bluuebunny/crossref_metadata_2025 huggingface dataset:
Accurate theoretical near-equilibrium potential energy and dipole moment surfaces of HgClO and HgBrO
Authors: Balabanov Nikolai B. , Peterson Kirk A.
Published: March 2004

The complexes HgBrO and HgClO have been previously determined by ab initio methods to be strongly bound and were suggested to be important intermediates during mercury depletions events observed in the polar troposphere. In the present work accurate near-equilibrium potential energy surfaces (PESs) of these species are reported. The PESs are determined using accurate coupled cluster methods and a series of correlation consistent basis sets with subsequent extrapolation to the complete basis set limit. Additive corrections for both core–valence correlation energy and relativistic effects are also included. The anharmonic ro-vibrational spectra of HgBrO and HgClO have been calculated in variational calculations. Strong infrared band strengths are predicted for all fundamentals in these species. The spin–orbit splitting dominates over the vibronic coupling effect in both HgClO and HgBrO. The Renner–Teller vibronic energy levels corresponding to the bending mode of these molecules are calculated via perturbation theory.
A constrained variational approach for energy derivatives in Fock-space multireference coupled-cluster theory
Authors: Shamasundar K. R. , Asokan Subashini , Pal Sourav
Published: March 2004

In this paper, we present a formulation based on constrained variational approach to enable efficient computation of energy derivatives using Fock-space multireference coupled-cluster theory. Adopting conventional normal ordered exponential with Bloch projection approach, we present a method of deriving equations when general incomplete model spaces are used. Essential simplifications arise when effective Hamiltonian definition becomes explicit as in the case of complete model spaces or some special quasicomplete model spaces. We apply the method to derive explicit generic expressions upto third-order energy derivatives for [0,1], [1,0], and [1,1] Fock-space sectors. Specific diagrammatic expressions for zeroth-order Lagrange multiplier equations for [0,1], [1,0], and [1,1] sectors are presented.
An electron in a finite-dipole potential
Authors: Patil S. H.
Published: March 2004

Some structural properties of the energy eigenfunctions of an electron in a finite-dipole field are analyzed, in particular the asymptotic behavior when the electron is far away, and the coalescence and cusp properties when it is close to the dipole charges. Model wave functions incorporating these properties are developed, which give accurate values for the energies and some other quantities, and a useful insight into the physical structure of the system. Critical radius for the existence of the bound states is obtained from the Wentzel–Kramers–Brillouin approach. These considerations are extended to the description of the system in two dimensions.
An application of error reduction and harmonic inversion schemes to the semiclassical calculation of molecular vibrational energy levels
Authors: Kunikeev Sharif D. , Atilgan Erdinç , Taylor Howard S. , Kaledin Alexey L. , Main Jörg
Published: March 2004

A singular value decomposition based harmonic inversion signal processing scheme is applied to the semiclassical initial value representation (IVR) calculation of molecular vibrational states. Relative to usual IVR procedure of Fourier analysis of a signal made from the Monte Carlo evaluation of the phase space integral in which many trajectories are needed, the new procedure obtains acceptable results with many fewer trajectories. Calculations are carried out for vibrational energy levels of H2O to illustrate the overall procedure.
A theoretical and experimental study of the SO22+ dication
Authors: Hochlaf M. , Eland J. H. D.
Published: March 2004

The double photoionization spectrum of SO2 has been measured using the TOF–PEPECO technique and contains one resolved band. Detailed electronic structure calculations and experimental comparisons allow the resolved band to be identified as the A 1A2 state of the SO22+ dication, with its adiabatic ionization energy at 35.284±0.02 eV. According to the most accurate calculations, the ground state level of SO22+ must be located near 33.48 eV, well below the range accessed by vertical transitions from neutral SO2. Transient SO22+ molecules detected by mass spectrometry may be identified either as the sharp levels of the A 1A2 state or as ground state levels populated by nonvertical ionization pathways.
High resolution photoabsorption and photofragment fluorescence spectroscopy of water between 10.9 and 12 eV
Authors: Fillion J.-H. , Ruiz J. , Yang X.-F. , Castillejo M. , Rostas F. , Lemaire J.-L.
Published: March 2004

This work presents absorption and photofragment fluorescence spectra of water (H2O and D2O) simultaneously recorded at rotational resolution and at room temperature, by means of a synchrotron radiation source in the range 10.9–12 eV, covering the nd intense series from n=3 to 8. The Rydberg states observed are assigned in the light of the most advanced theoretical work available [M. S. Child, Philos. Trans. R. Soc. London, Ser. A 355, 1623 (1997)], and by reference to the stretching and bending mode progressions. Comparison between absorption and fluorescence spectra is shown to reveal a fast predissociation mechanism involving the linear 3pb2 1B2 state, and permits the identification of its (0,14,0) vibrational level observed in the absorption spectra.
Static dipole polarizability and binding energy of sodium clusters Nan (n=1–10): A critical assessment of all-electron based post Hartree–Fock and density functional methods
Authors: Chandrakumar K. R. S. , Ghanty Tapan K. , Ghosh Swapan K.
Published: March 2004

A systematic all electron post Hartree–Fock as well as density functional theory (DFT) based calculations for the polarizability and binding energy of sodium metal clusters have been performed and an in-depth analysis of the discrepancy between the experimental and theoretical results is presented. A systematic investigation for the assessment of different DFT exchange–correlation functionals in predicting the polarizability values has also been reported. All the pure DFT functionals have been found to considerably underestimate the calculated polarizability values as compared to the MP2 results. DFT calculations using the full Hartree–Fock exchange along with one-parameter progressive correlation functional have, however, been shown to yield results in good agreement with the MP2 and experimental results. The possible sources of error present in the experimental measurements as well as in the different theoretical methods have also been analyzed. One of the most important conclusions of the present study is that the effect of electron correlation plays a significant role in determining the polarizability of the clusters and the MP2 method can be considered to be one of the most reliable methods for their prediction. It has also been noted that the polarizability value of the lower member clusters (Na2 and Na4) calculated by highly sophisticated methods such as, CCSD and CCSD(T) are found to be very close to the corresponding MP2 values. The polarizability and the binding energy of the clusters are found to be inversely related to each other and their correlation is rationalized by invoking the minimum polarizability principle. A good linear correlation between the polarizability and volume of the cluster has also been found to exist.
Relativistic correlating basis sets for the sixth-period d-block atoms from Lu to Hg
Authors: Osanai You , Noro Takeshi , Miyoshi Eisaku , Sekiya Masahiro , Koga Toshikatsu
Published: March 2004

Contracted Gaussian-type function sets to describe valence correlation are developed for the sixth-period d-block atoms Lu through Hg. A segmented contraction scheme is employed for their compactness and efficiency. Contraction coefficients and exponents are determined by minimizing the deviation from accurate natural orbitals generated from configuration interaction calculations, in which relativistic effects are incorporated through the third-order Douglas–Kroll approximation. The present basis sets yield more than 99% of atomic correlation energies predicted by accurate natural orbital sets of the same size. Relativistic model core potential calculations with the present correlating sets give the spectroscopic constants of the AuH molecule in excellent agreement with experimental results.
Potential energy surface and rovibrational states of the ground Ar–HI complex
Authors: Prosmiti Rita , López-López Sergio , Garcı́a-Vela Alberto
Published: March 2004

A potential energy surface for the ground electronic state of the Ar–HI van der Waals complex is calculated at the coupled-cluster with single and double excitations and a noniterative perturbation treatment of triple excitations [CCSD(T)] level of theory. Calculations are performed using for the iodine atom a correlation consistent triple-ζ valence basis set in conjunction with large-core Stuttgart–Dresden–Bonn relativistic pseudopotential, whereas specific augmented correlation consistent basis sets are employed for the H and Ar atoms supplemented with an additional set of bond functions. In agreement with previous studies, the equilibrium structure is found to be linear Ar–I–H, with a well depth of 205.38 cm−1. Another two secondary minima are also predicted at a linear and bent Ar–H–I configurations with well depths of 153.57 and 151.57 cm−1, respectively. The parametrized CCSD(T) potential is used to calculate rovibrational bound states of Ar–HI/Ar–DI complexes, and the vibrationally averaged structures of the different isomers are determined. Spectroscopic constants are also computed from the CCSD(T) surface and their comparison with available experimental data demonstrates the quality of the present surface in the corresponding configuration regions.
Optimal control of quantum non-Markovian dissipation: Reduced Liouville-space theory
Authors: Xu Ruixue , Yan YiJing , Ohtsuki Yukiyoshi , Fujimura Yuichi , Rabitz Herschel
Published: March 2004

An optimal control theory for open quantum systems is constructed containing non-Markovian dissipation manipulated by an external control field. The control theory is developed based on a novel quantum dissipation formulation that treats both the initial canonical ensemble and the subsequent reduced control dynamics. An associated scheme of backward propagation is presented, allowing the efficient evaluation of general optimal control problems. As an illustration, the control theory is applied to the vibration of the hydrogen fluoride molecule embedded in a non-Markovian dissipative medium. The importance of control-dissipation correlation is evident in the results.
Annealing contour Monte Carlo algorithm for structure optimization in an off-lattice protein model
Authors: Liang Faming
Published: March 2004

We present a space annealing version for a contour Monte Carlo algorithm and show that it can be applied successfully to finding the ground states for an off-lattice protein model. The comparison shows that the algorithm has made a significant improvement over the pruned-enriched-Rosenbluth method and the Metropolis Monte Carlo method in finding the ground states for AB models. For all sequences, the algorithm has renewed the putative ground energy values in the two-dimensional AB model and set the putative ground energy values in the three-dimensional AB model.
Kolmogorov–Sinai entropy for the A+B→P reaction in transitional flows
Authors: Rogberg P. , Cvetkovic V.
Published: March 2004

We study global decay of the bimolecular reaction A+B→P as c(t)∼t−α in a nonlinear transitional flow. A relationship is established between the decay exponent α, and a modified Kolmogorov–Sinai entropy, hr. We find that for dynamic conditions which induce relatively strong mixing, the decay exponent is α∝ln ψ−Bhr with B being a characteristic reactive mix-down time for the system, and ψ is a space–time scaling parameter. Dynamic conditions which imply weak mixing lead to a degenerate dependence of α on hr. The proposed relationship between α on hr should be a useful link between the dynamical evolution of the flow field and reaction kinetics in vortex dominated flows.
Reduced photobleaching of chromophores close to a metal surface
Authors: Vasilev Krasimir , Stefani Fernando D. , Jacobsen Volker , Knoll Wolfgang , Kreiter Maximilian
Published: March 2004

The photobleaching of chromophores in front of a metal film is measured by recording the emitted fluorescence intensity from an ensemble of chromophores as a function of time. A strong dependence of the photostability on the distance from the metal surface is found. The experimental data are well described in a classical electromagnetic model with the additional assumption that photobleaching occurs at a constant rate from the excited state. The metal interface influences the photostability of the chromophores in two ways, first by altering the excitation rate by local enhancement of the electromagnetic field and second by altering the electromagnetic decay rate.
Effects of the Gaussian energy dispersion on the statistics of polarons and bipolarons in conducting polymers
Authors: Bisquert Juan , Garcia-Belmonte Germà , Garcı́a-Cañadas Jorge
Published: March 2004

We discuss the interpretation of usually broad oxidation peaks observed in electronically conducting polymers, in terms of the statistical distributions functions of polarons and bipolarons. The analysis is based on examining the chemical capacitance, that relates the change of concentration to a modification of the chemical potential of a given species, for different statistical models. We first review the standard models for single energy species that provide a nernstian dependence, and the limitations of these models are discussed. A new model that assumes a Gaussian distribution of energies related to molecular geometry fluctuations is suggested, and this model shows excellent agreement with the results of electrochemical oxidation of polypyrrole in quasiequilibrium conditions. From a fit of the data, it is found that the density of conjugated chain segments in polypyrrole, Ns≈1021 cm−3, shows a Gaussian distribution of half width σ≈170 meV, tentatively attributed to bipolaron formation energies.
Density-functional study of organic–inorganic hybrid single crystal ZnSe(C2H8N2)1/2
Authors: Fu Huaxiang , Li Jing
Published: March 2004

Unusual properties (i.e., strong band dispersion, high carrier mobility, wide absorption-energy window, and sharp band-edge transition) that are desirable for hybrid-material electronics and for solar electric energy conversion are predicted to exist in the organic–inorganic chalcogenide single crystal ZnSe(C2H8N2)1/2 by using density-functional calculations. A simple mechanism, namely that the band-edge electronic states of the hybrid composite is predominantly determined by the inorganic constituent, is revealed to be responsible for governing these properties. Suggestions for further engineering hybrid semiconductors are also provided.
Dielectric response of modified Hubbard models with neutral-ionic and Peierls transitions
Authors: Soos Zoltan G. , Bewick Sharon A. , Peri Andrea , Painelli Anna
Published: March 2004

The dipole P(F) of systems with periodic boundary conditions in a static electric field F is applied to one-dimensional Peierls–Hubbard models for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for finite systems of N=14 and 16 sites that are almost converged to infinite chains in deformable lattices subject to a Peierls transition. The electronic polarizability per site, αel=(∂P/∂F)0, of rigid stacks with alternating transfer integrals t(1±δ) diverges at the neutral-ionic transition for δ=0 but remains finite for δ>0 in dimerized chains. The Peierls or dimerization mode couples to charge fluctuations along the stack and results in large vibrational contributions αvib that are related to ∂P/∂δ and that peak sharply at the Peierls transition. The extension of P(F) to correlated electronic states yields the dielectric response κ of models with neutral-ionic or Peierls transitions, where κ peaks >100 are found with parameters used previously for variable ionicity ρ and vibrational spectra of CT salts. The calculated κ accounts for the dielectric response of CT salts based on substituted TTF’s (tetrathiafulvalene) and substituted CA’s (chloranil). The role of lattice stiffness appears clearly in models: soft systems have a Peierls instability at small ρ and continuous crossover to large ρ, while stiff stacks such as TTF–CA have a first-order transition with discontinuous ρ that is both a neutral-ionic and Peierls transition. The transitions are associated with tuning the electronic ground state of insulators via temperature or pressure in experiments, or via model parameters in calculations.
Topology of the distribution of zeros of the Husimi function in the LiNC/LiCN molecular system
Authors: Arranz F. J. , Benito R. M. , Borondo F.
Published: March 2004

Phase space representations of quantum mechanics constitute useful tools to study vibrations in molecular systems. Among all possibilities, the Husimi function or coherent state representation is very widely used, its maxima indicating which regions of phase space are relevant in the dynamics of the system. The corresponding zeros are also a good indicator to investigate the characteristics of the eigenstates, and it has been shown how the corresponding distributions can discriminate between regular, irregular, and scarred wave functions. In this paper, we discuss how this result can be understood in terms of the overlap between coherent states and system eigenfunctions.
Interactions of Au cluster anions with oxygen
Authors: Sun Qiang , Jena Puru , Kim Young Dok , Fischer Matthias , Ganteför Gerd
Published: March 2004

Experimental and theoretical evidence is presented for the nondissociative chemisorption of O2 on free Au cluster anions (Aun−, n=number of atoms) with n=2, 4, 6 at room temperature, indicating that the stabilization of the activated di-oxygen species is the key for the unusual catalytic activities of Au-based catalysts. In contrast to Aun− with n=2, 4, 6, O2 adsorbs atomically on Au monomer anions. For the Au monomer neutral, calculations based on density functional theory reveal that oxygen should be molecularly bound. On Au dimer and tetramer neutrals, oxygen is molecularly bound with the O–O bond being less activated with respect to their anionic counterparts, suggesting that the excess electron in the anionic state plays a crucial role for the O–O activation. We demonstrate that interplay between experiments on gas phase clusters and theoretical approach can be a promising strategy to unveil mechanisms of elementary steps in nanocatalysis.
Test of the quantum instanton approximation for thermal rate constants for some collinear reactions
Authors: Ceotto Michele , Miller William H.
Published: March 2004

Two variants of the recently developed quantum instanton (QI) model for calculating thermal rate constants of chemical reactions are applied to several collinear atom–diatom reactions with various skew angles. The results show that the original QI version of the model is consistently more accurate than the “simplest” quantum instanton version (both being applied here with one “dividing surface”) and thus to be preferred. Also, for these examples (as with other earlier applications) the QI results agree well with the correct quantum rates (to within ∼20% or better) for all temperatures >200 K, except for situations where dynamical corrections to transition state theory (i.e., “re-crossing” dynamics) are evident. (Since re-crossing effects are substantially reduced in higher dimensionality, this is not a cause for serious concern.) A procedure is also described which facilitates use of the METROPOLIS algorithm for evaluating all quantities that appear in the QI rate expression by Monte Carlo path integral methods.
Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures
Authors: Rakvin B. , Maltar-Strmečki N. , Ramsey C. M. , Dalal N. S.
Published: March 2004

With the view of understanding the low frequency (40–50 cm−1) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8–20 K). The phase memory time, TM, and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1),  ⋅CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4–20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.
Photodissociation of bromobenzene, dibromobenzene, and 1,3,5-tribromobenzene
Authors: Liu Ya-Jun , Persson Petter , Karlsson Hans O. , Lunell Sten , Kadi Malin , Karlsson Daniel , Davidsson Jan
Published: March 2004

Quantum chemical calculations have been performed on the ground state and several low-lying excited states of bromobenzene, ortho-, meta-, and para-dibromobenzene, and 1,3,5-tribromobenzene using high-level ab initio and hybrid density-functional methods. Experimental observations of ultrafast predissociation in these molecules are clarified from extensive theoretical information about all low-energy potential-energy curves together with symmetry arguments. The intriguing observation that o- and m-dibromobenzene have two ultrafast predissociation channels while bromobenzene, p-dibromobenzene, and 1,3,5-tribromobenzene only have one such channel is explained from the calculated potential-energy curves. These show that the lowering of point-group symmetry from C2v to Cs along the main photodissociation reaction coordinate, which only occurs in o- and m-dibromobenzene, opens up a new predissociation channel. Dynamical quantum simulations based on the calculated potential-energy curves are used to estimate the coupling strength at the intersystem crossing point in bromobenzene.
Helium nanodroplet isolation spectroscopy of perylene and its complexes with oxygen
Authors: Çarçabal Pierre , Schmied Roman , Lehmann Kevin K. , Scoles Giacinto
Published: March 2004

This note reports the observation of the spectra of the S1←S0 electronic transition of complexes formed by perylene and molecular oxygen embedded in superfluid helium clusters. Because of the fluorescence quenching property of molecular oxygen, the observation of this system has not been reported previously.
Observation of the 3 3Σ+–X 1Σ+ transition of KRb by resonance enhanced two-photon ionization in a pulsed molecular beam: Hyperfine structures of K39Rb85 and K39Rb87 isotopomers
Authors: Lee Yonghoon , Yoon Youngjee , Kim Bongsoo , Li Li , Lee Sungyul
Published: March 2004

The 3 3Σ+–X 1Σ+ transition of KRb is observed by resonance enhanced two-photon ionization in a pulsed molecular beam. Hyperfine splittings of K39Rb85 and K39Rb87 isotopomers are observed. From the magnitude of hyperfine splittings, we found that the main hyperfine structure was dominated by the Fermi contact interaction between the Rb nuclear spin and the unpaired electron spin. The Fermi contact interaction constants were determined to be 291 MHz for K39Rb85 and 665 MHz for K39Rb87. In the KRb 3 3Σ+ state the electron spin couples more strongly with the Rb nuclear spin than with other angular momenta, and the energy level structure is well described by the hyperfine angular momentum coupling scheme of the bβS case. The molecular constants and the Rydberg–Klein–Rees potential energy curve of the 3 3Σ+ state were determined.
Basis set and electron correlation effects on initial convergence for vibrational nonlinear optical properties of conjugated organic molecules
Authors: Torrent-Sucarrat Miquel , Solà Miquel , Duran Miquel , Luis Josep M. , Kirtman Bernard
Published: March 2004

Using three typical π-conjugated molecules (1,3,5-hexatriene, 1-formyl-6-hydroxyhexa-1,3,5- triene, and 1,1-diamino-6,6-dinitrohexa-1,3,5-triene) we investigate the level of ab initio theory necessary to produce reliable values for linear and nonlinear optical properties, with emphasis on the vibrational contributions that are known to be important or potentially important. These calculations are made feasible by employing field-induced coordinates in combination with a finite field procedure. For many, but not all, purposes the MP2/6-31+G(d) level is adequate. Based on our results the convergence of the usual perturbation treatment for vibrational anharmonicity was examined. Although this treatment is initially convergent in most circumstances, a problematic situation has been identified.
Towards an understanding of the heat capacity of liquids. A simple two-state model for molecular association
Authors: Cerdeiriña Claudio A. , González-Salgado Diego , Romani Luis , del Carmen Delgado Marı́a , Torres Luis A. , Costas Miguel
Published: March 2004

A model for the temperature dependence of the isobaric heat capacity of associated pure liquids Cp,mo(T) is proposed. Taking the ideal gas as a reference state, the residual heat capacity is divided into nonspecific Cpres,ns and associational Cpres,ass contributions. Statistical mechanics is used to obtain Cpres,ass by means of a two-state model. All the experimentally observed Cp,mo(T) types of curves in the literature are qualitatively described from the combination of the ideal gas heat capacity Cpid(T) and Cpres,ass(T). The existence of Cp,mo(T) curves with a maximum is predicted and experimentally observed, for the first time, through the measurement of Cp,mo(T) for highly sterically hindered alcohols. A detailed quantitative analysis of Cp,mo(T) for several series of substances (n-alkanes, linear and branched alcohols, and thiols) is made. All the basic features of Cp,mo(T) at atmospheric and high pressures are successfully described, the model parameters being physically meaningful. In particular, the molecular association energies and the Cpres,ns values from the proposed model are found to be in agreement with those obtained through quantum mechanical ab initio calculations and the Flory model, respectively. It is concluded that Cp,mo(T) is governed by the association energy between molecules, their self-association capability and molecular size.
Folding probabilities: A novel approach to folding transitions and the two-dimensional Ising-model
Authors: Lenz Peter , Zagrovic Bojan , Shapiro Jessica , Pande Vijay S.
Published: March 2004

The theoretical concept of folding probability, pfold, has proven to be a useful means to characterize the kinetics of protein folding. Here, we illustrate the practical importance of pfold and demonstrate how it can be determined theoretically. We derive a general analytical expression for pfold and show how it can be estimated from simulations for systems where the transition rates between the relevant microstates are not known. By analyzing the Ising model we are able to determine the scaling behavior of the numerical error in the pfold estimate as function of the number of analyzed Monte Carlo runs. We apply our method to a simple, newly developed protein folding model for the formation of alpha helices. It is demonstrated that our technique highly parallelizes the calculation of pfold and that it is orders of magnitude more efficient than conventional approaches.
A new cell model for equation of state of polyethylene melt and n-alkane
Authors: Murakami Masahiko
Published: March 2004

The connectivity of successive carbon atoms in polymer decreases the degrees of freedom, and hence, the external degrees of freedom should correspond only to translational motion. We therefore, introduce a coarse-grained particle of a few successive monomers. A cell model (or the hard core model with attractive potential) for the particles can accordingly be used for the derivation of the equation of state of polymers. Modifying the classical cell model by Lennard-Jones and Devonshire, we construct a new equation of state for polyethylene melt and for liquid n-alkane; the free volume term is modified by using the Sutherland potential instead of the Lennard-Jones potential. The characteristic quantities P*, V0*, and T* in the equation of state are almost independent of temperature; the principle of corresponding state holds well. Since our equation of state contains the external degrees of freedom explicitly, we can evaluate the external degrees of freedom, c, for CH2. The value of c for the coarse-grained particle is equal to 1, and hence the particle is composed of 1/c repeating units. The linear length of the particle evaluated, 4.09 Å at 0 K, is consistent with that obtained by neutron and x-ray scatterings.
Synchronization of trajectories in canonical molecular-dynamics simulations: Observation, explanation, and exploitation
Authors: Uberuaga Blas P. , Anghel Marian , Voter Arthur F.
Published: March 2004

For two methods commonly used to achieve canonical-ensemble sampling in a molecular-dynamics simulation, the Langevin thermostat and the Andersen [H. C. Andersen, J. Chem. Phys. 72, 2384 (1980)] thermostat, we observe, as have others, synchronization of initially independent trajectories in the same potential basin when the same random number sequence is employed. For the first time, we derive the time dependence of this synchronization for a harmonic well and show that the rate of synchronization is proportional to the thermostat coupling strength at weak coupling and inversely proportional at strong coupling with a peak in between. Explanations for the synchronization and the coupling dependence are given for both thermostats. Observation of the effect for a realistic 97-atom system indicates that this phenomenon is quite general. We discuss some of the implications of this effect and propose that it can be exploited to develop new simulation techniques. We give three examples: efficient thermalization (a concept which was also noted by Fahy and Hamann [S. Fahy and D. R. Hamann, Phys. Rev. Lett. 69, 761 (1992)]), time-parallelization of a trajectory in an infrequent-event system, and detecting transitions in an infrequent-event system.
Mode-coupling theory for reaction dynamics in liquids
Authors: Shental Nurit , Rabani Eran
Published: March 2004

A theory for chemical reaction dynamics in condensed phase systems based on the generalized Langevin formalism of Grote and Hynes [J. Chem. Phys. 73, 2715 (1980)] is presented. A microscopic approach to calculate the dynamic friction is developed within the framework of a combination of kinetic and mode-coupling theories. The approach provides a powerful analytic tool to study chemical reactions in realistic condensed phase environments. The accuracy of the approach is tested for a model isomerization reaction in a Lennard-Jones fluid. Good agreement is obtained for the transmission coefficient at different solvent densities, in comparison with numerical simulations based on the reactive-flux approach.
Large effect of polydispersity on defect concentrations in colloidal crystals
Authors: Pronk Sander , Frenkel Daan
Published: March 2004

We compute the equilibrium concentration of stacking faults and point defects in polydisperse hard-sphere crystals. We find that, while the concentration of stacking faults remains similar to that of monodisperse hard-sphere crystals, the concentration of vacancies decreases by about a factor of 2. Most strikingly, the concentration of interstitials in the maximally polydisperse crystal may be some six orders of magnitude larger than in a monodisperse crystal. We show that this dramatic increase in interstitial concentration is due to the increased probability of finding small particles and that the small-particle tail of the particle size distribution is crucial for the interstitial concentration in a colloidal crystal.
Towards the Development of Quality of Life Indicators in the ‘Digital’ City
Authors: Craglia Massimo , Leontidou Lila , Nuvolati Giampaolo , Schweikart Jürgen
Published: March 2004

Cities are central to the economic and social development of European society, not only because over 80% of European citizens live in urban areas, but also because cities are at the same time centres of production, innovation, employment, and culture, and loci of segregation, deprivation, and ethnic conflict. The emergence of a European-wide urban policy, has given new impetus to the need for comparable indicators of the quality of life to monitor development and policy implementation. This paper reviews the literature on quality of life indicators, and argues that traditional measures of the quality of life need to be supplemented with two new dimensions that reflect more recent postmodernist thinking about the composition of urban landscapes, and the contribution to the quality of life of the emerging information society. We argue that the challenges of building appropriate indicators reflecting these new dimensions are considerable, even in urban environments so rich in information systems and data sources, if they are to qualify as ‘digital cities’. There are difficulties in finding common workable definitions of the indicators themselves, as well as definitions of the relevant populations, including city residents, and users. By raising these issues and suggesting possible avenues for addressing these challenges we contribute to a much-needed debate on how to define such indicators, which is the prerequisite for their development and use.
Delineation of Demographic Regions with GIS and Computational Intelligence
Authors: Hatzichristos Thomas
Published: March 2004

This paper presents a methodology for the creation of homogeneous demographic regions with geographical information systems (GIS) and computational intelligence. The proposed method is unsupervised fuzzy classification performed by neural networks using the fuzzy Kohonen algorithm. GIS technology offers a powerful set of tools for the input, management, and output of data, whereas computational intelligence is used for the analysis and the classification of the data. The proposed methodology is applied to the municipality of Athens, in Greece. Finally the advantages and disadvantages of the approach are discussed.
Evaluating Geographic Information Provision within Public Administration Websites
Authors: Campagna Michele , Deplano Giancarlo
Published: March 2004

The authors are concerned with the provision of public sector geographic information (GI) and the use of GI technologies (GITs) within public administration websites, with particular reference to planning. Results of a survey carried out by the authors on an Italian case study are evaluated with respect to the GI content and technology adopted. A rapid evaluation method is proposed to analyse and compare GI/GIT-diffusion frameworks within public administration at the national level.
The Construction of the Digital City
Authors: Couclelis Helen
Published: March 2004

In this paper I discuss the digital city as a comprehensive, web-based representation, or reproduction, of several aspects or functions of a specific real city, open to nonexperts. Much more than a technical and practical issue, the digital city has dimensions that are social, cultural, political, ideological, and of course also theoretical. I explore these broader conceptual underpinnings of the digital city idea. More specifically: where do urban ideologies, urban structures and functions, and digital urban representations meet? How may the digital city relate to the real city? On the basis of what choices might the digital city be constructed? What might the digital city mean to people? To be answered, these highly involved and interrelated questions require a coherent conceptual framework. By considering an overview of digital-city realizations, issues, and critiques, I adumbrate a theory of the digital city, seen as lying at the intersection of three domains: a physical urban area, the communities of people associated with that area, and the possibilities and constraints (technological, but also socioeconomic and ideological) of the evolving information society. A rough sketch is all that can be offered at this point because proper theories of the digital city cannot be developed independently of comprehensive theories of the real 21st-century city. I close with a brief discussion of what these ideas might imply for digital-city designers.
Raster Analysis of Urban Form
Authors: Ratti Carlo , Richens Paul
Published: March 2004

A new paradigm for investigating the environment consequences of urban texture is proposed. Using raster-based models and software algorithms derived from image processing the authors develop efficient methods of measuring geometric parameters and predicting radiation exchange. The possibilities of generating synthetic urban textures and of integrating cellular automata are explored. Results suggest the possibility of a raster-based urban model to inform planning and design.
Modelling Future Urban Scenarios in Developing Countries: An Application Case Study in Lagos, Nigeria
Authors: Barredo José I , Demicheli Luca , Lavalle Carlo , Kasanko Marjo , McCormick Niall
Published: March 2004

We consider urban sustainability issues in developing countries, with a focus on urban growth. The need for urban management tools that are able to provide prospective scenarios is addressed. Urban simulations can represent a useful approach to understanding the consequences of current planning policies—or their incompleteness. Nevertheless, simulations of future urban growth are usually quite difficult without tools which embrace the complexity of the urban system. We describe an urban-growth simulation for the city of Lagos in Nigeria, in which an urban cellular automata (CA) prototype is used. We propose a bottom-up approach which integrates land-use factors with the dynamic approach of CA for modelling future urban land-use scenarios. The model for Lagos was calibrated and tested with the aid of measured time-series data on land use, through a set of spatial metrics and κ-coefficients. A twenty-year simulation, until 2020, was run. The simulation results are realistic and achieve a high level of detail, confirming the effectiveness of the proposed model.
Comparative Analysis of Time-Space Prism Vertices for Out-of-Home Activity Engagement on Working and Nonworking Days
Authors: Yamamoto Toshiyuki , Kitamura Ryuichi , Pendyala Ram M
Published: March 2004

Out-of-home activities are engaged within time – space prisms, but the prisms themselves are unobservable. In this paper, stochastic frontier models with observed departure and arrival times as dependent variables are formulated to locate time – space prism vertices. Two possible distributions often used in frontier models for one-sided disturbance terms, the half-normal distribution and exponential distribution, are examined better to represent observed behavior. The locations of morning prism origin vertices and evening prism terminal vertices on working days and nonworking days are estimated with an empirical dataset obtained in southeast Florida. The results suggest that the exponential distribution has a better goodness of fit, but the coefficient estimates of explanatory variables are similar between models with the two distributions, suggesting that the model parameter estimates are robust regardless of the assumptions regarding the distribution of the one-sided random disturbance. It is found that the average heights of the morning prism before work and the evening prism after work on working days are estimated at about 2.5 hours and about 3 hours, respectively. It is also found that part-time workers have prism heights that are on average larger by half an hour than those of full-time workers on working days, and smaller by 1.5 hours on nonworking days.
In Pursuit of Sustainability? Challenges for Deliberative Democracy in a Tasmanian Local Government
Authors: Stratford Elaine , Jaskolski Martina
Published: March 2004

Sustainability may be viewed as a principled form of conduct. Among its effects is a growing emphasis on civil society and local governance through which the members of communities are encouraged to rethink democratic ideas and practices, and reconfigure how to live. Although normative, sustainability cannot properly be conceived as prescriptive; such a characteristic would undermine central elements of it, such as participation and equity. In this sense, requiring both mechanisms for community participation in decisionmaking and planning, and an ethic of engagement based on trust, reciprocity, and an acceptance of the rights of noncitizens and nonhuman nature, sustainability might also be construed as a deliberative form of democratic governance. Perhaps problematically, in the last decade this governmental aspect of sustainability has come to be associated with the procedures of communicative rationality. Supported by research conducted over three years in a local government in Tasmania, Australia, we argue that a deliberative and democratic praxis of sustainability may be effective only if and when underpinned by substantive changes to the exercise of power and leadership, and to the ways in which deliberative decisionmaking and planning are pursued. Communicative rationality alone is unlikely to achieve these ends.
Understanding Spatial and Temporal Processes of Urban Growth: Cellular Automata Modelling
Authors: Cheng Jianquan , Masser Ian
Published: March 2004

An understanding of the dynamic process of urban growth is a prerequisite to the prediction of land-cover change and the support of urban development planning and sustainable growth management. The spatial and temporal complexity inherent in urban growth requires the development of a new simulation approach, which should be process-oriented and have a strong interpretive element. In this paper the authors present an innovative methodology for understanding spatial processes and their temporal dynamics on two interrelated scales—the municipality and project scale—by means of a multistage framework and a dynamic weighting concept. The multistage framework is aimed at modelling local spatial processes and global temporal dynamics by the incorporation of explicit decisionmaking processes. It is divided into four stages: project planning, site selection, local growth, and temporal control. These four stages represent the interactions between top-down and bottom-up decisionmaking involved in land development in large-scale projects. Project-based cellular automata modelling is developed for interpreting the spatial and temporal logic between various projects that form the whole of urban growth. Use of dynamic weighting is an attempt to model local temporal dynamics at the project level as an extension of the local growth stage. As nonlinear function of temporal land development, dynamic weighting can link spatial processes and temporal patterns. The methodology is tested with reference to the urban growth of a fast growing city—Wuhan, in the People's Republic of China—from 1993 to 2000. The findings from this research suggest that this methodology can be used to interpret and visualise the dynamic process of urban growth temporally and transparently, globally and locally.
A Way Forward for Land-Use Planning to Achieve Policy Goals by Using Spatial Modelling Scenarios
Authors: Pettit Christopher , Pullar David
Published: March 2004

This paper investigates how demographic (socioeconomic) and land-use (physical and environmental) data can be integrated within a decision support framework to formulate and evaluate land-use planning scenarios. A case-study approach is undertaken with land-use planning scenarios for a rapidly growing coastal area in Australia, the Shire of Hervey Bay. The town and surrounding area require careful planning of the future urban growth between competing land uses. Three potential urban growth scenarios are put forth to address this issue. Scenario A (‘continued growth’) is based on existing socioeconomic trends. Scenario B (‘maximising rates base’) is derived using optimisation modelling of land-valuation data. Scenario C (‘sustainable development’) is derived using a number of social, economic, and environmental factors and assigning weightings of importance to each factor using a multiple criteria analysis approach. The land-use planning scenarios are presented through the use of maps and tables within a geographical information system, which delineate future possible land-use allocations up until 2021. The planning scenarios are evaluated by using a goal-achievement matrix approach. The matrix is constructed with a number of criteria derived from key policy objectives outlined in the regional growth management framework and town planning schemes. The authors of this paper examine the final efficiency scores calculated for each of the three planning scenarios and discuss the advantages and disadvantages of the three land-use modelling approaches used to formulate the final scenarios.
Accessibility Analyst: An Integrated GIS Tool for Accessibility Analysis in Urban Transportation Planning
Authors: Liu Suxia , Zhu Xuan
Published: March 2004

The authors present an integrated GIS tool, Accessibility Analyst, for accessibility analysis in urban transportation planning, built as an extension to the desktop GIS software package, ArcView. Accessibility Analyst incorporates a number of accessibility measures, ranging from catchment profile analysis to cumulative-opportunity measures, gravity-type measures, and utility-based measures, contains several travel-impedance measurement tools for estimating the travel distance, time, or cost by multiple travel modes along actual travel routes, and interoperates with GIS data-management and data-integration, spatial-analysis, network-analysis, surface-modelling, and spatial-visualisation functions. Undertaking accessibility analysis with use of Accessibility Analyst allows the user to take full advantage of a GIS to produce spatial distributions of accessibility over a region. It can be applied to a wide range of issues in urban transportation planning, such as for studies on the relationship between transportation and land use, evaluation of transportation network efficiency, transportation infrastructure planning, and for impact assessments relating to transportation policies.
Topological Analysis of Urban Street Networks
Authors: Jiang Bin , Claramunt Christophe
Published: March 2004

The authors propose a topological analysis of large urban street networks based on a computational and functional graph representation. This representation gives a functional view in which vertices represent named streets and edges represent street intersections. A range of graph measures, including street connectivity, average path length, and clustering coefficient, are computed for structural analysis. In order to characterise different clustering degrees of streets in a street network they generalise the clustering coefficient to a k-clustering coefficient that takes into account k neighbours. Based on validations applied to three cities, the authors show that large urban street networks form small-world networks but exhibit no scale-free property.