bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulation of biphenyl dissolved in a liquid crystalline solvent: A test of theoretical methods of deriving rotational potentials from partially averaged nuclear spin dipolar couplings
Authors: Palke W. E. , Catalano D. , Celebre G. , Emsley J. W.
Published: July 2002

A molecular dynamics simulation has been carried out of biphenyl dissolved in a solvent comprised of rigid particles interacting with the Gay–Berne potential. The solution is investigated in isotropic, nematic, and smectic phases, and the probability distribution, PLC(φ) obtained, where φ is the angle between the two ring normals. This is compared with Pmol(φ), the distribution calculated for an isolated molecule, and it is found that the positions, φmax of the maxima of the two distributions differ by about 2°. The molecular dynamics trajectory is used to calculate averaged nuclear spin dipolar couplings, Dij, and these are used to test the maximum entropy (ME) and additive potential (AP) theoretical models which have been used previously to obtain the distribution PLC(φ) from dipolar couplings obtained on real solutions of biphenyl in liquid crystalline solvents. It is concluded that the AP method is able to recover the true distribution PLC(φ) from the simulated Dij with good precision at all the temperatures studied, whereas the ME method achieves good precision only when the orientational order is high. The AP method also succeeds in obtaining the correct difference between PLC(φ) and Piso(φ), the distribution for an isotropic phase at the same temperature.
Intrinsic viscosity of small spherical polyelectrolytes: Proof for the intermolecular origin of the polyelectrolyte effect
Authors: Antonietti Markus , Briel Andreas , Förster Stephan
Published: July 2002

Spherical polystyrenesulfonate particles in the size range between 7 nm<R<50 nm are synthesized via crosslinking copolymerization in microemulsion and subsequent sulfonation via polymer reactions. These model polyelectrolytes, when carefully purified, show the qualitative aspects of the polyelectrolyte effect, i.e., large excess viscosities with a strong increase of the intrinsic viscosity with decreasing concentration. A quantitative evaluation of these data on the basis of a modified Hess–Klein relation reveals that the complete dependence on polymer as well as on salt concentration can be fitted with one parameter only, the effective charge number per particle, Zeff. The specific viscosity increases with decreasing particle size and inverse particle density, but no simple explanations for the found relations can be given. Since conformational changes play only a minor role for spherical systems, the comparison of the concentration dependence of the reduced viscosities of linear chains with those of the spherical polyelectrolytes allows for a differentiation between intra- and intermolecular effects. It is qualitatively shown that a major contribution to the polyelectrolyte effect is caused by intermolecular interactions, i.e., the increase of the electrostatic screening length and interparticle-coupling with decreasing concentration. The quantitative description of the concentration and molecular weight dependence of the reduced viscosity of linear polyelectrolytes in salt-free solution reveals that Zeff does apparently not depend on molecular weight, the found molecular weight dependence of the reduced viscosity is due to the increase of the hydrodynamic radius, only. In addition, our modified Hess–Klein model also describes some quantitative features of the viscosity curves, such as the molecular weight dependent shape of the maxima. Deviations between theoretical description and experimental data which become significant for smaller linear polyelectrolytes are attributed to a concentration dependent coil expansion.
Electrical properties of semiconducting (1−x)(90V2O5–10P2O5)+xBaTiO3 glass and glass–ceramics
Authors: Sadhukhan M. , Modak D. K. , Chaudhuri B. K.
Published: July 2002

Homogeneous (1−x)(90V2O5–10P2O5)+x(BaTiO3) glasses (abbreviated as VPBT) are formed with lower concentrations (x) of BaTiO3 (x<0.3). For x>0.3 microcrystallization of BaTiO3 appears during glass formation (corresponding samples are termed as glass–ceramics). All of these oxide glasses show semiconducting behavior. Electrical conductivity data measured between 80 and 450 K follow the adiabatic small polaron hopping conduction mechanism for glasses with lower concentrations (x<0.2), but for higher concentrations of BaTiO3 (x>0.2) the said conduction mechanism is changed to the nonadiabatic regime. Low temperature (T≤150 K) conductivity data of these glass samples can be well fitted with the variable range hopping model. At higher temperatures, Mott’s model of phonon assisted small polaron hopping between nearest neighbors is consistent with the conductivity data. The polaron hopping model of Schnakenberg can predict the conductivity data in the entire temperature range of measurement. However, the percolation model, valid for the similar V2O5–Bi2O3+SrTiO3 glass (denoted by VBBT), is not consistent with the experimental results of VPBT glass samples. Again the frequency dependent ac conductivity data of the VPBT glasses follow overlapping large polaron hopping model, while those of the VBBT glasses satisfy the correlated barrier hopping model.
Inelastic neutron scattering study of low-energy excitations in vapor-deposited glassy propylene
Authors: Yamamuro Osamu , Tsukushi Itaru , Matsuo Takasuke , Takeda Kiyoshi , Kanaya Toshiji , Kaji Keisuke
Published: July 2002

We have developed a novel cryostat in which glassy states of molecular substances are formed by deposition of their vapor on a cold substrate. The glassy samples are subjected to in situ inelastic neutron scattering experiments. By the use of this cryostat, the glassy state of propylene(CH2=CHCH3) prepared at 20 K was examined on an inverted geometry time-of-flight neutron spectrometer in the energy range below 10 meV. The measurement was performed also on the glassy sample annealed at the glass transition temperature (55 K). A broad excitation peak (boson peak) was found at around 3 meV in the S(2θ,ω) spectra of the as-deposited and annealed samples. The absolute densities of vibrational states G(ω) were derived by combining the neutron spectra and the heat capacity data of a similar compound 1-butene (CH2=CHCH2CH3). The number of vibrational modes associated with the low-energy excitation was 1.64 per molecule for the as-deposited sample and 1.34 for the annealed sample. The boson peak energy depends on the molecular weight in a systematic way for the five molecular glasses (propylene, 1-butene, 3-methylpentane, ethylbenzene, ortho-terphenyl) studied so far; the larger the molecular weight, the smaller the boson peak energy. This indicates that the low energy excitations of molecular glasses are associated with the overall motions, probably rotational one of the molecules. A model for the low energy excitations of molecular glasses is proposed based on the results obtained so far.
Long range corrections to thermodynamic properties of inhomogeneous systems with planar interfaces
Authors: Guo Mingxue , Lu Benjamin C.-Y.
Published: July 2002

Expressions for the long range corrections to configurational energy, normal pressure, surface tension, and chemical potential of an inhomogeneous system with two planar interfaces are developed. Applying the Euler forward difference scheme to correlate the density at any height to that of a central spatial element separates the corrections into two parts: one relates directly to the central local density, and another is due to the density differences between the central local value and those around the central spatial element. An analytical expression is obtained for the first part when the Lennard-Jones potential function is adopted in the evaluation. Variations of these properties along the normal direction are illustrated in terms of the equilibrium density profiles obtained from Monte Carlo simulations performed at T*=0.90 and T*=1.15.
Translational and rotational excitation of the CO2(000) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene: Evidence for impulsive collisions accompanied by large energy transfers
Authors: Michaels Chris A. , Lin Zhen , Mullin Amy S. , Tapalian H. Charles , Flynn George W.
Published: July 2002

The relaxation of highly vibrationally excited perfluorobenzene (C6F6) by collisions with CO2 molecules has been investigated over the temperature range 243–364 K using diode laser transient absorption spectroscopy. Particular focus is placed on understanding both the dynamical features and the kinetics of collisions which are accompanied by large energy transfers into the CO2 rotational and translational degrees of freedom. Vibrationally hot perfluorobenzene (Evib=41 822 cm−1) was prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions (J=64–80) of the 0000 ground state of CO2 resulting from collisions with hot perfluorobenzene were probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the distributions of CO2 recoil velocities for individual rotational levels of the 0000 state. In addition, the temperature dependence of the state resolved, absolute rate constants for collisions populating high J states of CO2 was determined. The rotational distributions, distributions of recoil velocities, and quenching rates for production of CO2 high J states (J=64–80) exhibit a very weak temperature dependence. The slight temperature dependence indicates that CO2 molecules which scatter into high J states of the ground vibrationless level originate from rotational levels near the mean of the pre-collision thermal rotational distribution. A gap law model is used to estimate the probability of collisions which are accompanied by large energy transfers yielding values less than 2×10−5/cm−1 for ΔE>2000  cm−1.
Linear scaling computation of the Fock matrix. II. Rigorous bounds on exchange integrals and incremental Fock build
Authors: Schwegler Eric , Challacombe Matt , Head-Gordon Martin
Published: July 2002

A new linear scaling method for computation of the Cartesian Gaussian-based Hartree-Fock exchange matrix is described, which employs a method numerically equivalent to standard direct SCF, and which does not enforce locality of the density matrix. With a previously described method for computing the Coulomb matrix [J. Chem. Phys. 106, 5526 (1997)], linear scaling incremental Fock builds are demonstrated for the first time. Microhartree accuracy and linear scaling are achieved for restricted Hartree-Fock calculations on sequences of water clusters and polyglycine α-helices with the 3-21G and 6-31G basis sets. Eightfold speedups are found relative to our previous method. For systems with a small ionization potential, such as graphitic sheets, the method naturally reverts to the expected quadratic behavior. Also, benchmark 3-21G calculations attaining microhartree accuracy are reported for the P53 tetramerization monomer involving 698 atoms and 3836 basis functions.
Quantum control of I2 in the gas phase and in condensed phase solid Kr matrix
Authors: Bardeen Christopher J. , Che Jianwei , Wilson Kent R. , Yakovlev Vladislav V. , Apkarian V. A. , Martens C. C. , Zadoyan R. , Kohler Bern , Messina Michael
Published: July 2002

We present experimental results and theoretical simulations for an example of quantum control in both gas and condensed phase environments. Specifically, we show that the natural spreading of vibrational wavepackets in anharmonic potentials can be counteracted when the wavepackets are prepared with properly tailored ultrafast light pulses, both for gas phase I2 and for I2 embedded in a cold Kr matrix. We use laser induced fluorescence to probe the evolution of the shaped wavepacket. In the gas phase, at 313 K, we show that molecular rotations play an important role in determining the localization of the prepared superposition. In the simulations, the role of rotations is taken into account using both exact quantum dynamics and nearly classical theory. For the condensed phase, since the dimensionality of the system precludes exact quantum simulations, nearly classical theory is used to model the process and to interpret the data. Both numerical simulations and experimental results indicate that a properly tailored ultrafast light field can create a localized vibrational wavepacket which persists significantly longer than that from a general non-optimal ultrafast light field. The results show that, under suitable conditions, quantum control of vibrational motion is indeed possible in condensed media. Such control of vibrational localization may then provide the basis for controlling the outcome of chemical reactions.
Vibrational anharmonicity and multilevel vibrational dephasing from vibrational echo beats
Authors: Rector K. D. , Kwok A. S. , Ferrante C. , Tokmakoff A. , Rella C. W. , Fayer M. D.
Published: July 2002

Vibrational echo experiments were performed on the IR active CO stretching modes (∼2000 cm−1) of rhodium dicarbonylacetylacetonate [Rh(CO)2acac] and tungsten hexacarbonyl [W(CO)6] in dibutylphthalate and a mutant of myoglobin-CO (H64V-CO) in glycerol–water using ps IR pulses from a free electron laser. The echo decays display pronounced beats and are nonexponential. The beats and nonexponential decays arise because the bandwidths of the laser pulses exceed the vibrational anharmonicities, leading to the excitation and dephasing of a multilevel coherence. From the beat frequencies, the anharmonicities are determined to be 14.7, 13.5, and 25.4 cm−1, for W(CO)6, Rh(CO)2acac, and H64V-CO, respectively. From the components of the nonexponential decays, the vibrational dephasing at very low temperature of both the v=0–1 and v=1–2 transitions are determined. At the lowest temperatures, T2≈2T1, so the v=2 lifetimes are obtained for the three molecules. These are found to be significantly shorter than the v=1 lifetimes. Although the v=1 lifetimes are similar for the three molecules, there is a wide variation in the v=2 lifetimes.
Nonrandom behavior in multicomponent lattice mixtures: Effects of solute size and shape
Authors: Aranovich G. L. , Hocker T. , Wu D. W. , Donohue M. D.
Published: July 2002

A new lattice theory is proposed to describe nonrandom behavior for multicomponent mixtures of monomers, for mixtures of monomers interacting with a polymer, and for mixtures of monomers at a surface. Based on concepts first proposed by Ono and Kondo, this new approach allows one to derive local densities around each species taking into account molecular interactions as well as molecular geometry and lattice structure. This approach can be used to describe a number of very different systems in the framework of a single model. The generalizations presented here are rigorous in that no assumptions beyond those of the original (binary) theory are needed in order to treat multicomponent mixtures of molecules of different sizes and shapes.
Bond energies of MgKr+ and MgXe+ from resonant two-color photoionization thresholds
Authors: Kaup John G. , Breckenridge W. H.
Published: July 2002

The ionization thresholds for the metastable Mg(3s3p 3P0)⋅RG(3Π0−,v′′=0) states (RG=Kr,Xe) have been determined by two-color photoionization via several Mg(3s4s 3S1)⋅RG(3Σ+,v′) intermediate resonance states. From other experimental measurements and ab initio calculations, D0(Mg(3s3p 3P0)⋅RG(3Π0−))=250±80 cm−1 and 500±150 cm−1 for RG=Kr,Xe, respectively, from which D0(MgKr+) and D0(MgXe+) are calculated to be 1891±80 cm−1 and 2848±150 cm−1. Bonding in the MgRG+ ions is discussed and compared to the bonding in the analogous NaRG+ and AlRG+ ions.
The MaxFlux algorithm for calculating variationally optimized reaction paths for conformational transitions in many body systems at finite temperature
Authors: Huo Shuanghong , Straub John E.
Published: July 2002

An algorithm for the calculation of reaction paths between known reactant and product states in systems of low or high dimension is described. The optimal reaction path is defined as the path of maximum flux for a diffusive dynamics assuming isotropic friction. The resulting reaction path is temperature dependent and independent of the magnitude of the friction. Comparison is made with a number of algorithms designed for the calculation of minimum-energy reaction paths. Applications to two model potentials and an extended atom model of a dipeptide are presented. The applications demonstrate the ability of the algorithm to isolate a temperature-dependent optimal reaction path for a high dimensional molecular system.
An application of classical thermodynamics to solid-fluid equilibrium in hard sphere mixtures
Authors: Cottin X. , Monson P. A.
Published: July 2002

We consider the phase diagrams of hard sphere mixtures from the point of view of some well known approximations in classical thermodynamics which have been used to estimate solid-fluid equilibrium. We consider two cases. First, when both the fluid and the solid phases are ideal solutions. This approximation is applied when the particle size ratio is close to unity. Second, where an ideal fluid mixture is in equilibrium with a pure solid. This approximation is applied when substitutionally disordered solid solutions are not stable with respect to phase separated pure solids. We compare the predictions of these simple approximations with those from an accurate molecular theory which we have recently developed for these systems. In particular we show that the phase diagram is most significantly influenced by the nonideality in the solid phase. Solid-fluid phase diagrams for hard sphere mixtures can be accurately predicted by assuming ideal solution behavior in the fluid phase.
Fitting potential-energy surfaces: A search in the function space by directed genetic programming
Authors: Makarov Dmitrii E. , Metiu Horia
Published: July 2002

We propose new procedures by which genetic programming can be used to find the best functional form and the best set of parameters to fit the energies and the energy derivatives provided by ab initio calculations. Our main contribution is a new procedure, which we call a directed genetic search, which is more efficient and more stable than a “traditional” genetic program.
Static and dynamic polarizability calculations for the polyyne series (C2nH2) with extrapolation to the infinite chain
Authors: Dalskov Erik K. , Oddershede Jens , Bishop David M.
Published: July 2002

We have calculated static and dynamic longitudinal polarizabilities, αzz(−ω;ω), for the polyyne series, C2nH2, using both the uncorrelated random phase approximation and the correlated second-order polarization propagator approximation. The calculated polarizabilities are extrapolated to the value for the infinitely long chain using seven different extrapolation techniques. We employ both conventional schemes, such as the fitting of simple polynomials, as well as new schemes, such as the fitting of a Padé approximant, or purely mathematically motivated nonlinear sequence transformations which have not previously been used in connection with this property. For the direct fits, where the number of parameters is the same as the number of points, we find that the most stable and reliable extrapolation schemes are to be found among the latter.
Quantum mechanical calculation of the rate constant for the reaction H+O2→OH+O
Authors: Viel Alexandra , Leforestier Claude , Miller William H.
Published: July 2002

Quantum rate calculations for the H+O2→HO+O combustion reaction, as well as for the reverse reaction, are reported. Using the DMBE IV potential energy surface, the cumulative reaction probability N0(E) has been directly computed for total angular momentum J=0, by means of the Seideman, Manthe and Miller Lanczos-based absorbing boundary condition method [J. Chem. Phys. 96, 4412 (1992); 99, 3411 (1993)]. Special attention has been paid to the definition of the molecular basis set, and to the sensitivity of the results to the absorbing potentials used in the asymptotic regions. The N0(E) results show very good overall agreement with the coupled channel calculations of Pack et al. [J. Chem. Phys. 102, 5998 (1995)], although the highly oscillatory behavior just above threshold renders such a comparison difficult in that energy range. The behavior of the J≠0 cumulative reaction probability has been estimated from calculations using the Jz-conserving approximation for J values in the range 10–70. This allowed us to define which reference geometry should be used in the J-shifting approximation, in order to compute the cumulative reaction probability NJ(E) for any J value. By imposing conservation of the total energy within this approximation, the rate constants are shown to display better agreement with the experimental results.
First principles study of adsorbed Cun (n=1–4) microclusters on MgO(100): Structural and electronic properties
Authors: Musolino V. , Selloni A. , Car R.
Published: July 2002

We present a density functional study of the structural and electronic properties of small Cun (n=1,4) aggregates on defect-free MgO(100). The calculations employ a slab geometry with periodic boundary conditions, supercells with up to 76 atoms, and include full relaxation of the surface layer and of all adsorbed atoms. The preferred adsorption site for a single Cu adatom is on top of an oxygen atom. The adsorption energy and Cu–O distance are ES−A=0.99 eV and dS−A=2.04 Å using the Perdew–Wang gradient corrected exchange correlation functional. The saddle point for surface diffusion is at the “hollow” site, with a diffusion barrier of around 0.45 eV. For the adsorbed copper dimer, two geometries, one parallel and one perpendicular to the surface, are very close in energy. For the adsorbed Cu3, a linear configuration is preferred to the triangular geometry. As for the tetramer, the most stable adsorbed geometry for Cu4 is a rhombus. The adsorption energy per Cu atom decreases with increasing the size of the cluster, while the Cu–Cu cohesive energy increases, rapidly becoming more important than the adsorption energy.
Time-dependent quantum mechanical calculations on H+O2 for total angular momentum J>0
Authors: Meijer Anthony J. H. M. , Goldfield Evelyn M.
Published: July 2002

The H+O2→OH+O reaction has been studied with a time-dependent wave packet method for total angular momentum J=0, 1, 2, and 5, using the Coriolis coupled method [E. M. Goldfield and S. K. Gray, Comp. Phys. Commun. 98, 1 (1996)] on parallel computers. We find that at higher energies the total reaction probability decreases by a factor of 2 in going from a J=0 calculation to a J=1 calculation. The effect for higher J with respect to J=1 is less dramatic. We investigated the decrease in reaction probability for J>0 by examining the different initial conditions with respect to Ω, the projection of J onto the body-fixed z axis for the J>0 calculations. We conclude that the reaction probability is a strong function of Ω. If Ω=0 for J>0, collision geometries are accessible that lead to an enhanced reaction probability.
High-Rydberg fragment formation via core dissociation of superexcited Rydberg molecules
Authors: Pinnaduwage Lal A. , Zhu Yifei
Published: July 2002

Formation of high-Rydberg iodine atoms via core dissociation of ArF-excimer-laser excited methyl iodide high-Rydberg molecules is observed using a time-resolved, mass-analyzed, pulsed field ionization technique. This observation confirms that the Rydberg electron is essentially a spectator in the core dissociation process.
The isocyanopolyynes HC4NC and HC6NC: Microwave spectra and ab initio calculations
Authors: Botschwina P. , Heyl Ä. , Chen W. , McCarthy M. C. , Grabow J.-U. , Travers M. J. , Thaddeus P.
Published: July 2002

Rotational spectra of HC4NC and HC6NC, linear molecules of interest to interstellar cloud chemistry, were recorded by Fourier transform microwave spectroscopy, and the ground state rotational constants were determined to be 1401.18227(7) and 582.5203(1) MHz. Nitrogen quadrupole hyperfine structure could be observed for HC4NC. On the basis of coupled cluster calculations including connected triple substitutions accurate equilibrium structures (uncertainty in bond lengths ca. 0.0005 Å) could be established for both species. The equilibrium dipole moments, predicted to be −3.25 and −3.49 D for HC4NC and HC6NC, respectively, exhibit large correlation effects of 30% and 33%.
Dynamical Monte Carlo study of equilibrium polymers: Static properties
Authors: Wittmer J. P. , Milchev A. , Cates M. E.
Published: July 2002

We report results of extensive dynamical Monte Carlo investigations on self-assembled equilibrium polymers (EP) without loops in good solvent. (This is thought to provide a good model of giant surfactant micelles.) Using a novel algorithm we are able to describe efficiently both static and dynamic properties of systems in which the mean chain length 〈L〉 is effectively comparable to that of laboratory experiments (up to 5000 monomers, even at high polymer densities). We sample up to scission energies of E/kBT=15 over nearly three orders of magnitude in monomer density φ, and present a detailed crossover study ranging from swollen EP chains in the dilute regime up to dense molten systems. Confirming recent theoretical predictions, the mean-chain length is found to scale as 〈L〉∝φαexp(δE) where the exponents approach αd=δd=1/(1+γ)≈0.46 and αs=1/2[1+(γ−1)/(νd−1)]≈0.6,δs=1/2 in the dilute and semidilute limits respectively. The chain length distribution is qualitatively well described in the dilute limit by the Schulz-Zimm distribution p(s)≈sγ−1 exp(−s) where the scaling variable is s=γL/〈L〉. The very large size of these simulations allows also an accurate determination of the self-avoiding walk susceptibility exponent γ≈1.165±0.01. As chains overlap they enter the semidilute regime where the distribution becomes a pure exponential p(s)=exp(−s) with the scaling variable now s=L/〈L〉. In addition to the above results we measure the specific heat per monomer cv. We show that the average size of the micelles, as measured by the end-to-end distance and the radius of gyration, follows a crossover scaling that is, within numerical accuracy, identical to that of conventional monodisperse quenched polymers. Finite-size effects are discussed in detail.
A study of Gen− and Gen (n=2–6) using B3LYP-DFT and CCSD(T) methods: The structures and electron affinities of small germanium clusters
Authors: Archibong Edet F. , St-Amant Alain
Published: July 2002

The structures of the anionic germanium Gen− clusters and the corresponding neutral Gen clusters (n=2–6) have been investigated using B3LYP-DFT and CCSD(T) methods. The 6-311+G(3df ) basis set is employed for the dimers and trimers, while the smaller 6-311+G(d) basis set is used for clusters with n>3. The most stable structures for the germanium cluster anions Ge3−, Ge4−, Ge5−, and Ge6− are found to be C2v(2A1), D2h(2B2g), D3h(2A2″), and D4h(2A2u), respectively. In the case of Ge2−, our calculations show that the low lying Πu2 and Σg+2 states are within 1 kcal/mol of each other and both states are candidates for the ground state of the anion. The adiabatic electron affinities calculated for the Gen clusters with n=2,3,4,6 are within 0.1 eV of the corresponding experimental values. Furthermore, the adiabatic excitation energies computed at the CCSD(T) level for the low lying states of Ge3 and Ge4 compare quite well with the assignments of the bands observed in the photoelectron spectra of Ge3− and Ge4− by Burton, Xu, Arnold, and Neumark [J. Chem. Phys. 104, 2757 (1996)].
Comment on “On the importance of the fragment relaxation energy terms in the estimation of the basis set superposition error correction to the intermolecular interaction energy” [J. Chem. Phys. 104, 8821 (1996)]
Authors: Szalewicz Krzysztof , Jeziorski Bogumil
Published: July 2002

In a recent communication [J. Chem. Phys. 104, 88821 (1996)], Xantheas argued that the omission of the fragment relaxation energy terms introduces errors in the estimation of the basis set superposition error (BSSE) correction to the intermolecular interaction energy. It is shown that this conclusion is unwarranted and results from an application of an inappropriate definition of the interaction energy.
Homogeneous and mixed UF6 clusters with Ar: Calculations of structures and vibrational spectra
Authors: Beu T. A. , Onoe J. , Takeuchi K.
Published: July 2002

A recently developed site–site intermolecular potential for UF6, featuring exchange, dispersion, and electrostatic terms, is used to calculate minimum energy structures of homogeneous UF6 clusters up to the decamer. The structures of mixed (UF6)2–Arn clusters are also calculated by adding appropriate interaction terms. The IR spectra corresponding to the determined cluster structures in the region of the ν3 vibrational mode of the monomer (at 627.724 cm−1) are calculated using a second-order line shift formalism, treating the anharmonic intramolecular force field and the intermolecular potential as a perturbation. The leading interaction mechanism responsible for the line shifts of the ν3 mode is found to be the electrostatic one (implicitly the resonant dipole–dipole coupling). The theoretical spectra are shown to satisfactorily describe the peaks around 623, 632, and 640 cm−1 found in the recently measured Fourier transform IR spectra in a continuous supersonic Laval nozzle flow and attributed to the clusters formed by UF6.
An experimental and theoretical study of the vibrationally mediated photodissociation of hydroxylamine
Authors: Luckhaus David , Scott Jacqueline L. , Fleming Crim F.
Published: July 2002

We present a detailed investigation of the photodissociation of hydroxylamine following direct single-photon and vibrationally mediated two-photon excitation below 42 000 cm−1. In all cases the lowest dissociation channel [NH2(X̃ 2B1)+OH(X̃ 2Π)] dominates. Single-photon dissociation at 240 nm releases most of the excess energy (20 550 cm−1) into relative translation (53%) and NH2 internal energy (40%, mostly vibrational). OH carries little internal energy (7%), most of it in the form of rotational excitation. Torsional excitation during the dissociation step leads to rotational alignment of the OH fragments and a preferential population of the Π(A″) component of the lambda doublet. Both are lost after isoenergetic two-photon excitation via O–H stretching overtones of NH2OH, also leading to higher internal excitation of the NH2 fragments (∼50%) at the expense of relative translation. At lower total excitation energies the relative translation takes up an increasing fraction of the total excess energy (⩾80% at 5820 cm−1 of excess energy). The results are discussed in terms of ab initio calculations using complete active space second-order perturbation theory with augmented triple-ζ basis sets for the lowest excited singlet states. One- and two-dimensional potential functions explain the OH product state distributions observed in different experiments in terms of the geometry relaxation of NH2OH upon electronic excitation. Crossing between the lowest excitated A′ and A″ singlet states in the Franck–Condon region leads to a barrier of ∼0.5 eV to dissociation in S1, which dominates the photodissociation dynamics.
Classical trajectory calculations of collision energy dependence of total and partial Penning ionization cross sections for He*(23S)+N2→He+N2++e−
Authors: Ogawa Tetsuji , Ohno Koichi
Published: July 2002

Classical trajectory calculations are performed for the Penning ionization system He*(23S)+N2→He(1 1S)+N2+(X̃ 2Σg+,Ã 2Πu,B̃ 2Σu+)+e−. Anisotropic model potentials of He*(2 3S)+N2 are adapted to reproduce collision-energy dependence of ionic-state-resolved ionization cross sections observed by two-dimensional Penning ionization electron spectroscopy. Results of trajectory calculations are compared with those of ab initio potential surfaces obtained by Ishida. Opacity functions are demonstrated to be strongly dependent on ionic states, collision energy, and anisotropic varieties of trajectories.
Adhesion energy of Cu atoms on the MgO(001) surface
Authors: Lopez Nuria , Illas Francesc , Rösch Notker , Pacchioni Gianfranco
Published: July 2002

We have studied theoretically the interaction of an isolated Cu atom adsorbed on the oxygen sites of the regular MgO (001) surface with the aim of providing an accurate estimate of the adhesion energy. We performed cluster model calculations using a variety of first principles quantum-chemical approaches; local (spin) density approximation [L(S)DA], density functionals that include density gradient corrections (GC-DF), hybrid density functional (B3LYP), and explicitly correlated wave functions. Various combinations of exchange-correlation functionals and different methods to introduce electron correlation, including MP2 and CCSD(T), have been considered. The dependence of the results on cluster and basis set size has been carefully checked. We found that the hybrid DF method, B3LYP, and explicitly correlated wave functions, CCSD(T), give similar results with an adhesion energy of about 0.40±0.05 eV; GC-DF methods suggest a higher binding energy of 0.6 eV. Therefore, Cu atoms can be considered to bind to oxygen centers of the ideal MgO (001) surface in a relatively weak chemical fashion, in broad agreement with the low sticking coefficient measured experimentally.
Trihydrogen cation solvated by rare gas atoms: RgnH3+
Authors: Beyer Martin , Savchenko Elena V. , Niedner-Schatteburg Gereon , Bondybey Vladimir E.
Published: July 2002

The triatomic hydrogen cation solvated by rare gases, RgnH3+, n=1–5, Rg=He, Ne, Ar, Kr, Xe, is investigated by density functional theory. The results indicate that while the first solvent atom distorts and destabilizes the H3+ center, the ion is restabilized by additional solvation. For the n=3 species, the symmetric D3h structure is the global minimum for all rare gases except Xe; the n=4,5 solvent atoms are placed on the C3 axis of the molecule. The computed potential energy surface of the isomerization reaction RgnH3+→H2Rg2H+Rgn−2, n=2–4, provides insight into the possible H3+ formation and destruction mechanisms in rare gas matrices. As expected, solid neon is the most suitable medium for H3+ studies, with its stabilization becoming progressively more difficult in the heavier rare gas solids.
Kinetic energy conserving integrators for Gaussian thermostatted SLLOD
Authors: Zhang Fei , Searles Debra J. , Evans Denis J. , den Toom Hansen Jan S. , Isbister Dennis J.
Published: July 2002

A new integration scheme is developed for nonequilibrium molecular dynamics simulations where the temperature is constrained by a Gaussian thermostat. The utility of the scheme is demonstrated by its application to the SLLOD algorithm which is the standard nonequilibrium molecular dynamics algorithm for studying shear flow. Unlike conventional integrators, the new integrators are constructed using operator-splitting techniques to ensure stability and that little or no drift in the kinetic energy occurs. Moreover, they require minimum computer memory and are straightforward to program. Numerical experiments show that the efficiency and stability of the new integrators compare favorably with conventional integrators such as the Runge–Kutta and Gear predictor–corrector methods.
Constraints upon natural spin orbital functionals imposed by properties of a homogeneous electron gas
Authors: Cioslowski Jerzy , Pernal Katarzyna
Published: July 2002

The expression Vee[Γ1]=(1/2)∑p≠q[npnqJpq−Ω(np,nq)Kpq], where {np} are the occupation numbers of natural spin orbitals, and {Jpq} and {Kpq} are the corresponding Coulomb and exchange integrals, respectively, generalizes both the Hartree–Fock approximation for the electron–electron repulsion energy Vee and the recently introduced Goedecker–Umrigar (GU) functional. Stringent constraints upon the form of the scaling function Ω(x,y) are imposed by the properties of a homogeneous electron gas. The stability and N-representability of the 1-matrix demand that 2/3<β<4/3 for any homogeneous Ω(x,y) of degree β [i.e., Ω(λx,λy)≡λβΩ(x,y)]. In addition, the Lieb–Oxford bound for Vee asserts that β⩾βcrit, where βcrit≈1.1130, for Ω(x,y)≡(xy)β/2. The GU functional, which corresponds to β=1, does not give rise to admissible solutions of the Euler equation describing a spin-unpolarized homogeneous electron gas of any density. Inequalities valid for more general forms of Ω(x,y) are also derived.
Test of a simple and flexible molecule model for α-, β-, and γ-S8 crystals
Authors: Pastorino C. , Gamba Z.
Published: July 2002

S 8 is the most stable compound of elemental sulfur in solid and liquid phases, at ambient pressure and below 400 K. Three crystalline phases of S8 have been clearly identified in this range of thermodynamic parameters, although no calculation of its phase diagram has been performed yet. α- and γ-S8 are orientationally ordered crystals while β-S8 is measured as orientationally disordered. In this paper we analyze the phase diagram of S8 crystals, as given by a simple and flexible molecule model, via a series of molecular dynamics (MD) simulations. The calculations are performed in the constant pressure–constant temperature ensemble, using an algorithm that is able to reproduce structural phase transitions.
Observation of the singlet–triplet pair of the 4p Rydberg state and assignment of the Rydberg series of SO2
Authors: Xue Bing , Chen Yang , Dai Hai-Lung
Published: July 2002

The triplet g̃ Rydberg state of SO2 with an origin at 78 245.8 cm−1, 526 cm−1 below the previously detected singlet G̃ state origin, has been observed in a two-color resonantly enhanced multiphoton ionization experiment. The bending vibrational levels (v=0–5 with intervals of 400±10 cm−1) has been identified and reported. Based on the symmetry, bending frequencies, quantum defects, and term values, this G̃ state singlet–triplet pair was assigned to the sulfur 4p state belonging to the series converging to the SO2+ X̃ 2A1 state. The same procedure also allows the reassignment of the Rydberg state previously observed in the 111.5–115 nm region to the sulfur 4p orbital (converging to the SO2+ Ã 2A2 state) and the one in the 106.3–110.4 nm region to the 5p orbital (converging to the SO2+ X̃ 2A1 state). The spectral patterns also indicate that the G̃ states have a detectable increase in the dissociation rate above the v2=2 level. The triplet state near the 193 nm region is dissociative with a homogeneous width of ∼13 cm−1.
Crystal field influence on the 8S7/2 ground state splitting of Bk4+ in CeF4
Authors: Brito H. F. , Liu G. K.
Published: July 2002

The one-particle models of crystal-field theory provide a qualitative interpretation for the observed ground state splitting of four Kramers doublets of the 8S7/2 of Bk4+ doped into CeF4. A set of nine nonzero (Bqk) parameters corresponding a C2v point symmetry provide a very good correlation between the experimental data and simulated energy level schemes within a rms deviation of 13.7 cm−1. The calculated and experimental energy values have the same order-of-magnitude for the ground state and excited components. The total ground state splitting of the S-state ions of f-elements such as Bk4+ in CeF4 is larger when compared with Cm3+:LaCl3 and Gd3+:La(C2H5SO4)3⋅9H2O ions. The so-called crystal-field strength parameter, Nv, increases as a function of the increasing maximum splitting of the ground state due to a decrease in the participation of the pure 8S7/2 in the final composition of the ground state eigenvector.
Scalar field propagators in anti-de Sitter space-time
Authors: Dullemond C. , van Beveren E.
Published: July 2002

Simple expressions are found for the retarded, advanced, and Feynman propagators, as well as several other auxiliary invariant functions, for scalar fields of arbitrary mass in anti-de Sitter space-time.
Linear independence of renormalization counterterms in curved space-times of arbitrary dimensionality
Authors: Jack Ian , Parker Leonard
Published: July 2002

The counterterms in the Lagrangian of a renormalizable quantum field theory that involve the Riemann curvature tensor are considered. It is proved that for six or fewer dimensions the counterterms not containing derivatives of the Riemann tensor are linearly independent. It is shown that there appears to be a maximum space-time dimension for which identities can exist among invariants involving the product of n Riemann tensors (without derivatives acting on them.) Space-times that would require these products as counterterms for a renormalizable theory have a dimensionality which is one higher than this maximum dimension. This makes it plausible that the required set of counterterms not involving derivatives of the Riemann tensor is linearly independent in arbitrary dimensions. In the Appendix, a relation cubic in the Weyl tensor, which relates invariants cubic in the Riemann tensor in five dimensions, is proved.
SL(2,C), SU(2), and Chebyshev polynomials
Authors: Bacry Henri
Published: July 2002

When expressed in terms of the trace, the characters of SU(2) are known to be related with the Chebyshev polynomials of second kind. It is shown that those of the first kind also play a fundamental role. If A∈SU(2) and t=Tr A, then fn(t)=Tr(An), fn (2 cos θ)=sin nθ/sin θ, ln(t) =Tr(An), ln (2 cos θ)=2 cos nθ, where An denotes the representative of A in the irrep of dimension n. Other polynomials related with them are of interest. They are (i) the ‘‘primordial’’ polynomials Pn (every fn or ln can be expressed in a unique way in terms of Pd, where d is a divisor of n), (ii) the ‘‘factorial’’ polynomials fn!=f1 f2⋅⋅⋅fn which occur in a natural way in the representations, (iii) the gn polynomials appearing in the generating functions of powers of fn.
Invariants for the time-dependent harmonic oscillator
Authors: Colegrave R. K. , Mannan M. A.
Published: July 2002

A systematic method is presented for the construction of invariants for the damped oscillator under the action of a driving force and for the N-dimensional isotropic or anisotropic oscillator. Invariants for time-dependent oscillators are obtained by canonical transformation. The treatment holds in both classical and quantum mechanics.
Colliding gravitational plane waves with noncollinear polarization. III
Authors: Ernst Frederick J. , García D. Alberto , Hauser Isidore
Published: July 2002

The Hauser–Ernst homogeneous Hilbert problem (HHP) approach, formerly used in connection with the derivation of stationary axisymmetric fields, is here adapted to the derivation of colliding gravitational plane wave solutions of the vacuum Einstein equations. Proceeding from Kasner metrics, and using a double-Harrison transformation, the HHP approach yields a three-parameter generalization of a two-parameter family of colliding wave solutions found recently by Ferrari, Ibañez, and Bruni. In the present paper we provide the details concerning the derivation of this new family of solutions, and we set the stage for future applications of the HHP approach in connection with gravitational waves.
Rarita–Schwinger fields in algebraically special vacuum space-times
Authors: Torres del Castillo G. F.
Published: July 2002

It is shown that previous results concerning test massless fields on algebraically special vacuum backgrounds can be extended to the case of massless spin- (3)/(2) Rarita–Schwinger fields. A decoupled equation is derived from the Rarita–Schwinger equation on an algebraically special vacuum space-time and it is shown that all the components of the field can be obtained from a scalar potential that obeys a wavelike equation. In the case of type D metrics, identities of the Teukolsky–Starobinsky-type are obtained. Some relations induced by Killing spinors are also included.
Tight frames of compactly supported affine wavelets
Authors: Lawton Wayne M.
Published: July 2002

This paper extends the class of orthonormal bases of compactly supported wavelets recently constructed by Daubechies [Commun. Pure Appl. Math. 41, 909 (1988)]. For each integer N≥1, a family of wavelet functions ψ having support [0,2N−1] is constructed such that {ψjk(x)=2j/2ψ(2jx−k) kj,k∈Z} is a tight frame of L2(R), i.e., for every f∈L2(R), f=c∑jk 〈ψjk‖f〉ψjk for some c>0. This family is parametrized by an algebraic subset VN of R4N. Furthermore, for N≥2, a proper algebraic subset WN of VN is specified such that all points in VN outside of WN yield orthonormal bases. The relationship between these tight frames and the theory of group representations and coherent states is discussed.
Relationship between S
N and U(n) isoscalar factors and higher-order U(n) invariants
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

General relationships expressing U(n) and SN coupling and transformation isoscalar factors in terms of U(n) Racah and 9λ coefficients are derived. The absolute values of U(n) Racah coefficients involving at most k-column irreducible representations are shown to be identical with SU(k) Racah coefficients. In particular, an explicit relationship is established between the U(n) and SU(2) approaches to the many-electron correlation problem.
Necessary and sufficient conditions for constructing orthonormal wavelet bases
Authors: Lawton Wayne M.
Published: July 2002

This paper proves a previous conjecture of the author characterizing sequences h∈l2(Z) that yield orthonormal wavelet bases of L2(R) in terms of the multiplicity of the eigenvalue 1 of an operator associated to h. The proof utilizes a result of Cohen characterizing these sequences in terms of the real zeros of their Fourier transforms. The mapping from sequences to wavelets is shown to define a continuous mapping from a subset of l2(Z) into L2(R). Related conjectures are discussed.
Multiphase interchange inequalities
Authors: Nesi V.
Published: July 2002

Phase interchange inequalities have been studied since the early work of Keller [J. Math. Phys. 5, 548 (1964)]. They constrain the effective conductivity of composite materials which are obtained from each other, for fixed configuration, by interchanging the position of the phases. Optimal results in this direction for the case of a two-phase composite are due to Keller in spatial dimension d=2 and to Avellaneda et al. [J. Appl. Phys. 63, 4989 (1988)] in dimension d=3. In this paper new inequalities in spatial dimension d=2 and d=3, which are valid when an arbitrary number of phases is present, are proven. When specialized to two-phase composites, they agree with those of Keller in d=2 and of Avellaneda et al. in d=3, respectively.
Concerning the gauge invariance and the apparent nonlocality of the real form of Schrödinger’s equation
Authors: Chen Robert L. W.
Published: July 2002

This paper will amend an earlier paper that derives the real form of Schrödinger’s equation. It will remove certain difficulties that are: (1) the problem of gauge invariance and (2) the apparent nonlocality in the inverse Hamiltonian operator. The mathematical tools used here are fairly simple: method for the real dispersive waves of Sommerfeld and Brillouin and the Green’s theorem.
Hamiltonian formulation of a theory with constraints
Authors: Kulshreshtha Usha
Published: July 2002

The Hamiltonian formulation of a constrained dynamical system with four degrees of freedom, the quantization of which has recently been discussed by Hojman and Shepley [J. Math. Phys. 32, 142 (1991)], is considered. The theory possesses four primary constraints which are all second class. The system is quantized using the Dirac’s procedure. Next the theory is reduced to the three-dimensional one by setting q4=k=const. It is found that the theory now possesses only three primary constraints which are all first class; and consequently the theory now becomes a gauge theory. This three-dimensional theory is then quantized using the Dirac’s procedure again, under different gauge-fixing conditions.