bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiple-collision rotational energy transfer in Na2
Authors: Zalicki P. , Kolwas K. , Kolwas M.
Published: July 2002

Multiple-collision redistribution of the population of the optically excited Na2 B 1Πu (v=6, j=43) level over adjacent rotational levels with j+Δj=49, 45, 41, 39, and 37 is studied. Fluorescence intensities from these levels corresponding to Q transitions were observed as a function of perturber gas pressure (He) up to 500 Torr. Resulting multiple-collision transfer coefficients increase linearly with pressure, then tend to saturation, and again increase with pressure. We present the theoretical description of the process, based on rate equations and including multiple-collision population and depopulation rates. Effective cross sections for the rotational transfer are found showing the increasing role of multiple collisions with increasing ‖Δj‖ value.
Time-of-flight neutron diffraction studies on the different phases of CsOH, H2O
Authors: Marx R. , Dachs H. , Ibberson R. M.
Published: July 2002

Neutron diffraction studies on the two dimensional ice CsOH, H2O were performed at the High Resolution Powder Diffractometer at the spallation source ISIS in England. The structure at the highest temperature comprises an O (and H) disorder perpendicular to the O2H−3 nets as well as an H disorder along an O–O connection line within the nets. The room temperature phase comprises ordered, corrugated nets, very likely built of ordered groups of OH− and H2O. The O order disorder transition can be assigned to the phase transition at 340 K. For a deuterated sample the different behavior of the OD− and the D2O group is determined. The OD− group must be described by a model with O off the threefold axis. O–D⋅⋅⋅ O bridges of different strengths result.
A molecular dynamics study of ion bombardment induced ejection of C2H4 from Ni(111): Relationships of the angle distributions of the translational and rotational angular momentum of ejected C2H4 to surface structure
Authors: Lauderback L. L. , Ang M. L. , Murray H. C.
Published: July 2002

The angular distributions of the translational and rotational angular momentum of C2H4 molecules ejected from a Ni(111) surface by ion bombardment have been calculated from molecular dynamics simulations of the ion–surface collision process. Both the translational and rotational momentum of ejected C2H4 tend to orient in preferred directions which are dependent on the C2H4 bonding site geometry. Most C2H4 molecules are ejected by a collision with a single energetic Ni atom. It is shown that the translational angle distribution is governed primarily by the distribution of Ni–C2H4 collision geometries. A preference for Ni atoms to eject in the directions between surrounding nearest neighbor Ni atoms gives rise to preferred Ni–C2H4 collision geometries that cause C2H4 to eject in preferred directions. The preferred orientations of the rotational angular momentum of ejected C2H4 are shown to be governed directly by the orientation of the C–C bond axis of the original chemisorbed C2H4 molecules, independent of the Ni–C2H4 collision geometry.
Reaction cross sections by time profile measurements under crossed-beam conditions: The Ca(3P,1D)+N2O→CaO*+N2 reaction
Authors: Verdasco E. , Ureña A. González
Published: July 2002

The collision energy dependence of the Ca(3P,1D)+N2O→CaO*+N2 reaction has been determined by time profile measurements under crossed-beam conditions. A narrow velocity distribution N2O beam collides with a pulsed beam of excited metastable Ca(3P,1D) atoms, produced by low-voltage discharge. Time-of-flight spectra were used to monitor both the reagent translational and the chemiluminescence yield of the CaO* product (green arc band). The chemiluminescence cross section for excitation of the CaO green arc bands decreases with increasing translational energy with 1D reagent, but increases for the 3P state. The combination of a narrow velocity distribution of the supersonic N2O beam with short pulses of the Ca* beam leads to collision energy resolution of the order of magnitude of a few meV. This could be important not only for determining high resolution reaction thresholds, but perhaps for the onset of quantum effect in the total reaction cross section.
Fluctuation dominated recombination kinetics with traps
Authors: Rasaiah Jayendran C. , Zhu Jianjun , Hubbard Joseph B. , Rubin Robert J.
Published: July 2002

Theoretical and computer simulation studies of annihilation reactions with traps on two and in three dimensional lattice systems are reported for the following reactions: (1) Bimolecular trapping/annihilation: A+A→*; A+T→AT; A+AT→T; (2) unimolecular trapping/annihilation: A+A→*; A→AT; A+AT→*. The mean field analysis and combinatorial calculations of the rate constants given previously for a square lattice are generalized to lattices in two and three dimensions. It is found that the kinetics of trapped A’s can be described by mean field theory for bimolecular but not for unimolecular trapping reactions. The kinetics of free A’s obeys mean field theory at short times, but at longer times and at low trap densities the free A population decays as a stretched exponential at when large density fluctuations dominate the reaction. This stretched exponential behavior of the Donsker–Varadhan from A(t)∼exp(−td/(d+2)), where d is the dimensionality, already found for the reactant decay in A–A annihilation reactions with traps on a square lattice [Rasaiah et al., J. Phys. Chem. 94, 652 (1990)] was tested for universality by studying triangular and hexagonal lattices in two dimensions (2D) and a cubic lattice in three dimensions (3D). The same behavior is also observed when the free particle annihilation is turned off. The effect of a finite staying probability ps on the kinetics of these reactions are also investigated.
Quasielastic gamma-ray scattering from polydimethylsiloxane in benzene solutions
Authors: Hammouda B. , Schupp G. , Maglic S.
Published: July 2002

Quasielastic gamma-ray scattering of 46.5-keV Mössbauer photons by polydimethylsiloxane has been studied at room temperature as a function of dilution in benzene. The high energy resolution of this novel technique allowed the separation of the scattering signal into a narrow component associated with stiff motions along the polymer chain backbone and a quasielastic component associated with softer side group motions. The narrow component disappears upon dilution in benzene while the intensity of the quasielastic component grows proportionately. This result is interpreted as a softening of the backbone normal modes upon dilution.
Visible and uv spectra of a polydiacetylene with a side group conjugated to the main chain
Authors: Ichimura K. , Kobayashi T. , Matsuda H. , Nakanishi H. , Kato M.
Published: July 2002

The polarized reflection spectra of single crystals of PDA-CPDO [poly-1-(N-carbazolyl)penta-1, 3-diyn-5-ol], which are π conjugated between the side groups and the main chain, have been measured in the photon energy region from 1.38 to 32 eV for the first time with a polarized synchrotron radiation source. Absorption spectra have been calculated using the Kramers–Kronig relation. Transitions in the visible absorption spectrum, which have been previously attributed to an interband transition in a similar polydiacetylene, have been found to be highly dichroic with respect to the direction along the polymer backbone. The absorption spectrum in this region reveals two broad (ΔE≳0.4 eV) peaks at 1.9 and 2.7 eV with an almost equal absorption coefficient of 7.5×104 cm−1. The spectral features in the range from 3 to 8 eV result from electronic transitions of the carbazolyl side groups. A single broad (ΔE∼10 eV) absorption band observed at 18 eV is due to either transitions of σ electrons to higher σ* or π* states of the carbazolyl group, or to ionization processes.
Pseudopotential approaches to Ca, Sr, and Ba hydrides. Why are some alkaline earth MX2 compounds bent?
Authors: Kaupp M. , Schleyer P. v. R. , Stoll H. , Preuss H.
Published: July 2002

Quasirelativistic and nonrelativistic 10-valence-electron pseudopotentials for Ca, Sr, and Ba are presented. Results of calculations with 6s6p5d basis sets for MH, MH+, and MH2 are compared with all-electron and 2-valence-electron pseudopotential calculations with and without core-polarization potentials. The 10-valence-electron pseudopotential approach agrees well with all-electron calculations. It circumvents problems for the 2-valence-electron pseudopotentials arising from an incomplete separation of valence and subvalence shells in polar molecular systems due to strongly contracted occupied (n−1)-d orbitals. All higher-level calculations show SrH2 and BaH2 to be bent with angles of ∼140° and 120°, respectively, while CaH2 is linear with a flat potential-energy surface for the bending motion. The use of a core-polarization potential together with the 2-valence-electron pseudopotential approach allows an investigation of the relative importance of core-polarization vs direct d-orbital bonding participation as reasons for the bent structures. The calculations strongly suggest that both contribute to the bending in SrH2 and BaH2. Even at the Hartree–Fock level of theory 10-valence-electron pseudopotential calculations given reasonable angles when the potential-energy surface is not exceedingly flat, and only moderately contracted basis sets including both compact d functions and diffuse p functions are used. The effect of core-valence correlation and the importance of f functions also are discussed.
Deexcitation electron spectroscopy of core-excited CO2: Comparison of experiment with theory
Authors: Carroll T. X. , Thomas T. D.
Published: July 2002

The electron–electron coincidence technique has been used to measure the spectra of autoionizing (Auger) electrons that are emitted following excitation of carbon and oxygen 1s electrons in CO2 to the 2πu orbital. The experimental spectra are compared with theoretical calculations of Porwol et al. Agreement between theory and experiment is not good, in contrast to the situation for the photoionization spectrum, where there is good agreement. Possible causes for discrepancies are discussed.
Reactions of H2, D2 with Hg(6 3P) or Hg(6 3P2): Product rotational state distributions
Authors: Bras N. , Jeannet J. C. , Butaux J. , Perrin D.
Published: July 2002

The initial rotational state distributions of the HgH(v=0) and HgD(v=0) molecules produced in the reactions of Hg(6 3P0) with H2, D2 have been determined. These distributions differ from the ones previously determined with Hg in the 6 3P1 state. This difference may be due to the decrease of the reaction exothermicity when Hg(6 3P1) is replaced by Hg(6 3P0). As for the reaction with Hg(6 3P2), it seems from a time analysis that there is some production of molecules in the higher rotational levels, nevertheless more work is necessary to conclude about the Hg(6 3P2) reactivity.
Sustained patterns in chlorite–iodide reactions in a one-dimensional reactor
Authors: Ouyang Q. , Castets V. , Boissonade J. , Roux J. C. , De Kepper P. , Swinney Harry L.
Published: July 2002

We describe an open one-dimensional reaction–diffusion system, the Couette reactor, which provides a permanent feed of fresh reactants and simultaneously preserves the diffusion process. Different compositions of variants of chlorite–iodide reactions are fed at both ends, but there is no net axial mass flux in the reactor. The effective diffusion coefficient is the same for all chemical species, and can easily be varied from 10−2 to 1 cm2/s, permitting large size reaction–diffusion structures. Spatial bistability was obtained with the minimal chlorite–iodide reaction in symmetric feed conditions. A larger variety of patterns was observed with the chlorite–iodide–malonic acid reaction and asymmetric feed conditions: a single steady chemical front, two steady fronts, three steady fronts, a single oscillating front, two oscillating fronts, three oscillating fronts, simple colliding fronts, and a bursting pattern. The patterns were studied as a function of several parameters to determine the sequences of bifurcations. Our observations demonstrate the existence of steady chemical patterns other than Turing patterns.
Fully optimized contracted Gaussian basis sets for atoms Li to Kr
Authors: Schäfer Ansgar , Horn Hans , Ahlrichs Reinhart
Published: July 2002

Various contracted Gaussian basis sets for atoms up to Kr are presented which have been determined by optimizing atomic self-consistent field ground state energies with respect to all basis set parameters, i.e., orbital exponents and contraction coefficients.
Nosé–Hoover chains: The canonical ensemble via continuous dynamics
Authors: Martyna Glenn J. , Klein Michael L. , Tuckerman Mark
Published: July 2002

Nosé has derived a set of dynamical equations that can be shown to give canonically distributed positions and momenta provided the phase space average can be taken into the trajectory average, i.e., the system is ergodic [S. Nosé, J. Chem. Phys. 81, 511 (1984), W. G. Hoover, Phys. Rev. A 31, 1695 (1985)]. Unfortunately, the Nosé–Hoover dynamics is not ergodic for small or stiff systems. Here a modification of the dynamics is proposed which includes not a single thermostat variable but a chain of variables, Nosé–Hoover chains. The ‘‘new’’ dynamics gives the canonical distribution where the simple formalism fails. In addition, the new method is easier to use than an extension [D. Kusnezov, A. Bulgac, and W. Bauer, Ann. Phys. 204, 155 (1990)] which also gives the canonical distribution for stiff cases.
Gaussian basis sets for use in correlated molecular calculations. III. The atoms aluminum through argon
Authors: Woon David E. , Dunning Thom H.
Published: July 2002

Correlation consistent and augmented correlation consistent basis sets have been determined for the second row atoms aluminum through argon. The methodology, originally developed for the first row atoms [T. H. Dunning, Jr., J. Chem. Phys. 90, (1989)] is first applied to sulfur. The exponents for the polarization functions (dfgh) are systematically optimized for a correlated wave function (HF+1+2). The (sp) correlation functions are taken from the appropriate HF primitive sets; it is shown that these functions differ little from the optimum functions. Basis sets of double zeta [4s3p1d], triple zeta [5s4p2d1f], and quadruple zeta [6s5p3d2f1g] quality are defined. Each of these sets is then augmented with diffuse functions to better describe electron affinities and other molecular properties: s and p functions were obtained by optimization for the anion HF energy, while an additional polarization function for each symmetry present in the standard set was optimized for the anion HF+1+2 energy. The results for sulfur are then used to assist in determining double zeta, triple zeta, and quadruple zeta basis sets for the remainder of the second row of the p block.
A new mixing of Hartree–Fock and local density-functional theories
Authors: Becke Axel D.
Published: July 2002

Previous attempts to combine Hartree–Fock theory with local density-functional theory have been unsuccessful in applications to molecular bonding. We derive a new coupling of these two theories that maintains their simplicity and computational efficiency, and yet greatly improves their predictive power. Very encouraging results of tests on atomization energies, ionization potentials, and proton affinities are reported, and the potential for future development is discussed.
Microwave spectra of the D2+ and HD+ ions near their dissociation limits
Authors: Carrington Alan , Leach Christine A. , Marr Andrew J. , Moss Richard E. , Pyne Christopher H. , Steimle Timothy C.
Published: July 2002

We have observed and measured seven microwave transitions in D2+, which are vibration–rotation components of the 2pσu–1sσg electronic band system. We also report two microwave rotational components involving the 1sσ v=21 level of HD+, which lies close to the first dissociation limit. Our experiments use ion beam techniques in which state selection is achieved by electric field dissociation. The techniques and theory of electric field dissociation are discussed, and fragment ion kinetic energy spectra described. Ab initio calculations of the transition frequencies for both D2+ and HD+ are in excellent agreement with experiment, and the hyperfine structure of the rotational transitions in HD+ confirms previous demonstrations of the increasing asymmetry in the electron distribution as the dissociation limit is approached. The electronic transition in D2+ which involves the highest bound level of the ground state shows an unexpected hyperfine splitting, which is interpreted in terms of g/u symmetry breaking by the deuteron Fermi contact interaction.
Conservation of zero-point energy in classical trajectory computations by a simple semiclassical correspondence
Authors: Ben-Nun M. , Levine R. D.
Published: July 2002

A simple practical procedure which ensures that the energy in a molecular vibrational mode does not decrease below its zero-point value is discussed and applied. The method is based on taking the classical limit of the Hamiltonian and thereby deriving classical equations of motion which are solved via a standard classical trajectory computation. We refer to this as the ‘‘reference’’ trajectory. It is argued that the reference solution differs from what one would obtain if one were to begin with a classical description of the problem; the difference being that the reference computation puts the zero of energy at the correct, quantum-theoretic, zero, i.e., at the zero point. To obtain a fully classical-like solution one needs to shift the energy and period of the reference trajectory and the different ways of doing this are discussed. The resulting, energy, and phase shifted, equivalent classical trajectory cannot, by construction, lose the zero-point energy from the modes in which it is placed. The method is discussed first for the obvious case of a single oscillator, including the role of the anharmonicity, and is then applied to a variety of dimers [I2He, ArHBr, (HF)2] where a higher frequency mode is coupled to a low-frequency one and the problem is to prevent the (high) zero-point energy from being made available for transfer to the far weaker mode. Other advantages of the proposed scheme, such as the correct frequency dependence of the power spectrum, and its application to an unbound motion in the continuum are also discussed.
A simple method for the derivation of exact quantum-mechanical vibration–rotation Hamiltonians in terms of internal coordinates
Authors: Lukka Tuomas J.
Published: July 2002

By considering infinitesimal rotations, the well-known G-matrix method is extended for the derivation of exact quantum-mechanical vibration–rotation Hamiltonians in arbitrary vibrational coordinates and molecule-fixed coordinate systems. All rovibrational coefficients can be calculated by dot products, with considerably less algebra than by using conventional methods. Coordinate transformations from one molecule-fixed coordinate system to another are discussed. Hamiltonians are partly derived for XH2 and XH3 (without inversion) type molecules to demonstrate the ease of this approach.
A smooth particle mesh Ewald method
Authors: Essmann Ulrich , Perera Lalith , Berkowitz Max L. , Darden Tom , Lee Hsing , Pedersen Lee G.
Published: July 2002

The previously developed particle mesh Ewald method is reformulated in terms of efficient B-spline interpolation of the structure factors. This reformulation allows a natural extension of the method to potentials of the form 1/rp with p≥1. Furthermore, efficient calculation of the virial tensor follows. Use of B-splines in place of Lagrange interpolation leads to analytic gradients as well as a significant improvement in the accuracy. We demonstrate that arbitrary accuracy can be achieved, independent of system size N, at a cost that scales as N log(N). For biomolecular systems with many thousands of atoms this method permits the use of Ewald summation at a computational cost comparable to that of a simple truncation method of 10 Å or less.
Microwave dielectric characterization of binary mixtures of water, methanol, and ethanol
Authors: Bao Jian-Zhong , Swicord Mays L. , Davis Christopher C.
Published: July 2002

The complex dielectric constants of binary mixtures of water–methanol and water–ethanol in the frequency range from 45 MHz to 26.5 GHz, and binary mixtures of methanol–ethanol in the range from 200 MHz to 26.5 GHz have been measured with various volume fractions around room temperature by means of an open-ended coaxial sensor and a network analyzer. Methanol–ethanol mixtures display a near-Debye dispersion while water–alcohol mixtures show a Cole–Davidson dispersion. The logarithm of relaxation time log τ  and dielectric decrement Δε for methanol–ethanol mixtures show a good linear relation with the volume fraction of methanol, while log τ and Δε extracted with the Debye function for water–alcohol mixtures display a near-linear relation with volume fraction of water. Two simple formulas are proposed for identifying the volume fractions of the components in binary mixtures of alcohol–alcohol and water–alcohol from a knowledge of τ and Δε for the pure liquids and the mixtures. The validity of these formulas has been demonstrated with three blind tests. The relation between the mole fraction of water and log τ for water–methanol and water–ethanol mixtures extracted by the use of a Cole–Davidson function clearly shows two linear regions, which implies a change of relaxation mechanism with mole fraction.
The integral-direct coupled cluster singles and doubles model
Authors: Koch Henrik , Sánchez de Merás Alfredo , Helgaker Trygve , Christiansen Ove
Published: July 2002

An efficient and highly vectorized implementation of the coupled cluster singles and doubles (CCSD) model using a direct atomic integral technique is presented. The minimal number of n6 processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of integral prescreening.
The determination of apparent memory functions from dielectric relaxation data for amorphous polymers and glass-forming liquids: Theory and experiment
Authors: Williams Graham , Fournier Jérôme
Published: July 2002

Interrelationships between time-correlation functions for molecular reorientation, their apparent memory functions and dielectric permittivity for the α relaxation in glass-forming liquids and amorphous polymers are considered briefly and a method is given for the determination of an apparent first-order memory function K̃1(ω) in the frequency domain from dielectric relaxation data. For the Kohlrausch–Williams–Watts (KWW) function and the Cole–Cole function and for experimental data for three amorphous polymers it is shown that the real and imaginary components of K̃1(ω) increase without limit as frequency f(=ω/2π) is increased. The physical significance of such apparent memory functions is considered in view of this behavior and the recent experimental studies of the α relaxation in polymers by Schmidt–Rohr and Spiess and in glass-forming liquids by Cicerone and Ediger, who suggest that a ‘‘dynamic heterogeneity’’ is present that leads to a broad distribution of relaxation times where each elementary process has no proper memory. For such cases it appears that the apparent memory functions are simply related to the parameters characterizing the distribution and may have no physical significance for the α process in glass-forming materials.
Size-selected vibrational spectra of phenol-(H2O)n (n=1–4) clusters observed by IR–UV double resonance and stimulated Raman-UV double resonance spectroscopies
Authors: Watanabe Takeshi , Ebata Takayuki , Tanabe Shigeki , Mikami Naohiko
Published: July 2002

OH and CH stretching vibrations of bare phenol, phenol-(H2O)n clusters (n=1–4), and partially deuterated clusters in the S0 state were observed by using IR–UV double resonance and stimulated Raman-UV double resonance spectroscopies. Characteristic spectral features of the OH stretching vibrations of the phenol as well as of the H2O sites were observed, which are directly related to their structures. The cluster structures were investigated by comparing the observed spectra with the calculated ones obtained by the ab initio molecular orbital calculation with (self-consistent field) SCF 6-31G and SCF 6-31G* basis sets given by Watanabe and Iwata. It was found that for the clusters with n≥2, the isomer of ring form hydrogen-bonded structure is most stable and the simulated IR spectra based on the calculated structure showed good agreements with the observed ones. For a particular cluster, which was assigned as an isomer of the n=4 cluster, an anomalous IR spectrum was observed. Two forms of the isomer are proposed with respect to the structure of water moiety: (1) an ‘‘ice’’ structure and (2) an ‘‘ion-pair’’ structure. The relative IR absorption cross sections of each bands were also investigated for the clusters with n=1 to 4. It was found that the IR absorption cross section of the phenolic OH stretching vibration of the n=1 cluster increases by a factor of 6 compared to that of bare phenol and it further increases with the cluster size.
Morphology of miktoarm star block copolymers of styrene and isoprene
Authors: Tselikas Y. , Iatrou H. , Hadjichristidis N. , Liang K. S. , Mohanty K. , Lohse D. J.
Published: July 2002

The morphologies of several styrene–isoprene miktoarm star copolymers have been examined by small angle x-ray scattering and electron microscopy. Three of these polymers were of the three-armed A2B type, and six were four-armed A3B miktoarms. The ordered microphase-separated morphologies displayed by these polymers were seen to be highly dependent on the architecture of the chains, and were quite different from those that occur in the corresponding linear block copolymers. These results can be quite well explained by a theory of Milner which is based on the bending energy of the microphase interface and the ability of the chains to stretch away from the interface between the microphases. We speculate that the small differences between the observations and theoretical predictions are due to the effects of the compression of chains in small gaps between adjacent domains.
Crossed-beam reaction of carbon atoms with hydrocarbon molecules. I. Chemical dynamics of the propargyl radical formation, C3H3 (X2B2), from reaction of C(3P
j) with ethylene, C2H4(X1A
g)
Authors: Kaiser R. I. , Lee Y. T. , Suits A. G.
Published: July 2002

The reaction between ground-state carbon atoms, C(3Pj), and ethylene, C2H4(X1Ag), was studied at average collision energies of 17.1 and 38.3 kJmol−1 using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of m/e=39 were recorded. Forward-convolution fitting of the results yields a maximum energy release as well as angular distributions consistent with the formation of the propargyl radical in its X2B2 state. Reaction dynamics inferred from the experimental data indicate two microchannels, both initiated by attack of the carbon atom to the π-orbital of the ethylene molecule via a loose, reactant like transition state located at the centrifugal barrier. Following Cs symmetry on the ground state 3A″ surface, the initially formed triplet cyclopropylidene complex rotates in a plane roughly perpendicular to the total angular momentum vector around its C-axis, undergoes ring opening to triplet allene, and decomposes via hydrogen emission through a tight transition state to the propargyl radical. The initial and final orbital angular momenta L and L′ are weakly coupled and result in an isotropic center-of-mass angular distribution. A second microchannel arises from A-like rotations of the cyclopropylidene complex, followed by ring opening and H-atom elimination. In this case, a strong L-L′ correlation leads to a forward-scattered center-of-mass angular distribution. The explicit identification of C3H3 under single collision conditions represents a single, one-step mechanism to build up hydrocarbon radicals. Our findings strongly demand incorporation of distinct product isomers of carbon atom-neutral reactions in reaction networks simulating chemistry in combustion processes, the interstellar medium, as well as in outflows of carbon stars, and open the search for the hitherto unobserved interstellar propargyl radical.
A simple nonequilibrium molecular dynamics method for calculating the thermal conductivity
Authors: Müller-Plathe Florian
Published: July 2002

A nonequilibrium molecular dynamics method for calculating the thermal conductivity is presented. It reverses the usual cause and effect picture. The “effect,” the heat flux, is imposed on the system and the “cause,” the temperature gradient is obtained from the simulation. Besides being very simple to implement, the scheme offers several advantages such as compatibility with periodic boundary conditions, conservation of total energy and total linear momentum, and the sampling of a rapidly converging quantity (temperature gradient) rather than a slowly converging one (heat flux). The scheme is tested on the Lennard-Jones fluid.
Absolute and convective instabilities in a one-dimensional Brusselator flow model
Authors: Kuznetsov Sergey P. , Mosekilde Erik , Dewel Guy , Borckmans Pierre
Published: July 2002

The paper considers a one-dimensional Brusselator model with a uniform flow of the mixture of reaction components. An absolute as well as a convective instability can arise for both the Hopf and the Turing modes. The corresponding linear stability analysis is presented and supported by the results of computer simulations of the nonlinear equations. Finally, the condition for spatially undamped tails (the Cherenkov condition) is obtained. This represents a new mechanism for pattern formation in chemical reaction-diffusion systems.
Evaluation of quantum transition rates from quantum-classical molecular dynamics simulations
Authors: Prezhdo Oleg V. , Rossky Peter J.
Published: July 2002

The impact of quantum decoherence and zero point motion on non-adiabatic transition rates in condensed matter systems is studied in relation to non-adiabatic (NA) molecular dynamics (MD) techniques. Both effects, and decoherence in particular, strongly influence the transition rate, while neither is accounted for by straightforward quantum-classical approaches. Quantum corrections to the quantum-classical results are rigorously introduced based on Kubo’s generating function formulation of Fermi’s Golden rule and the frozen Gaussian approximation for the nuclear wave functions. The development provides a one-to-one correspondence between the decoherence function and the Franck–Condon factor. The decoherence function defined in this paper corrects an error in our previous work [J. Chem. Phys. 104, 5942 (1996)]. The relationship between the short time approach and the real time NA MD is investigated and a specific prescription for incorporating quantum decoherence into NA simulations is given. The proposed scheme is applied to the hydrated electron. The rate of excited state non-radiative relaxation is found to be very sensitive to the decoherence time. Quantum coherence decays about 50% faster in H2O than in D2O, providing a theoretical rationalization for the lack of experimentally observed solvent isotope effect on the relaxation rate. Microscopic analysis of solvent mode contributions to the coherence decay shows that librational degrees of freedom are primarily responsible, due to the strong coupling between the electron and molecular rotations and to the small widths of the wave packets describing these modes. Zero point motion of the O–H bonds decreases the life time of the excited state of the hydrated electron by a factor of 1.3–1.5. The implications of the use of short time approximations for the NA transition rate and for the evolution of the nuclear wave functions are considered.
Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory: Characterization and correction of the time-dependent local density approximation ionization threshold
Authors: Casida Mark E. , Jamorski Christine , Casida Kim C. , Salahub Dennis R.
Published: July 2002

This paper presents an evaluation of the performance of time-dependent density-functional response theory (TD-DFRT) for the calculation of high-lying bound electronic excitation energies of molecules. TD-DFRT excitation energies are reported for a large number of states for each of four molecules: N2, CO, CH2O, and C2H4. In contrast to the good results obtained for low-lying states within the time-dependent local density approximation (TDLDA), there is a marked deterioration of the results for high-lying bound states. This is manifested as a collapse of the states above the TDLDA ionization threshold, which is at −εHOMOLDA (the negative of the highest occupied molecular orbital energy in the LDA). The −εHOMOLDA is much lower than the true ionization potential because the LDA exchange-correlation potential has the wrong asymptotic behavior. For this reason, the excitation energies were also calculated using the asymptotically correct potential of van Leeuwen and Baerends (LB94) in the self-consistent field step. This was found to correct the collapse of the high-lying states that was observed with the LDA. Nevertheless, further improvement of the functional is desirable. For low-lying states the asymptotic behavior of the exchange-correlation potential is not critical and the LDA potential does remarkably well. We propose criteria delineating for which states the TDLDA can be expected to be used without serious impact from the incorrect asymptotic behavior of the LDA potential.
Consistent treatment of quantum-mechanical and classical degrees of freedom in mixed quantum-classical simulations
Authors: Müller Uwe , Stock Gerhard
Published: July 2002

A mixed quantum-classical formulation of nonadiabatic molecular processes is outlined. Based on a recently introduced mapping formalism [Stock and Thoss, Phys. Rev. Lett. 78, 578 (1997)], the formulation employs a quantum-mechanically exact mapping of discrete electronic states onto continuous variables, thus describing the dynamics of both electronic and nuclear degrees of freedom by continuous variables. It is shown that the classical evaluation of the mapping formalism results in a self-consistent description of electronic and nuclear degrees of freedom, which treats both types of dynamical variables in a completely equivalent way. The applicability of the approach is thus solely determined by the validity of the classical approximation and does not rest on additional assumptions such as the ad hoc combination of classical and quantum-mechanical theories. The observation of unrestricted flow of zero-point energy in the electronic degrees of freedom indicates the limits of the classical approximation. However, it is shown that this problem can virtually be removed by restricting the classically accessible phase-space. Adopting a multidimensional model of the internal-conversion process in the benzene cation, it is demonstrated that the classical mapping approach is able to account for the branching of classical trajectories in the presence of multiple surface crossings. The classical simulations are found to match the exact quantum-mechanical reference calculations quite accurately. The virtues and limitations of various mixed quantum-classical descriptions are discussed by comparing the mapping approach to the classical-path, the classical electron-analog, and the surface-hopping formulation, respectively.
The hyperpolarizability of trans-butadiene rerevisited
Authors: Rozyczko Piotr B. , Bartlett Rodney J.
Published: July 2002

It has been stated that TDHF results for the dynamic second hyperpolarizability γ∥ of trans-butadiene provide superior results to those obtained with electron correlation [P. Norman et al. J. Chem. Phys. 106, 1827 (1997)]. Using the newly developed correlated frequency dependent EOM-CCSD method we critically address this question. Using three geometries and three basis sets to assess the importance of the right choice of these parameters, and estimating orbital relaxation, triple excitations and vibrational contributions, unlike TDHF, the EOM-CCSD method provides excellent agreement with experiment. The average γ∥ at the experimental geometry in our best basis is 20.3 at ω=0.043 a.u. and 28.0 at ω=0.0656 a.u. compared to experimental values of 20.2±0.1 and 27.7±1.6, respectively.
Generalized gradient approximation to the angle- and system-averaged exchange hole
Authors: Ernzerhof Matthias , Perdew John P.
Published: July 2002

A simple analytic model is proposed for the angle- and system-averaged exchange hole of a many-electron system. The model hole depends on the local density and density gradient. It recovers a nonoscillatory local-spin density (LSD) approximation to the exchange hole for a vanishing density gradient. The model hole reproduces the exchange energy density of the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) for exchange, and facilitates a detailed understanding of the PBE GGA. The hole model is applied to atoms and molecules, and a comparison is made to exact and LSD angle- and system-averaged exchange holes. We find that the GGA hole model significantly improves upon the LSD model. Furthermore, the GGA hole model accurately describes the change in the exchange hole upon the formation of single bonds, but is less accurate for the formation of multiple bonds, where it misses the appearance of a long-range tail.
Analytical energy gradients in second-order Mo/ller–Plesset perturbation theory for extended systems
Authors: Hirata So , Iwata Suehiro
Published: July 2002

The spin-restricted formulas for the analytical gradients of the second-order Mo/ller–Plesset perturbation (MP2) energy are presented within the framework of ab initio crystal orbital theory of infinite one-dimensional lattices (polymers). The coupled perturbed Hartree–Fock equation for polymers is solved iteratively using the atomic-orbital-based algorithms. The MP2 energy and its gradient contributions are evaluated by the disk-based algorithms with the aid of the two-particle density matrix. The analytical-gradient method at the MP2 level, as well as the analytical first- and second-derivative methods at the Hartree–Fock (HF) level, is applied to calculate the equilibrium structures and harmonic vibrational frequencies of all-trans polyacetylene. The deviations of the calculated frequencies from the observed ones for the in-phase C=C stretching modes are reduced by about 70% on going from HF/6-31G to MP2/6-31G theory.
An efficient implementation of time-dependent density-functional theory for the calculation of excitation energies of large molecules
Authors: Stratmann R. Eric , Scuseria Gustavo E. , Frisch Michael J.
Published: July 2002

Time-dependent density-functional (TDDFT) methods are applied within the adiabatic approximation to a series of molecules including C70. Our implementation provides an efficient approach for treating frequency-dependent response properties and electronic excitation spectra of large molecules. We also present a new algorithm for the diagonalization of large non-Hermitian matrices which is needed for hybrid functionals and is also faster than the widely used Davidson algorithm when employed for the Hermitian case appearing in excited energy calculations. Results for a few selected molecules using local, gradient-corrected, and hybrid functionals are discussed. We find that for molecules with low lying excited states TDDFT constitutes a considerable improvement over Hartree–Fock based methods (like the random phase approximation) which require comparable computational effort.
Derivation of coupled cluster excited states response functions and multiphoton transition moments between two excited states as derivatives of variational functionals
Authors: Hättig Christof , Jo/rgensen Poul
Published: July 2002

Based on an analysis of the second residues of the coupled cluster ground state response functions, general expressions are derived for multiphoton transition moments between two excited states and for excited states response functions. The second residues are rewritten as derivatives of variational functionals and computational efficient expressions are obtained in accordance with the 2n+1 and 2n+2 rules. For the excited state response functions secular divergent terms are removed by reformulating the expressions that are identified from the second residues of the ground state response functions as derivatives of an excited state quasienergy Lagrangian. Explicit expressions are given for coupled cluster one-, two- and three-photon transition moments between two excited states and for excited states first-order properties and linear and quadratic response functions.
Spectrum of fast dynamics in glass forming liquids: Does the “knee” exist?
Authors: Gapiński J. , Steffen W. , Patkowski A. , Sokolov A. P. , Kisliuk A. , Buchenau U. , Russina M. , Mezei F. , Schober H.
Published: July 2002

A knee-shaped feature observed earlier in light scattering spectra of Ca0.4K0.3(NO3)1.4 (CKN) below Tc is used as a strong argument in favor of mode-coupling theory of the glass transition (MCT). Our careful measurements reveal no “knee” in the spectra of two glass forming liquids, CKN and ortho-terphenyl. Instead of the knee the spectra show nontrivial broadening and an increase of the intensity with a temperature increase. Both variations are confirmed by neutron scattering measurements on CKN and are neither expected in the asymptotic MCT predictions nor in any other model.
The chemisorption and dissociation of ethylene on Pt{111} from first principles
Authors: Ge Q. , King D. A.
Published: July 2002

First principles density functional theory calculations have been performed for the chemisorption and dissociation of ethylene on Pt{111}, providing a direct comparison with experimental structures and bond energies.
Assessment of the Perdew–Burke–Ernzerhof exchange-correlation functional
Authors: Ernzerhof Matthias , Scuseria Gustavo E.
Published: July 2002

In order to discriminate between approximations to the exchange-correlation energy EXC[ρ↑,ρ↓], we employ the criterion of whether the functional is fitted to a certain experimental data set or if it is constructed to satisfy physical constraints. We present extensive test calculations for atoms and molecules, with the nonempirical local spin-density (LSD) and the Perdew–Burke–Ernzerhof (PBE) functional and compare our results with results obtained with more empirical functionals. For the atomization energies of the G2 set, we find that the PBE functional shows systematic errors larger than those of commonly used empirical functionals. The PBE ionization potentials, electron affinities, and bond lengths are of accuracy similar to those obtained from empirical functionals. Furthermore, a recently proposed hybrid scheme using exact exchange together with PBE exchange and correlation is investigated. For all properties studied here, the PBE hybrid gives an accuracy comparable to the frequently used empirical B3LYP hybrid scheme. Physical principles underlying the PBE and PBE hybrid scheme are examined and the range of their validity is discussed.
Toward reliable density functional methods without adjustable parameters: The PBE0 model
Authors: Adamo Carlo , Barone Vincenzo
Published: July 2002

We present an analysis of the performances of a parameter free density functional model (PBE0) obtained combining the so called PBE generalized gradient functional with a predefined amount of exact exchange. The results obtained for structural, thermodynamic, kinetic and spectroscopic (magnetic, infrared and electronic) properties are satisfactory and not far from those delivered by the most reliable functionals including heavy parameterization. The way in which the functional is derived and the lack of empirical parameters fitted to specific properties make the PBE0 model a widely applicable method for both quantum chemistry and condensed matter physics.
Geometry optimizations in the zero order regular approximation for relativistic effects
Authors: van Lenthe Erik , Ehlers Andreas , Baerends Evert-Jan
Published: July 2002

Analytical expressions are derived for the evaluation of energy gradients in the zeroth order regular approximation (ZORA) to the Dirac equation. The electrostatic shift approximation is used to avoid gauge dependence problems. Comparison is made to the quasirelativistic Pauli method, the limitations of which are highlighted. The structures and first metal-carbonyl bond dissociation energies for the transition metal complexes W(CO)6, Os(CO)5, and Pt(CO)4 are calculated, and basis set effects are investigated.
The spin-forbidden reaction CH(2Π)+N2→HCN+N(4S) revisited. II. Nonadiabatic transition state theory and application
Authors: Cui Qiang , Morokuma Keiji , Bowman Joel M. , Klippenstein Stephen J.
Published: July 2002

Transition state theory is extended straightforwardly to treat nonadiabatic processes and applied to study the rate constant of the spin-forbidden reaction CH(2Π)+N2→HCN+N(4S). A one-dimensional model was set up to calculate the intersystem crossing probability with the distorted wave approximation and using an ab initio value of the spin–orbit coupling. The effect of orthogonal degrees of freedom was then considered by energy convolution with the vibrational frequencies, obtained from ab initio calculations, orthogonal to the crossing seam at the minimum of the seam of crossing (MSX), also obtained from ab initio calculations. An expression for the cumulative reaction probability, N(E), of the reaction was obtained by a straightforward extension of the unified statistical theory, where the MSX was treated as a transition state. The calculated thermal rate constant, k(T), seems to be too low by two orders of magnitude compared to experimental measurements and an empirical transition state study where empirical vibrational frequencies at the MSX are lower by a factor of 2 than those derived here. The disagreement strongly suggests that the current treatment of the multidimensional dynamics needs to be improved. In particular, it may be a poor assumption that spin-forbidden transition takes place with uniform probability on the seam in the case we are considering.