1 states of CH2 on the rotational structure of the ã 1A1 bending vibrational levels has been observed. Renner–Teller coupling causes a decrease in the A rotational constant of the ã 1A1 (0,5,0) level, at 6400 cm−1 above the zero-point level, compared to the value extrapolated from the (0,v2,0) v2=0–4 levels. Excellent agreement is obtained between the experimentally determined A value for (0,5,0) and that predicted by ab initio calculations of Green et al. [J. Chem. Phys. 94, 118 (1991)]. The effect of Renner–Teller coupling on the A rotational constant of bending levels as low as v2=3 has also been detected. The barrier height to linearity in the ã 1A1 state was also estimated by fitting the ã 1A1 bending level term values to a harmonic plus Gaussian perturbation potential function, where the effects of orbital angular momentum were explicitly included to account for electronic-rotational coupling in the calculation. The value of 8600±400 cm−1 obtained is in good agreement with the ab initio result of 8800 cm−1.
Reaction-coordinate-dependent friction in classical activated barrier crossing dynamics: When it matters and when it doesn’t
Authors: Haynes George R. , Voth Gregory A.
Published: July 2002

The impact of the symmetry of the reaction-coordinate dependence of the solvent friction on the thermally activated barrier crossing rate is examined. Possible symmetry forms are defined for the reaction-coordinate dependence of the solvent friction. The implications in the effective Grote–Hynes theory of Voth [J. Chem. Phys. 97, 5908 (1992)] and the theory recently presented by Haynes, Voth, and Pollak [J. Chem. Phys. 101, 7811 (1994)] of a spatially antisymmetric solvent friction are illustrated. Surprisingly, no correction to the Kramers–Grote–Hynes theory for the transmission coefficient is predicted, although an antisymmetric spatial dependence of the solvent friction is a strong departure from the usual spatially independent friction-based generalized Langevin equation. The results from the analytical theories are compared to numerically exact generalized Langevin equation simulation results for a simple model system and found to agree well for a wide range of damping strengths and friction time scales, confirming the interesting predictions of these theories.
Photoinduced electron transfer reactions in mixed films of π-conjugated polymers and a homologous series of tetracyano-p-quinodimethane derivatives
Authors: Janssen René A. J. , Christiaans Marwijn P. T. , Hare Casey , Martin Nazario , Sariciftci N. Serdar , Heeger Alan J. , Wudl Fred
Published: July 2002

Near-steady-state photoinduced absorption (PIA) and photoluminescence studies are presented on photoinduced electron transfer reactions from poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[3-(2-(3-methylbutoxy)ethyl)thiophene] (P3MBET) as donors (D) onto a homologous series of tetracyano-p-quinodimethane (TCNQ) derivatives containing fused aromatic rings as acceptors (A) to systematically study the effect of acceptor electron affinity. We observe that composite films of these D/A couples give rise to long-lived charge separated states upon photoexcitation, as evidenced from the formation of polaron bands (radical cation absorption) in the PIA and the concomitant loss of the metastable triplet PIA and photoluminescence that are observed in pristine MEH-PPV and P3MBET. We find that the efficiency of the photoinduced electron transfer reaction correlates with the reduction potential of the acceptors.
Relaxation in filled polymers: A fractional calculus approach
Authors: Metzler Ralf , Schick Winfried , Kilian Hanns-Georg , Nonnenmacher Theo F.
Published: July 2002

In recent years the fractional calculus approach to describing dynamic processes in disordered or complex systems such as relaxation or dielectric behavior in polymers or photo bleaching recovery in biologic membranes has proved to be an extraordinarily successful tool. In this paper we apply fractional relaxation to filled polymer networks and investigate the dependence of the decisive occurring parameters on the filler content. As a result, the dynamics of such complex systems may be well–described by our fractional model whereby the parameters agree with known phenomenological models.
Structural flexibility of carbon nanotubes
Authors: Iijima Sumio , Brabec Charles , Maiti Amitesh , Bernholc Jerzy
Published: July 2002

We report high resolution electron microscope (HREM) observations and atomistic simulations of the bending of single and multi-walled carbon nanotubes under mechanical duress. Single and multiple kinks are observed at high bending angles. Their occurrence is quantitatively explained by the simulations, which use a realistic many-body potential for the carbon atoms. We show that the bending is fully reversible up to very large bending angles, despite the occurrence of kinks and highly strained tube regions. This is due to the remarkable flexibility of the hexagonal network, which resists bond breaking and bond switching up to very high strain values.
Elastic constants of ice VI by Brillouin spectroscopy
Authors: Tulk C. A. , Gagnon R. E. , Kiefte H. , Clouter M. J.
Published: July 2002

The elastic constants of ice VI have been determined at −2 °C and 7.2 kbar by single crystal Brillouin spectroscopy. The adiabatic elastic moduli were found to be C11=26.8, C12=14.5, C13=12.8, C33=26.2, C44=6.3, and C66=10.4×104 bar, within approximately 1.5%. Large (125 mm3) single crystals were grown in a high pressure cell and in situ Brillouin spectra were collected at various crystal orientations about the laboratory z-axis. Ice VI crystals were oriented, while under pressure, by using the birefringent properties and Brillouin spectral behavior. Several polycrystalline elastic properties are derived using the bulk adiabatic moduli and a comparison with previous results is made.
An extended x-ray absorption fine structure study by employing molecular dynamics simulations: Bromide ion in methanolic solution
Authors: D’Angelo P. , Di Nola A. , Mangoni M. , Pavel N. V.
Published: July 2002

X-ray absorption spectroscopy is widely employed in the structural analysis of disordered systems. In the standard extended x-ray absorption fine structure (EXAFS) analysis the coordination of the photoabsorber is usually defined by means of Gaussian shells. It is known that this procedure can lead to significant errors in the determination of the coordination parameters for systems which present anharmonic thermal vibrations or interatomic asymmetric pair distribution functions. An efficient method has been recently employed in the study of the hydration shells of bromide and rubidium ions and brominated hydrocarbon molecules in diluted aqueous solutions. According to this method, pair distribution functions [g(r)] obtained from molecular dynamics simulations can be used as relevant models in the calculation of the EXAFS signals. Moreover, asymmetric shells modeled on the g(r) first peaks, have been employed in the EXAFS analysis and the parameters defining the asymmetric peaks have been optimized during the minimization procedure. In the present paper this new procedure has been used to investigate the coordination of Br− in methanol. The analysis of this system is particularly interesting due to the presence of three well separated coordination shells. We show that the inclusion of the hydrogen signal is essential to perform a reliable analysis. A comparison of the analysis with asymmetric and Gaussian shells shows how the accuracy of the EXAFS data analysis is improved by using asymmetric shells.
A comparison of models for calculating nuclear magnetic resonance shielding tensors
Authors: Cheeseman James R. , Trucks Gary W. , Keith Todd A. , Frisch Michael J.
Published: July 2002

The direct (recomputation of two-electron integrals) implementation of the gauge-including atomic orbital (GIAO) and the CSGT (continuous set of gauge transformations) methods for calculating nuclear magnetic shielding tensors at both the Hartree-Fock and density functional levels of theory are presented. Isotropic 13C, 15N, and 17O magnetic shielding constants for several molecules, including taxol (C47H51NO14 using 1032 basis functions) are reported. Shielding tensor components determined using the GIAO and CSGT methods are found to converge to the same value at sufficiently large basis sets; however, GIAO shielding tensor components for atoms other than carbon are found to converge faster with respect to basis set size than those determined using the CSGT method for both Hartree-Fock and DFT. For molecules where electron correlation effects are significant, shielding constants determined using (gradient-corrected) pure DFT or hybrid methods (including a mixture of Hartree-Fock exchange and DFT exchange-correlation) are closer to experiment than those determined at the Hartree-Fock level of theory. For the series of molecules studied here, the RMS error for 13C chemical shifts relative to TMS determined using the B3LYP hybrid functional with the 6-311+G(2d,p) basis is nearly three times smaller than the RMS error for shifts determined using Hartree-Fock at this same basis. Hartree-Fock 13C chemical shifts calculated using the 6-31G* basis set give nearly the same RMS error as compared to experiment as chemical shifts obtained using Hartree-Fock with the bigger 6-311+G(2d,p) basis set for the range of molecules studied here. The RMS error for chemical shifts relative to TMS calculated at the Hartree-Fock 6-31G* level of theory for taxol (C47H51NO14) is 6.4 ppm, indicating that for large systems, this level of theory is sufficient to determine accurate 13C chemical shifts.
Linear-scaling semiempirical quantum calculations for macromolecules
Authors: Lee Tai-Sung , York Darrin M. , Yang Weitao
Published: July 2002

A linear-scaling method to carry out semiempirical quantum mechanical calculations for large systems has been developed based on the density matrix version of the divide-and-conquer approach. The method has been tested and demonstrated to be accurate and efficient. With this implementation, semiempirical quantum mechanical calculations are made possible for large molecules over 9000 atoms on a typical workstation. For biological macromolecules, solvent effects are included with a dielectric continuum model.