bluuebunny/crossref_metadata_2025 huggingface dataset:
Annealing processes of vacancy-type defects in electron-irradiated and as-grown 6H-SiC studied by positron lifetime spectroscopy
Authors: Kawasuso Atsuo , Itoh Hisayoshi , Okada Sohei , Okumura Hajime
Published: July 2002

Annealing processes of vacancy-type defects in 3 MeV electron-irradiated and as-grown 6H-SiC have been studied by positron lifetime spectroscopy. Vacancy-type defects giving rise to a positron lifetime of 183 ps were detected in as-grown n-type specimens. They were found to be annealed at around 1400 °C and were related to silicon vacancies, possibly complexes of silicon vacancies and nitrogen atoms. Defects related to carbon vacancies, silicon vacancies, and divacancies were found to be created by electron irradiation. The defects related to carbon vacancies and divacancies were found to be annealed up to 500 °C. The defects related to silicon vacancies were found to be annealed at around 750 and 1400 °C. The former annealing stage was inferred to be due to migration of silicon vacancies to internal sinks or nitrogen atoms to form complexes of silicon vacancies and nitrogen atoms. The latter annealing stage was explained as due to annihilations of the complexes as well as the case of as-grown specimens.
Effective thermal conductivity of particulate composites with interfacial thermal resistance
Authors: Nan Ce-Wen , Birringer R. , Clarke David R. , Gleiter H.
Published: July 2002

A methodology is introduced for predicting the effective thermal conductivity of arbitrary particulate composites with interfacial thermal resistance in terms of an effective medium approach combined with the essential concept of Kapitza thermal contact resistance. Results of the present model are compared to existing models and available experimental results. The proposed approach rediscovers the existing theoretical results for simple limiting cases. The comparisons between the predicted and experimental results of particulate diamond reinforced ZnS matrix and cordierite matrix composites and the particulate SiC reinforced Al matrix composite show good agreement. Numerical calculations of these different sets of composites show very interesting predictions concerning the effects of the particle shape and size and the interfacial thermal resistance.
Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals
Authors: Park Seung-Eek , Shrout Thomas R.
Published: July 2002

The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3, morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s)>2500 pC/N and subsequent strain levels up to >0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for 〈001〉 oriented rhombohedral crystals, although 〈111〉 is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to an E-field induced phase transformation. High electromechanical coupling (k33)>90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.
Ellipsometric determination of optical constants for silicon and thermally grown silicon dioxide via a multi-sample, multi-wavelength, multi-angle investigation
Authors: Herzinger C. M. , Johs B. , McGahan W. A. , Woollam J. A. , Paulson W.
Published: July 2002

Optical constant spectra for silicon and thermally grown silicon dioxide have been simultaneously determined using variable angle of incidence spectroscopic ellipsometry from 0.75 to 6.5 eV. Spectroscopic ellipsometric data sets acquired at multiple angles of incidence from seven samples with oxide thicknesses from 2 to 350 nm were analyzed using a self-contained multi-sample technique to obtain Kramers–Kronig consistent optical constant spectra. The investigation used a systematic approach utilizing optical models of increasing complexity in order to investigate the need for fitting the thermal SiO2 optical constants and including an interface layer between the silicon and SiO2 in modeling the data. A detailed study was made of parameter correlation effects involving the optical constants used for the interface layer. The resulting thermal silicon dioxide optical constants were shown to be independent of the precise substrate model used, and were found to be approximately 0.4% higher in index than published values for bulk glasseous SiO2. The resulting silicon optical constants are comparable to previous ellipsometric measurements in the regions of overlap, and are in agreement with long wavelength prism measurements and transmission measurements near the band gap.
Enhanced mechanical hardness in epitaxial nonisostructural Mo/NbN and W/NbN superlattices
Authors: Madan A. , Wang Yun-yu , Barnett S. A. , Engström C. , Ljungcrantz H. , Hultman L. , Grimsditch M.
Published: July 2002

Epitaxial Mo/NbN and W/NbN superlattices with modulation wavelengths Λ ranging from 1.3 to 120 nm were grown on MgO (001) substrates by dc reactive magnetron sputtering in Ar/N2 mixtures. The superlattices were shown to be epitaxial with nearly planar layers using high- and low-angle x-ray diffraction and transmission electron microscopy. Computer simulation fits of the x-ray data indicated that interface widths were ⩽0.3 nm. The epitaxial relationship between the layers was (001)metal∥(001)NbN and [110]metal∥[100]NbN. The nanoindenter microhardness values from W/NbN and Mo/NbN superlattices with 50 vol % metal were nearly identical. The largest hardnesses were 30 GPa, observed at superlattice periods Λ=2–3 nm, compared to rule-of-mixtures values of 10 GPa. The hardness decreased with increasing Λ above ≈3 nm, following the dependences H=10.3+26.70Λ−0.38 GPa for Mo/NbN and H=12.88+22.1Λ−0.3 GPa for W/NbN. Hardness versus metal volume fraction with Λ≈5 nm showed a flat-topped dependence. Brillouin scattering results for Mo/NbN superlattices showed a minor elastic anamoly at small Λ. The hardness results are compared with theories for strengthening of multilayers.
InGaAs/GaAs quantum dots on (111)B GaAs substrates
Authors: Tsai F.-Y. , Lee C. P.
Published: July 2002

InGaAs/GaAs quantum dots have been grown directly on (111)B GaAs substrates using molecular beam epitaxy. The island growth mode, which causes quantum dot formation, is due to the 2×2 surface reconstruction as the growth temperature is lowered. It is not related to the Stranski–Krastanov growth mechanism because the nominal thickness of InGaAs used is far below the critical thickness and no wetting layer is found. Atomic force microscope images indicate island formation with the growth temperature below 480 °C. The photoluminescence (PL) spectra of quantum dots show large peak shifts compared with the reference quantum well of the same nominal thickness of InGaAs. The PL emission from the quantum dots is strong and the peaks are narrow. A linewidth as low as 8.5 meV at 8 K has been measured, indicating excellent quality.
Magnetization of small arrays of interacting single-domain particles
Authors: Grundler D. , Meier G. , Broocks K.-B. , Heyn Ch. , Heitmann D.
Published: July 2002

We have prepared tailored small arrays of single-domain Ni nanomagnets exhibiting an easy-axis perpendicular to the substrate surface. Using ballistic Hall micromagnetometry we probe the magnetization under different tilt angles. For an array of 13 nanomagnets with 700 nm spacing, we observe distinct plateau values in the hysteresis loop indicating the stepwise switching of these particles. The observed hierarchy of reversal comes close to a model on the basis of the magnetostatic interparticle interaction.
Thickness dependence of structural and electrical properties in epitaxial lead zirconate titanate films
Authors: Nagarajan V. , Jenkins I. G. , Alpay S. P. , Li H. , Aggarwal S. , Salamanca-Riba L. , Roytburd A. L. , Ramesh R.
Published: July 2002

We have studied the effect of misfit strain on the microstructure and properties of ferroelectric lead zirconate titanate thin films. We have changed the misfit strain by varying the film thickness and studied the thickness effect on the domain formation of epitaxial PbZr0.2Ti0.8O3 (PZT) films grown by pulsed laser deposition on (001) LaAlO3 substrates with La0.5Sr0.5CoO3 (LSCO) electrodes. The nominal thickness of the PZT films was varied from 60 to 400 nm with the LSCO electrode thickness kept constant at 50 nm. X-ray diffraction experiments show that the films relax via the formation of a domains, the fraction of which increase with the ferroelectric film thickness. The c-axis lattice constant of PZT films calculated from the 002 reflection decreases with increasing film thickness and approaches the bulk value of ∼0.413 nm in the films thicker than 300 nm. Cross-sectional transmission electron microscopy images reveal that the a-domain fraction and period increase with increasing film thickness. The relaxation of misfit strain in the film is accompanied by systematic changes in the polarization properties, as well as the switching fields, quantified by the coercive field and the activation field.
Piezoelectric properties of zirconium-doped barium titanate single crystals grown by templated grain growth
Authors: Rehrig Paul W. , Park Seung-Eek , Trolier-McKinstry Susan , Messing Gary L. , Jones Beth , Shrout Thomas R.
Published: July 2002

Single crystals of Ba(ZrxTi1−x)O3 were grown by templated grain growth (TGG). Millimeter size single crystals of Ba(ZrxTi1−x)O3 were produced by heating a BaTiO3 crystal in contact with a sintered polycrystalline matrix of 4.5, 5.0, or 8.5 mol % Zr-doped barium titanate for 30 h at 1350 °C. To facilitate boundary migration, the ceramic compact was made 3 mol % TiO2 excess. The 4.5 and 5.0 mol % Zr-doped crystals were orthorhombic at room temperature, and for a pseudocubic (001) orientation, they showed remanent polarizations of 13 μC/cm2 and a high field d33 of 340–355 pC/N. The 8.5 mol % Zr-doped crystal [again oriented along the pseudocubic (001)] was rhombohedral at room temperature with a remanent polarization of 10 μC/cm2. A k33 value of 0.74 from resonance measurements was observed for the 4.5 mol % Zr-doped crystal.
Electroplated thick permanent magnet arrays with controlled direction of magnetization for MEMS application
Authors: Cho Hyoung J. , Bhansali Shekhar , Ahn Chong H.
Published: July 2002

We have successfully developed a technique to electroplate thick CoNiMnP-based permanent magnet arrays with controlled direction of magnetization and improved magnetic properties by applying external magnetic fields during electroplating. The magnet arrays with individual magnet shapes and sizes were fabricated on a Si substrate using micromachining and electroplating techniques. The magnetic properties of these magnets have been characterized with the vibrating sample magnetometer. The optimized processing conditions with external magnetic fields during electroplating have improved the coercivity and the retentivity of the magnets by more than 200% and 350%, respectively, comparing with those without external magnetic fields. This paper describes the process for fabricating the magnets and the effect of external magnetic fields in controlling and improving the properties of the magnets. In addition to the capability of full scale integration, the high vertical anisotropy of thick magnet arrays can be used for many MEMS device applications.
Thermoelectric and transport properties of β-Ag2Se compounds
Authors: Ferhat Marhoun , Nagao Jiro
Published: July 2002

The Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K. The results showed maxima in both Hall coefficient and the electrical resistivity just below the onset of the intrinsic conduction range. This anomaly was qualitatively explained by the deviation of the Coulomb scattering from the usual assumption of independence due to the degenerate nature of the samples. The estimated energy gap for different samples of about 160 meV seems to confirm the existence of the second low temperature phase β2. This second phase is a probable reason for the relatively high thermoelectric figure of merit observed.
Nuclear-motion corrections to Born–Oppenheimer barrier heights for chemical reactions
Authors: Garrett Bruce C. , Truhlar Donald G.
Published: July 2002

We use diatomics-in-molecules theory to estimate the nuclear-motion contributions to atom–diatom potential energy surfaces for reactive systems, with special emphasis on the saddle point region. We examine the reactions of H, O, F, and Cl with H2 and H with Cl2. Nuclear-motion corrections at the saddle point are in the range 0.007–0.07 kcal/mol for cases with one or two hydrogens and classical barrier heights in the range 0.7–3 kcal/mol and are about 0.2 kcal/mol for cases with two hydrogens and classical barrier heights in the range 8–12 kcal/mol. For F+H2 with parameters such that the predicted nuclear-motion correction is 0.03 kcal/mol at the saddle point, the correction is as large as 0.2 kcal/mol elsewhere on the surface. Isotopic substitution of D for H changes the classical barrier height by 0.003–0.1 kcal/mol for the cases studied.
Natural localized molecular orbitals
Authors: Reed Alan E. , Weinhold Frank
Published: July 2002

The method of natural localized molecular orbitals (NLMOs) is presented as a novel and efficient technique for obtaining LMOs for SCF and CI wave functions. It is an extension of the previously developed natural atomic orbital (NAO) and natural bond orbital (NBO) methods, and uses only the information contained in the one-particle density matrix. Results are presented for methane and cytosine to indicate that NLMOs closely resemble LMOs obtained by the Boys and Edmiston–Ruedenberg methods, with the exception that the NLMO procedure automatically preserves the MO σ–π separation in planar molecules. The computation time is modest, generally only a small fraction of the SCF computation time. In addition, the derivation of NLMOs from NBOs gives direct insight into the nature of the LMO ‘‘delocalization tails,’’ thus enhancing the role of LMOs as a bridge between chemical intuition and molecular wave functions.
Origin of the scattering peak in microemulsions
Authors: Teubner M. , Strey R.
Published: July 2002

From a Landau theory the static scattering intensity distribution I(q) of microemulsions is obtained. As essential ingredient we have included a negative gradient term in the free energy expression. The form of I(q)∼(a2+c1q2+c2q4)−1 yields for a2>0, c1<0, and c2>0 a single broad scattering peak and a q−4 decay at large q, both properties experimentally observed for a variety of microemulsions containing comparable amounts of water and oil. The peak originates from the modulation in the corresponding space correlation function given by γ(r)=(d/2πr)⋅e−r/ξ⋅sin(2πr/d). It is shown that the scattering intensity relation describes experimental literature data remarkably well, using only three fit parameters.
TheoreticalK-shell ionization spectra of N2 and CO by a fourth-order Green’s function method
Authors: Angonoa G. , Walter O. , Schirmer J.
Published: July 2002

A recently developed complete fourth-order Green’s function method referred to as ADC(4) was used to calculate ionization energies and relative intensities of 1s hole states in N2 and CO. Very accurate 1s electron binding energies and a good qualitative description of the shakeup spectra at low and medium energies have been obtained. For the N1s and C1s results comparison is made with previous large-scale configuration interaction treatments. New results are presented for the O1s shakeup spectrum of CO. Here striking differences with respect to the C1s spectrum are found which are explained in terms of charge–transfer effects induced by electronic excitation. We also present results for shakeup states of symmetries other than 2Σ+ which are not observed in the usual photoemission experiment.
Hindered internal rotation in jet cooled H2HF complexes
Authors: Lovejoy Christopher M. , Nelson David D. , Nesbitt David J.
Published: July 2002

The vibration–rotation spectrum of the HF stretch mode in ortho-H2HF complexes has been obtained via infrared laser direct absorption detection in a slit supersonic jet expansion. The spectrum resembles a Ka =1←1 parallel band of a prolate near-symmetric top and can be reasonably well fit with a Watson A-type Hamiltonian; however, no rigid molecular structure can reproduce the observed Ka splittings without invoking unphysically large changes in the constituent bond lengths upon complexation. The splittings are more correctly analyzed in terms of a j=1 hindered H2 rotor in an anisotropic potential, with a minimum energy T-shaped geometry. Matrix calculations determine barriers to H2 rotation between 120 and 170 cm−1 that depend systematically both on vibrational and rotational state in the complex. These data are consistent with a strong increase in potential anisotropy with decreasing intermolecular separation, with both upper and lower vibrational states close to the dissociation limit. No evidence for a corresponding Σ←Σ para-H2HF spectrum is observed, despite adequate experimental sensitivity. The matrix calculations indicate that the ground Σ state of para-H2HF is less stabilized by the potential anisotropy than the ground Π state in ortho-H2HF, and may therefore be much less efficiently formed in the jet expansion. The preferential observation of a ground Π vs Σ state in ortho-H2HF clearly indicates a minimum in the potential surface for a T-shaped vs collinear geometry. The observed rotational constants strongly suggest a H2⋅⋅⋅H–F ordering. The results provide direct evidence for vibrationally averaged structure, internal rigidity, and intermolecular bond strength that are significantly quantum state dependent, but can be qualitatively understood in terms of simple steric interactions between the H2 and HF subunits.
Nuclear spin relaxation by intermolecular magnetic dipole coupling in the gas phase. 129Xe in oxygen
Authors: Jameson Cynthia J. , Jameson A. Keith , Hwang Joseph K.
Published: July 2002

The nuclear spin relaxation times (T1) of 129Xe in xenon–O2 gas mixtures have been measured as a function of temperature and density at different magnetic fields. This system is used to characterize the intermolecular dipolar relaxation of nuclear spins in the gas phase. An empirical Boltzmann-averaged collision cross section associated with the collision-induced transitions between 129Xe nuclear spin states is obtained as a function of temperature.
Exact quantization of the scattering from a classically chaotic repellor
Authors: Gaspard Pierre , Rice Stuart A.
Published: July 2002

The S matrix for the scattering of a point particle from three hard discs fixed on a plane is calculated exactly using Green’s theorem. The S matrix is obtained explicitly from S=I−iCM−1D, where the matrix M describes the multiple scattering between the three discs and the matrices C and D describe the free propagation from the edges of the three discs to large distances. The scattering resonances are located in the complex wave number plane as the zeros of the determinant of the matrix M, and their symmetry representations are determined. The results of this calculation are compared with the results obtained from the semiclassical approximation described in the preceding paper.
Critical properties of bimodal polymer solutions
Authors: Broseta Daniel , Leibler Ludwik
Published: July 2002

The phase diagrams and the critical properties of solutions of binary polymer mixtures are investigated. Both polymer species are chemically identical and differ only in their molecular weights. Depending on the relative concentration and the relative molecular weights of the two polymer species, different behaviors may be found. Thus, the addition to a monodisperse polymer solution of chains of smaller or comparable molecular weight does not change the phase behavior: in particular, the critical exponents remain of the Ising type. On the contrary, adding chains of larger molecular weight deeply transforms the phase behavior: the location of the critical points is deeply modified, a third phase may even appear, and the critical exponents are renormalized. A simple criterion for the crossover between the Ising and the renormalized behaviors is given.
Two- and three-body forces in the interaction of He atoms with Xe overlayers adsorbed on (0001) graphite
Authors: Aziz R. A. , Buck U. , Jónsson H. , Ruiz-Suárez J.-C. , Schmidt B. , Scoles G. , Slaman M. J. , Xu J.
Published: July 2002

In order to address the problem of three-body interactions in gas–surface scattering, we considered the collision of a He atom with the (0001) surface of graphite coated by a monolayer of Xe. To eliminate the uncertainties connected with errors in the two-body He–Xe interaction, we determined the latter by crossed-beam differential collision cross-section measurements performed at two energies (67.2 and 22.35 meV). These scattering data together with room-temperature bulk diffusion data are then fitted with a Hartree–Fock–dispersion–type function to yield an interaction potential that explains most of the properties of this system within the experimental errors and represents an improvement on previously published He–Xe potentials. Helium diffraction measurements are then carried out from the Xe overlayer and the dependence of the specular intensity from the angle of incidence is carefully determined. Further, a He–surface potential is constructed by adding together the following terms: (1) the He–Xe pairwise sum, (2) the long-range He–(0001)C interaction, (3) the three-body contribution generated by the Axilrod–Teller–Muto term, (4) the so-called surface-mediated three-body interaction He–Xe–(0001)C first considered by A. D. McLachlan [Mol. Phys. 7, 381 (1964)], and finally (5) a small correction which is meant to take into account the nonstationary nature of the surface. Using this potential, well-converged close-coupling scattering calculations are carried out, and their results compared with the data. In general, good agreement is obtained. The agreement can, however, be improved by (a) an increase of about 30% in the contribution of three-body forces, (b) the lowering of the He–graphite long-range attraction coefficient by about 15%, or (c) a reduction of the two-body interaction well depth of 1.6% (the experimental error) together with any combination of the factors under (a) and (b) reduced by an adequate amount. Elimination of the contribution of the graphite surface by studying Xe multilayers is hindered by the uncertainties in the ‘‘thermal correction’’ [point (5) above] which, due to the multilayer increased ‘‘softness,’’ becomes an appreciable source of uncertainty.
Error estimates on averages of correlated data
Authors: Flyvbjerg H. , Petersen H. G.
Published: July 2002

We describe how the true statistical error on an average of correlated data can be obtained with ease and efficiency by a renormalization group method. The method is illustrated with numerical and analytical examples, having finite as well as infinite range correlations.
The heat capacity and derived thermophysical properties of the high T
C superconductor YBa2Cu3O7−δ from 5.3 to 350 K
Authors: Shaviv R. , Westrum E. F. , Brown R. J. C. , Sayer M. , Yu X. , Weir R. D.
Published: July 2002

The heat capacity of the perovskite high-TC superconductor YBa2Cu3O7−δ was measured from 5.3 to 350 K in an adiabatic calorimetric cryostat. A break in the heat-capacity curve, associated with the critical temperature for superconductivity was observed between 90.09 and 92.59 K. The transition temperature was identified as 91.44 K, and ΔCp,m was calculated to be 0.559R at that temperature. The lattice heat capacity was evaluated by means of the recently developed Komada/Westrum phonon distribution model and the apparent characteristic temperature ΘKW was calculated to be 107.7 K. The excess electronic heat capacity for the superconducting phase was evaluated and the energy gap was identified as 234. R K. Excess contribution, resulting from magnetic impurities, was noted below 20 K. Thermodynamic properties at selected temperatures are presented.
The photochemistry of ozone at 193 and 222 nm
Authors: Turnipseed Andrew A. , Vaghjiani Ghanshyam L. , Gierczak Tomasz , Thompson John E. , Ravishankara A. R.
Published: July 2002

The quantum yields for the formation of O(1D) and O(3P) from the photolysis of ozone have been measured at 222 and 193 nm. The quantum yield for O(3P) was observed to be Φ(3P)=0.13±0.02 at 222 nm, and the primary quantum yield for O(1D) was found to be Φ(1D)=0.87±0.04. This measurement is consistent with other studies of O3 photolysis within the Hartley band which indicate that a significant portion (5–15%) of the products are formed in the ground state. At 193 nm the quantum yield for the production of excited state O(1D) atoms is Φ(1D)=0.46±0.29, which is significantly less than what is observed during photolysis within the lower energy Hartley band. The quantum yield for O(3P) atoms at 193 nm was found to be 0.57±0.14. We have also observed that the quantum yield for O atoms [O(1D)+O(3P)] is greater than unity [Φ(O)=1.20±0.15] at 193 nm indicating the presence of a channel which produces three O(3P) atoms. O2(b 1Σ) has also been detected and quantified, Φ(b 1Σ)=0.50±0.38, from O3 photolysis at 193 nm. These measurements suggest that O(1D) and O2(b1Σ) are co-produced.
Diffusion and interaction in gels and solutions. 4. Hard sphere Brownian dynamics simulations
Authors: Johansson Lennart , Löfroth Jan-Erik
Published: July 2002

Results from Brownian dynamics simulations of hard sphere diffusion in polymer networks of wormlike chains are presented. The influence of the size of the diffusing particles, the polymer concentration, the polymer radius, and the persistence length of the chains were investigated. The diffusion coefficients obtained in systems of stiff chains were compared to a theory [Macromolecules 24, 6024 (1991)]. The agreement between the simulation data and the theory was satisfactory, except for dense systems where the theory predicted somewhat too high diffusion coefficients. To account for the diffusion in systems of more flexible polymers, we derived a semiphenomenological theory based on ideas from the concept of fractals. By this approach we could show how the available volume fraction for particles in polymer networks scales with the particle radii. The scaling is not a power law, but instead is a stretched exponential, and is related to the local structure of the polymer chains. Finally, the inherent assumptions in the theoretical descriptions, e.g., the negligence of the hydrodynamic interactions, and the applicability of the theory to real polymer systems, are thoroughly discussed.
Effective core potential methods for the lanthanides
Authors: Cundari Thomas R. , Stevens Walter J.
Published: July 2002

In this paper a complete set of effective core potentials (ECPs) and valence basis sets for the lanthanides (Ce to Lu) are derived. These ECPs are consistent not only within the lanthanide series, but also with the third-row transition metals which bracket them. A 46-electron core was chosen to provide the best compromise between computational savings and chemical accuracy. Thus, the 5s and 5p are included as ‘‘outer’’ core while all lower energy atomic orbitals (AOs) are replaced with the ECP. Generator states were chosen from the most chemically relevant +3 and +2 oxidation states. The results of atomic calculations indicate that the greatest error vs highly accurate numerical potential/large, even-tempered basis set calculations results from replacement of the large, even-tempered basis sets with more compact representations. However, the agreement among atomic calculations remains excellent with both basis set sizes, for a variety of spin and oxidation states, with a significant savings in time for the optimized valence basis set. It is expected that the compact representation of the ECPs and valence basis sets will eventually encourage their use by computational chemists to further explore the bonding and reactivity of lanthanide complexes.
The effect of long-range electrostatic interactions in simulations of macromolecular crystals: A comparison of the Ewald and truncated list methods
Authors: York Darrin M. , Darden Tom A. , Pedersen Lee G.
Published: July 2002

Simulations of the HIV-1 protease unit cell using a 9 Å cutoff, 9/18 Å ‘‘twin-range’’ cutoff, and full Ewald sums have been carried out to 300 ps. The results indicate that long-range electrostatic interactions are essential for proper representation of the HIV-1 protease crystal structure. The 9 Å simulation did not converge in 300 ps. Inclusion of a 9/18 Å ‘‘twin-range’’ cutoff showed significant improvement. Simulation using the Ewald summation convention gave the best overall agreement with x-ray crystallographic data, and showed the least internal differences in the time average structures of the asymmetric units. The Ewald simulation represents an efficient implementation of the Particle Mesh Ewald method [Darden et al., J. Chem. Phys. 98, 10 089 (1993)], and illustrates the importance of including long-range electrostatic forces in large macromolecular systems.
An ab initio study of CuCO
Authors: Bauschlicher Charles W.
Published: July 2002

Modified coupled-pair functional (MCPF) calculations and coupled cluster singles and doubles calculations, which include a perturbational estimate of the connected triples [CCSD(T)], yield a bent structure for CuCO, thus, supporting the prediction of a nonlinear structure based on density functional (DF) calculations. Our best estimate for the binding energy is 4.9±1.4 kcal/mol; this is in better agreement with experiment (6.0±1.2 kcal/mol) than the DF approach which yields a value (19.6 kcal/mol) significantly larger than experiment.
Crystal field effects on the topological properties of the electron density in molecular crystals: The case of urea
Authors: Gatti C. , Saunders V. R. , Roetti C.
Published: July 2002

The Quantum Theory of Atoms in Molecules, due to Bader, is applied to periodic systems. Results for molecular and crystalline urea are presented. Changes in both bond critical points and atomic properties due to changes of chemical environment are described. A rationale for the different lengths of the in-plane and out-of-plane hydrogen bonds and for the lengthening of the CO bond in bulk urea is provided in terms of the properties of the Laplacian of the oxygen atom electron density distribution. An evaluation of molecular and atomic volume changes indicates that the decrease of molecular volume upon change of phase from gas to solid originates primarily from a contraction of the atomic basins directly involved in hydrogen bonds. Other atoms show a small expansion. The considerable decrease of oxygen and hydrogen atomic volumes is related to the mutual penetration of their van der Waals envelopes following hydrogen bond formation. The results confirm that urea is more polar in the solid phase.
Relativistic total energy using regular approximations
Authors: van Lenthe E. , Baerends E. J. , Snijders J. G.
Published: July 2002

In this paper we will discuss relativistic total energies using the zeroth order regular approximation (ZORA). A simple scaling of the ZORA one-electron Hamiltonian is shown to yield energies for the hydrogenlike atom that are exactly equal to the Dirac energies. The regular approximation is not gauge invariant in each order, but the scaled ZORA energy can be shown to be exactly gauge invariant for hydrogenic ions. It is practically gauge invariant for many-electron systems and proves superior to the (unscaled) first order regular approximation for atomic ionization energies. The regular approximation, if scaled, can therefore be applied already in zeroth order to molecular bond energies. Scalar relativistic density functional all-electron and frozen core calculations on diatomics, consisting of copper, silver, and gold and their hydrides are presented. We used exchange-correlation energy functionals commonly used in nonrelativistic calculations; both in the local-density approximation (LDA) and including density-gradient (‘‘nonlocal’’) corrections (NLDA). At the NLDA level the calculated dissociation energies are all within 0.2 eV from experiment, with an average of 0.1 eV. All-electron calculations for Au2 and AuH gave results within 0.05 eV of the frozen core calculations.
Symmetry-adapted perturbation theory of nonadditive three-body interactions in van der Waals molecules. I. General theory
Authors: Moszynski Robert , Wormer Paul E. S. , Jeziorski Bogumil , van der Avoird Ad
Published: July 2002

Symmetry-adapted perturbation theory of pairwise nonadditive interactions in trimers is formulated, and pure three-body polarization and exchange components are explicitly separated out. It is shown that the three-body polarization contributions through the third order of perturbation theory naturally separate into terms describing the pure induction, mixed induction–dispersion, and pure dispersion interactions. Working equations for these components in terms of molecular integrals and linear and quadratic response functions are derived. These formulas have a clear, partly classical, partly quantum mechanical, physical interpretation. The asymptotic expressions for the second- and third-order three-body polarization contributions through the multipole moments and (hyper)polarizabilities of the isolated monomers are reported. Finally, assuming the random phase approximation for the response functions, explicit orbital formulas for the three-body polarization terms are derived. The exchange terms are also classified, and the simplest approximations (neglecting intramonomer correlation effects) are written as explicitly connected commutator expressions involving second-quantized operators. The corresponding orbital formulas are also reported.
High resolution UV spectroscopy of phenol and the hydrogen bonded phenol-water cluster
Authors: Berden Giel , Meerts W. Leo , Schmitt Michael , Kleinermanns Karl
Published: July 2002

The S1←S0 000 transitions of phenol and the hydrogen bonded phenol(H2O)1 cluster have been studied by high resolution fluorescence excitation spectroscopy. All lines in the monomer spectrum are split by 56±4 MHz due to the internal rotation of the −OH group about the a axis. The barrier for this internal motion is determined in the ground and excited states; V2″=1215 cm−1, and V2′=4710 cm−1. The rotational constants for the monomer in the ground state are in agreement with those reported in microwave studies. The excited state rotational constants were found to be A′=5313.7 MHz, B′=2620.5 MHz, and C′=1756.08 MHz. The region of the redshifted 000 transition of phenol(H2O)1 shows two distinct bands which are 0.85 cm−1 apart. Their splitting arises from a torsional motion which interchanges the two equivalent H atoms in the H2O moiety of the cluster. This assignment was confirmed by spin statistical considerations. Both bands could be fit to rigid rotor Hamiltonians. Due to the interaction between the overall rotation of the entire cluster and the internal rotation, both bands have different rotational constants. They show that V2′<V2″, and that the internal rotation axis is nearly parallel to the a-axis of the cluster. If it is assumed that the structure of the rotor part does not change upon electronic excitation, the internal motion becomes simply a rotation of the water molecule around its symmetry axis. Assuming this motion, barriers of 180 and 130 cm−1 could be estimated for the S0 and S1 states, respectively. The analysis of the rotational constants of the cluster yielded an O–O distance of the hydrogen bond of 2.93 Å in the ground state and 2.89 Å in the electronically excited state. In the equilibrium structure of the cluster, the plane containing phenol bisects the plane of the water molecule.
Density-functional thermochemistry. IV. A new dynamical correlation functional and implications for exact-exchange mixing
Authors: Becke Axel D.
Published: July 2002

A new dynamical correlation functional is constructed subject to a small number of simple, yet key, requirements not all satisfied by existing functionals in the literature. The new functional gives good atomic correlation energies, and, in conjunction with previous gradient-corrected exchange functionals and exact-exchange mixing, excellent thermochemistry in the G2 benchmarks of Pople and co-workers.
Ultrafast x-ray absorption probing of a chemical reaction
Authors: Ráksi Ferenc , Wilson Kent R. , Jiang Zhiming , Ikhlef Abdelaziz , Côté Christian Y. , Kieffer Jean-Claude
Published: July 2002

Ultrafast x-ray techniques can, in principle, allow us to more directly watch the time evolution of matter, with atomic spatial resolution and with time resolution on the scale of atomic motions such as the making and breaking of chemical bonds, in order to more directly observe the fundamental molecular dynamics underlying the concept of ‘‘mechanism’’ in inorganic, organic, and biochemical reactions. As a step toward this goal, we have observed a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast near-edge x-ray absorption spectroscopy with time resolutions of 1.5–3 ps, near the sulfur K edge at a photon energy of 2.48 keV (4.98 A).
Poling dynamics and investigation into the behavior of trapped charge in poled polymer films for nonlinear optical applications
Authors: Pauley M. A. , Guan H. W. , Wang C. H.
Published: July 2002

The behavior of surface and trapped charge that originate during contact electric-field poling of a model guest–host nonlinearly active polymer has been investigated. Charge effects in thin films of para-nitroaniline doped polymethylmethacrylate were studied during and after the poling process by simultaneous measurement of the current through the poling circuit and the second harmonic generation (SHG) signal from the polymer system. The poling current present in a fresh sample was found to differ in both magnitude and temporal dependence to that observed in the sample during subsequent polings. The magnitude of the steady-state current eventually reached during the poling process was found to depend on temperature and is related to the mobility of charge. A sharp drop-off in SHG signal intensity that occurred when the poling electrodes were grounded is shown to result from the removal of surface charge that orients chromophore dipoles near the surface of the film; the decay in the macroscopic polarization that then occurs is prolonged by charges trapped in the polymer matrix. The build-up of trapped charge explains a memory effect in which the SHG signal relaxation time gradually increased as the film was subjected to multiple polings until a steady-state value was reached. Finally, it is demonstrated that the charges trapped in the polymer matrix are released only after the polymer is heated to high above its temperature of glass transition and the SHG signal has completely decayed away.
Ionic solids at elevated temperatures and high pressures: MgF2
Authors: Barrera G. D. , Taylor M. B. , Allan N. L. , Barron T. H. K. , Kantorovich L. N. , Mackrodt W. C.
Published: July 2002

A combination of periodic Hartree–Fock theory, quasiharmonic lattice dynamics, and molecular dynamics is used to study the behavior of MgF2 at elevated temperatures and/or high pressures. Particular attention is paid to the pressure-induced transition from the rutile to the fluorite structure in view of earlier theoretical estimates of the transition pressure, which differ widely. It is shown that previously reported potentials obtained by fitting to empirical data fail to reproduce thermodynamic properties. To rectify this, a new set of consistent two-body potentials has been derived from ab initio periodic Hartree–Fock calculations. Lattice dynamics calculations in the quasiharmonic approximation based on these potentials has been used to study the two phases of MgF2 at high T and P. The resulting transition pressure and that obtained directly from Hartree–Fock calculations in the static limit are both ⩽30 GPa, which is close to the experimental value but appreciably lower than a previous molecular dynamics value of over 130 GPa. The variation of quantities such as (∂P/∂T)V which play a central role in the formulation of approximate equations of state is also considered.
A class IV charge model for molecular excited states
Authors: Li Jiabo , Williams Brian , Cramer Christopher J. , Truhlar Donald G.
Published: July 2002

We present a new parameterization for calculating class IV charges for molecules containing H, C, N, O, F, Si, P, S, Cl, Br, and I from wave functions calculated at the intermediate-neglect-ofdifferential-overlap-for-spectroscopy (INDO/S) level. First we readjust the oxygen parameters in INDO/S on the basis of electronic excitation energies; this yields a new set of parameters called INDO/S2. Then we parameterize the charge model. The new model, called charge model 2 for INDO/S2 (CM2/INDO/S2), is parameterized against the most accurate available data from both ab initio and experimental sources for dipole moments of ground and excited electronic states. For a training set containing 211 dipole moments of molecules in their ground states and 33 dipole moments of molecules in their first excited states, the CM2/INDO/S2 model leads to a root-mean-square (rms) error in dipole moments of 0.26 D for ground states and 0.40 D for the excited states. The new model, INDO/S2 with CM2, systematically improves the n→π* excitation energies and the dipole moments of the excited states of carbonyl compounds. We also parameterized a CM2 model for the standard INDO/S model (CM2/INDO/S), which predicts quite accurate dipole moments for ground states with an rms error of 0.24 D.
Relative formation rates of O350 and O352 in O16–18O mixtures
Authors: Janssen Christof , Guenther Juergen , Krankowsky Dieter , Mauersberger Konrad
Published: July 2002

Tunable diode laser (TDL) and mass spectrometry have been combined to measure relative formation rate coefficients of each of the four channels contributing to ozone of mass 50 u and 52 u produced in O16–18O mixtures. Only one channel has a large rate coefficient advantage causing almost exclusively the observed isotope enrichment. Collisions to form ozone are end-on reactions. Molecular symmetry plays no apparent role in the ozone isotope enrichment process, regardless, whether or not ozone is produced in collisions with homo- or heteronuclear molecular oxygen. The oxygen isotope exchange process may hold a key in explaining the rate coefficient results.
A rigged Hilbert space for the free radiation field
Authors: Gadella M.
Published: July 2002

We here construct a rigged Hilbert space suitable for the description of the free radiation field in a box and in the Coulomb gauge. We prove the continuity of the main observables on this structure.
Darboux transformation and solutions for an equation in 2+1 dimensions
Authors: Estévez P. G.
Published: July 2002

Painlevé analysis and the singular manifold method are the tools used in this paper to perform a complete study of an equation in 2+1 dimensions. This procedure has allowed us to obtain the Lax pair, Darboux transformation and τ functions in such a way that a plethora of different solutions with solitonic behavior can be constructed iteratively.
Solitary waves in compressible deep fluids
Authors: Miesen R. H. M. , Kamp L. P. J. , Sluijter F. W.
Published: July 2002

Solitary waves in incompressible deep fluids are described to the first order in the wave amplitude by the Benjamin–Davis–Ono equation [J. Fluid Mech. 29, 559 (1967); J. Fluid Mech. 29, 593 (1967); J. Phys. Soc. Jpn. 39, 1082 (1975)]. This equation describes the balance between dispersion and weak nonlinear effects for long internal waves in a density stratified layer of fluid confined in an infinitely deep fluid. A solution of this equation is the so-called algebraic solitary wave. In the present paper the modifications of this wave due to the compressibility of the fluid are investigated. It is found that the linear (eigenvalue) problem, determining the modal function and the possible phase speeds, as well as the coefficients of the Benjamin–Davis–Ono equation, change due to the compressibility. These changes are discussed considering three special cases in detail.
Long solitary waves in compressible shallow fluids
Authors: Miesen R. H. M. , Kamp L. P. J. , Sluijter F. W.
Published: July 2002

Long solitary waves in incompressible density stratified fluids are described, to the first order in wave amplitude by the Korteweg–de Vries equation. A well-known solution of this equation represents a single solitary wave that has the familiar ‘‘sech2 ’’ profile and a phase speed that varies linearly with the wave amplitude. In the present paper the modifications of these waves resulting from the compressibility are investigated. Solitary waves are again described by the Korteweg–de Vries equation. Three special cases are discussed and results are compared with the results of incompressible solitary wave theory. This shows that qualitative and quantitative differences may be profound, especially for an isothermal shearless fluid.
Low-dimensional models for complex geometry flows: Application to grooved channels and circular cylinders
Authors: Deane A. E. , Kevrekidis I. G. , Karniadakis G. E. , Orszag S. A.
Published: July 2002

Two-dimensional unsteady flows in complex geometries that are characterized by simple (low-dimensional) dynamical behavior are considered. Detailed spectral element simulations are performed, and the proper orthogonal decomposition or POD (also called method of empirical eigenfunctions) is applied to the resulting data for two examples: the flow in a periodically grooved channel and the wake of an isolated circular cylinder. Low-dimensional dynamical models for these systems are obtained using the empirically derived global eigenfunctions in the spectrally discretized Navier–Stokes equations. The short- and long-term accuracy of the models is studied through simulation, continuation, and bifurcation analysis. Their ability to mimic the full simulations for Reynolds numbers (Re) beyond the values used for eigenfunction extraction is evaluated. In the case of the grooved channel, where the primary horizontal wave number of the flow is imposed from the channel periodicity and so remains unchanged with Re, the models extrapolate reasonably well over a range of Re values. In the case of the cylinder wake, however, due to the significant spatial wave number changes of the flow with the Re, the models are only valid in a small neighborhood of the decompositional Reynolds number.
A dynamic subgrid-scale eddy viscosity model
Authors: Germano Massimo , Piomelli Ugo , Moin Parviz , Cabot William H.
Published: July 2002

One major drawback of the eddy viscosity subgrid-scale stress models used in large-eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity between the subgrid-scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid-scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near-wall region of a turbulent boundary layer. The results of large-eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.