bluuebunny/crossref_metadata_2025 huggingface dataset:
Photofragment imaging study of the CH2CCH2OH radical intermediate of the OH+allene reaction
Authors: Raman Arjun S. , Justine Bell M. , Lau Kai-Chung , Butler Laurie J.
Published: October 2007

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH2CCH2OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl+CH2CCH2OH photofragments, a spin-orbit branching ratio for Cl(P1∕22):Cl(P3∕22) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH2CCH2OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH2CCH2OH radical intermediate to the three most important product channels for the OH+allene reaction expected from this radical intermediate: formaldehyde+C2H3, H+acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH3 product channel. We compare our results to a previous theoretical study of the O+allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates.
Ab initio studies of aspartic acid conformers in gas phase and in solution
Authors: Chen Mingliang , Lin Zijing
Published: October 2007

Systematic and extensive conformational searches of aspartic acid in gas phase and in solution have been performed. For the gaseous aspartic acid, a total of 1296 trial canonical structures and 216 trial zwitterionic structures were generated by allowing for all combinations of internal single-bond rotamers. All the trial structures were optimized at the B3LYP∕6-311G* level and then subjected to further optimization at the B3LYP∕6-311++G** level. A total of 139 canonical conformers were found, but no stable zwitterionic structure was found. The rotational constants, dipole moments, zero-point vibrational energies, harmonic frequencies, and vertical ionization energies of the canonical conformers were determined. Single-point energies were also calculated at the MP2∕6-311++G** and CCSD∕6-311++G** levels. The equilibrium distributions of the gaseous conformers at various temperatures were calculated. The proton affinity and gas phase basicity were calculated and the results are in excellent agreement with the experiments. The conformations in the solution were studied with different solvation models. The 216 trial zwitterionic structures were first optimized at the B3LYP∕6-311G* level using the Onsager self-consistent reaction field model (SCRF) and then optimized at the B3LYP∕6-311++G** level using the conductorlike polarized continuum model (CPCM) SCRF theory. A total of 22 zwitterions conformers were found. The gaseous canonical conformers were combined with the CPCM model and optimized at the B3LYP∕6-311++G** level. The solvated zwitterionic and canonical structures were further examined by the discrete/SCRF model with one and two water molecules. The incremental solvation of the canonical and zwitterionic structures with up to six water molecules in gas phase was systematically examined. The studies show that combining aspartic acid with at least six water molecules in the gas phase or two water molecules and a SCRF solution model is required to provide qualitatively correct results in the solution.
Appearance of interatomic Coulombic decay in Ar, Kr, and Xe homonuclear dimers
Authors: Lablanquie P. , Aoto T. , Hikosaka Y. , Morioka Y. , Penent F. , Ito K.
Published: October 2007

Interatomic Coulombic decay (ICD) is observed in the rare gas homonuclear dimers Ar2, Kr2, and Xe2 with photoion spectroscopy techniques. Inner valence ionization of the outer ns shell of these systems is known to create a metastable state that dissociates to form a ground state ion and a neutral excited fragment. Inner valence ionization to form ns satellite states leads to similar dissociations, but the neutral fragment gets all the more excited as the internal energy of the ns satellite state increases. When enough excitation energy is transferred to reach the ionization potential, ICD occurs. ICD threshold is observed to coincide with the position of the A+A+ ground state in the Franck-Condon region.
Semiclassical wave packet study of anomalous isotope effect in ozone formation
Authors: Vetoshkin Evgeny , Babikov Dmitri
Published: October 2007

We applied the semiclassical initial value representation method to calculate energies, lifetimes, and wave functions of scattering resonances in a two-dimensional potential for O+O2 collision. Such scattering states represent the metastable O3* species and play a central role in the process of ozone formation. Autocorrelation functions for scattering states were computed and then analyzed using the Prony method, which permits one to extract accurate energies and widths of the resonances. We found that the results of the semiclassical wave packet propagation agree well with fully quantum results. The focus was on the O16O16O18 isotopomer and the anomalous isotope effect associated with formation of this molecule, either through the O16O16+O18 or the O16+O16O18 channels. An interesting correlation between the local vibration mode character of the metastable states and their lifetimes was observed and explained. New insight is obtained into the mechanism by which the long-lived resonances in the delta zero-point energy part of spectrum produce the anomalously large isotope effect.
Simultaneous photon absorption as a probe of molecular interaction and hydrogen-bond cooperativity in liquids
Authors: Woutersen Sander
Published: October 2007

We have investigated the simultaneous absorption of near-infrared photons by pairs of neighboring molecules in liquid methanol. Simultaneous absorption by two OH-stretching modes is found to occur at an energy higher than the sum of the two absorbing modes. This frequency shift arises from interaction between the modes, and its value has been used to determine the average coupling between neighboring methanol molecules. We find a rms coupling strength of 46±1cm−1, larger than can be explained from a transition-dipole coupling mechanism, suggesting that hydrogen-bond mediated interactions also contribute to the coupling. The most important aspect of simultaneous vibrational absorption is that it allows for a quantitative investigation of hydrogen-bond cooperativity. We derive the extent to which the hydrogen-bond strengths of neighboring molecules are correlated by comparing the line shape of the absorption band caused by simultaneous absorption with that of the fundamental transition. Surprisingly, neighboring hydrogen bonds in methanol are found to be strongly correlated, and from the data we obtain an estimate for the hydrogen-bond correlation coefficient of 0.69±0.12.
Electron attachment induced proton transfer in a DNA nucleoside pair: 2′-deoxyguanosine-2′-deoxycytidine
Authors: Gu Jiande , Xie Yaoming , Schaefer Henry F.
Published: October 2007

To elucidate electron attachment induced damage in the DNA double helix, electron attachment to the 2′-deoxyribonucleoside pair dG:dC has been studied with the reliably calibrated B3LYP∕DZP++ theoretical approach. The exploration of the potential energy surface of the neutral and anionic dG:dC pairs predicts a positive electron affinity for dG:dC [0.83eV for adiabatic electron affinity (EAad) and 0.16eV for vertical electron affinity (VEA)]. The substantial increases in the electron affinity of dG:dC (by 0.50eV for EAad and 0.23eV for VEA) compared to those of the dC nucleoside suggest that electron attachment to DNA double helices should be energetically favored with respect to the single strands. Most importantly, electron attachment to the dC moiety in the dG:dC pair is found to be able to trigger the proton transfer in the dG:dC−∙ pair, surprisingly resulting in the lower energy distonic anionic complex d(G-H)−:d(C+H)∙. The negative charge for the latter system is located on the base of dC in the dG:dC−∙ pair, while it is transferred to d(G-H) in d(G-H)−:d(C+H)∙, accompanied by the proton transfer from N1(dG) to N3(dC). The low energy barrier (2.4kcal∕mol) for proton transfer from dG to dC−∙ suggests that the distonic d(G-H)−:d(C+H)∙ pair should be one of the important intermediates in the process of electron attachment to DNA double helices. The formation of the neutral nucleoside radical d(C+H)∙ is predicted to be the direct result of electron attachment to the DNA double helices. Since the neutral radical d(C+H)∙ nucleotide is the key element in the formation of this DNA lesion, electron attachment might be one of the important factors that trigger the formation of abasic sites in DNA double helices.
Adam-Gibbs based model to describe the single component dynamics in miscible polymer blends under hydrostatic pressure
Authors: Schwartz Gustavo A. , Alegría Ángel , Colmenero Juan
Published: October 2007

We present in this work a new model to describe the component segmental dynamics in miscible polymers blends as a function of pressure, temperature, and composition. The model is based on a combination of the Adam-Gibbs (AG) theory and the concept of the chain connectivity. In this paper we have extended our previous approach [D. Cangialosi et al. J. Chem. Phys. 123, 144908 (2005)] to include the effects of pressure in the component dynamics of miscible polymer blends. The resulting model has been tested on poly(vinyl methyl ether) (PVME)/polystyrene (PS) blends at different concentrations and in the temperature range where the system is in equilibrium. The results show an excellent agreement between the experimental and calculated relaxation times using only one fitting parameter. Once this parameter is known the model allows calculating the size of the relevant length scale where the segmental relaxation of the dielectrically active component takes place, i.e., the so called cooperative rearrangement region (CRR) in the AG framework. Thus the size of the CRR for PVME in the blends with PS has been determined as well as its dependence with pressure, temperature, and concentration.
Matrix isolation and ab initio study of the HXeCCH⋯CO2 complex
Authors: Tanskanen Hanna , Johansson Susanna , Lignell Antti , Khriachtchev Leonid , Räsänen Markku
Published: October 2007

The HXeCCH⋯CO2 complex is studied experimentally and computationally. The complex is prepared in a low-temperature xenon matrix using UV photolysis of propiolic acid (HCCCOOH) and thermal mobilization of H atoms at 45K. Photolysis of propiolic acid leads to the HCCH⋯CO2 complex as one of the photolysis products. The HCCH⋯CO2 complex is further photolyzed to the HCC⋯CO2 complex. Thermal annealing leads to the formation of HXeCCH complexed with CO2. The H–Xe stretching absorption of the HXeCCH⋯CO2 complex is blueshifted (+31.9 and +5.8cm−1) from the value of the HXeCCH monomer in a xenon matrix. In the calculations, three HXeCCH⋯CO2 structures were found (one parallel and two linear structures) corresponding to the true energy minima on the potential energy surface. For the H–Xe stretching mode, the calculations give blueshifted values of +19.2 or +19.5cm−1 depending on the computational level [MP2∕6-311++G(2d,2p) and MP2/aug-cc-pVDZ] for the parallel structure and +19.4 or +27.9cm−1 for one linear structure. For the second linear structure, the H–Xe stretching frequency is redshifted by −8.6 or −9.4cm−1 at these levels of theory. Based on the calculations, the experimental band shifted by +5.8cm−1 (1492.2cm−1) most likely corresponds to the HXeCCH⋯CO2 parallel structure. The band with larger blueshift of +31.9cm−1 (1518.3cm−1) can be due to another matrix site of the same structure or to the blueshifting linear structure.
Comparison of displacement speed statistics of turbulent premixed flames in the regimes representing combustion in corrugated flamelets and thin reaction zones
Authors: Chakraborty Nilanjan
Published: October 2007

The curvature and strain rate dependence of local displacement speed Sd is compared between two three-dimensional simplified chemistry and transport based direct numerical simulation (DNS) datasets of turbulent premixed flames with comparable turbulent Reynolds number. One of these datasets has attributes of the corrugated flamelets regime (Karlovitz number Ka<1), and the combustion situation in the other belongs to the thin reaction zones regime (Ka>1). It has been found that the probability of finding negative displacement speed increases for the flame in the thin reaction zones regime in comparison to the flame in the corrugated flamelets regime. However, the mean displacement speed remains positive throughout the flame brush in both the flames. It is shown that the curved flame contribution in displacement speed St is principally responsible for the negative displacement speed in the flame representing the thin reaction zones regime. The alignment characteristics of scalar gradient with principal strain rates is found to be different in these two flames, which leads to significant differences in tangential strain rate and curvature dependence of the magnitude of the reaction progress variable gradient ∣∇c∣. This difference in ∣∇c∣ statistics leads to marked differences in the statistics of the combined reaction and normal diffusion component of displacement speed SLS=(Sr+Sn), which in turn leads to significant differences in Sd statistics between the corrugated flamelets and the thin reaction zones regimes. This difference has important implications in the context of flame surface density based reaction rate closure.
Amplification of nonlinear surface waves by wind
Authors: Leblanc Stéphane
Published: October 2007

A weakly nonlinear analysis is conducted to study the evolution of slowly varying wavepackets with small but finite amplitudes, that evolve at the interface between air and water under the effect of wind. In the inviscid assumption, wave envelopes are governed by cubic nonlinear Schrödinger or Davey-Stewartson equations forced by a linear term corresponding to Miles’ mechanism of wave generation. Under fair wind, it is shown that Stokes waves grow exponentially and that Benjamin-Feir instability becomes explosive.
What is the best definition of a liquid cluster at the molecular scale?
Authors: Wedekind Jan , Reguera David
Published: October 2007

We investigate the ability of different cluster definitions to serve as a good reaction coordinate in molecular simulations of nucleation. In particular, the most commonly used Stillinger criterion [J. Chem. Phys. 38, 1486 (1963)] is compared with the cluster definition introduced by ten Wolde and Frenkel [J. Chem. Phys. 109, 9901 (1998)]. The accuracy of these two different cluster definitions is tested by using molecular dynamics to study the vapor-liquid nucleation of Lennard-Jones argon as a model system. We are able to compare the size of the critical cluster identified by each cluster definition with a completely model-independent value provided by the nucleation theorem, aided by a recently introduced method that accurately extracts the location of the transition state directly from the kinetics. It is found that the Stillinger definition strongly overestimates the size of small molecular clusters by up to a factor of 2. A simple change of the Stillinger radius is unable to rectify this deficiency. On the contrary, the ten Wolde-Frenkel definition, while being only slightly more elaborate than a simple Stillinger criterion, is remarkably successful in identifying the correct molecular excess of the small clusters if the parameters are chosen adequately. The method described here can also be generalized to identify a proper reaction coordinate in other activated processes.
The appearance of an interval of energies that contain the whole diamagnetic contribution to NMR magnetic shieldings
Authors: Maldonado Alejandro , Aucar Gustavo A.
Published: October 2007

Working within relativistic polarization propagator approach, it was shown in a previous article that the electronic origin of diamagnetic contributions to NMR nuclear magnetic shielding, σd, are mostly excitations that fit in a well defined interval of energies such that 2mc2≦(εi−εs¯)<4mc2. That interval of energies does not have, in principle, any physical reason to be so well defined, and gives a large amount of the total contribution to σd, e.g., close to 98% of it. Then a further study is given in this article, where we show some of the main characteristics of that interval of energy, such as its universal appearance and basis set independence. Our main result is the finding that σd is completely described by that interval of excitation energies, i.e., there is no contribution arising from outside of it. Most of the contributions belonging to that interval arise from virtual electronic energies larger than −3mc2. For heavier atoms, there are few contributions from states with virtual negative energies smaller than −3mc2. The model systems under study were noble gases, XH (X=Br, I, and At), XH2 (X=O, S, Se, Te, and Po), XH3 (X=N, P, As, Sb, and Bi); XH4 (X=Sn and Pb), and SnXH3 (X=Br and I). The pattern of contributions of occupied molecular orbitals (MOs) is also shown, where the 1s1∕2 is the most important for excitations ending in the bottom half part of the above mentioned interval. On the other hand, the contribution of the other occupied MOs are more important than that of 1s1∕2 for the other part of such interval. We also show that σd is electron correlation independent within both relativistic and nonrelativistic domain. In the case of σp, we find out a clear dependence of electron correlation effects with relativistic effects, which is of the order of 30% for Pb in PbH4.
Probing the extent of the Sr2+ ion condensation to anionic polyacrylate coils: A quantitative anomalous small-angle x-ray scattering study
Authors: Goerigk G. , Huber K. , Schweins R.
Published: October 2007

The shrinking process of anionic sodium polyacrylate (NaPA) chains in aqueous solution induced by Sr2+ counterions was analyzed by anomalous small-angle x-ray scattering. Scattering experiments were performed close to the precipitation threshold of strontium polyacrylate. The pure-resonant scattering contribution, which is related to the structural distribution of the Sr2+ counterions, was used to analyze the extent of Sr2+ condensation onto the polyacrylate coils. A series of four samples with different ratios [Sr2+]∕[NaPA] (between 0.451 and 0.464) has been investigated. From the quantitative analysis of the resonant invariant, the amount of Sr cations localized in the collapsed phase was calculated with concentrations v¯ between 0.94×1017 and 2.01×1017cm−3 corresponding to an amount of Sr cations in the collapsed phase between 9% and 23% of the total Sr2+ cations in solution. If compared to the concentration of polyacrylate expressed in moles of monomers [NaPA], a degree of site binding of r=[Sr2+]∕[NaPA] between 0.05 and 0.11 was estimated. These values clearly differ from r=0.25, which was established from former light scattering experiments, indicating that the counterion condensation starts before the phase border is reached and increases rather sharply at the border.
Laser-induced nonlinear response in photoassisted resonant electronic transport
Authors: Urdaneta Inés , Keller Arne , Atabek Osman , Mujica Vladimiro
Published: October 2007

We consider an extension of our previous model for photoassisted electron transport in molecular and atomic junctions to the study of nonlinear processes, induced by a laser field, that are strongly influenced by a resonant level structure associated with the material structure between the two metal electrodes. Using a Floquet transformation to include the radiation field, we calculate the stationary Landauer current as a function of the intensity and frequency of the laser. The emphasis in this work is in the description of the interplay between the optical response of the junction and its transport properties. Our results may be then of importance in understanding the response of molecular junctions to multiphoton excitations, a regime where nonlinearities are dominant.
Correlation effects in R-matrix calculations of electron-F2 elastic scattering cross sections
Authors: Tarana Michal , Horáček Jiří
Published: October 2007

Correlation effects are studied in electron scattering off the fluorine molecule. Fixed-nuclei approximation R-matrix calculations of the elastic collision cross sections are presented for a set of internuclear distances at three levels of correlation. The aim of this work is to study the role of electronic correlation on the properties of the Σu2 resonance. The Feshbach-Fano R-matrix method of resonance-background separation is used to study the effect of inclusion of various levels of correlation on the energy and width of the Σu2 resonance. Data required for construction of the nonlocal resonance model (construction of a discrete state and its coupling to the continuum) which allows the calculation of inelastic processes such as dissociative electron attachment and vibrational excitation [W. Domcke, Phys. Rep. 208, 97 (1991)] including the correlation are presented.
Toward realistic modeling of dynamic processes in cell signaling: Quantification of macromolecular crowding effects
Authors: Sun Jian , Weinstein Harel
Published: October 2007

One of the major factors distinguishing molecular processes in vivo from biochemical experiments in vitro is the effect of the environment produced by macromolecular crowding in the cell. To achieve a realistic modeling of processes in the living cell based on biochemical data, it becomes necessary, therefore, to consider such effects. We describe a protocol based on Brownian dynamics simulation to characterize and quantify the effect of various forms of crowding on diffusion and bimolecular association in a simple model of interacting hard spheres. We show that by combining the elastic collision method for hard spheres and the mean field approach for hydrodynamic interaction (HI), our simulations capture the correct dynamics of a monodisperse system. The contributions from excluded volume effect and HI to the crowding effect are thus quantified. The dependence of the results on size distribution of each component in the system is illustrated, and the approach is applied as well to the crowding effect on electrostatic-driven association in both neutral and charged environments; values for effective diffusion constants and association rates are obtained for the specific conditions. The results from our simulation approach can be used to improve the modeling of cell signaling processes without additional computational burdens.
Nonvolatile programmable metallization cell memory switching element based on Ag-doped SbTe solid electrolyte
Authors: Park Young Sam , Lee Seung Yun , Yoon Sung Min , Jung Soon Won , Yu Byoung Gon , Lee Soo Jin , Yoon Soon Gil
Published: October 2007

We report that industrially qualified SbTe chalcogenide film can be applied to programmable metallization cell memory switching device. To fabricate the switching device, Sb35Te65, Ag, and W (top electrode) were consequently sputtered on TiW (bottom electrode)/SiO2∕Si substrate, and Ag diffusion process was not added. During Ag sputtering, it is apparent that Ag is diffused into Sb35Te65 film to form Ag-doped Sb35Te65 solid electrolyte, and that some of the diffused Ag reacts with Te to form Ag–Te bond in the solid electrolyte.
Current-induced cleaning of graphene
Authors: Moser J. , Barreiro A. , Bachtold A.
Published: October 2007

A simple yet highly reproducible method to suppress contamination of graphene at low temperature inside the cryostat is presented. The method consists of applying a current of several milliamperes through the graphene device, which is here typically a few microns wide. This ultrahigh current density is shown to remove contamination adsorbed on the surface. This method is well suited for quantum electron transport studies of undoped graphene devices, and its utility is demonstrated here by measuring the anomalous quantum Hall effect.
“Water bath” effect during the electrical underwater wire explosion
Authors: Oreshkin V. I. , Chaikovsky S. A. , Ratakhin N. A. , Grinenko A. , Krasik Ya. E.
Published: October 2007

The results of a simulation of underwater electrical wire explosion at a current density >109A∕cm2, total discharge current of ∼3MA, and rise time of the current of ∼100ns are presented. The electrical wire explosion was simulated using a one-dimensional radiation-magnetohydrodynamic model. It is shown that the radiation of the exploded wire produces a thin conducting plasma shell in the water in the vicinity of the exploding wire surface. It was found that this plasma shell catches up to 30% of the discharge current. Nevertheless, it was shown that the pressure and temperature of the wire material remain unchanged as compared with the idealized case of the electrical wire explosion in vacuum. This result is explained by a “water bath” effect.
Performance of the correlation consistent composite approach for transition states: A comparison to G3B theory
Authors: Grimes Thomas V. , Wilson Angela K. , DeYonker Nathan J. , Cundari Thomas R.
Published: October 2007

The correlation consistent composite approach (ccCA) was applied to the prediction of reaction barrier heights (i.e., transition state energy relative to reactants and products) for a standard benchmark set of reactions comprised of both hydrogen transfer reactions and nonhydrogen transfer reactions (i.e., heavy-atom transfer, SN2, and unimolecular reactions). The ccCA method was compared against G3B for the same set of reactions. Error metrics indicate that ccCA achieves “chemical accuracy” with a mean unsigned error (MUE) of 0.89kcal∕mol with respect to the benchmark data for barrier heights; G3B has a mean unsigned error of 1.94kcal∕mol. Further, the greater accuracy of ccCA for predicted reaction barriers is compared to other benchmarked literature methods, including density functional (BB1K, MUE=1.16kcal∕mol) and wavefunction-based [QCISD(T), MUE=1.10kcal∕mol] methods.
Ab initio density functional theory applied to quasidegenerate problems
Authors: Grabowski Ireneusz , Lotrich Victor , Bartlett Rodney J.
Published: October 2007

Ab initio density functional theory (DFT), previously applied primarily at the second-order many-body perturbation theory (MBPT) level, is generalized to selected infinite-order effects by using a new coupled-cluster perturbation theory (CCPT). This is accomplished by redefining the unperturbed Hamiltonian in ab initio DFT to correspond to the CCPT2 orbital dependent functional. These methods are applied to the Be-isoelectronic systems as an example of a quasidegenerate system. The CCPT2 variant shows better convergence to the exact quantum Monte Carlo correlation potential for Be than any prior attempt. When using MBPT2, the semicanonical choice of unperturbed Hamiltonian, plays a critical role in determining the quality of the obtained correlation potentials and obtaining convergence, while the usual Kohn-Sham choice invariably diverges. However, without the additional infinite-order effects, introduced by CCPT2, the final potentials and energies are not sufficiently accurate. The issue of the effects of the single excitations on the divergence in ordinary OEP2 is addressed, and it is shown that, whereas their individual values are small, their infinite-order summation is essential to the good convergence of ab initio DFT.
Real-time time-dependent density functional theory approach for frequency-dependent nonlinear optical response in photonic molecules
Authors: Takimoto Y. , Vila F. D. , Rehr J. J.
Published: October 2007

We present ab initio calculations of frequency-dependent linear and nonlinear optical responses based on real-time time-dependent density functional theory for arbitrary photonic molecules. This approach is based on an extension of an approach previously implemented for a linear response using the electronic structure program SIESTA. Instead of calculating excited quantum states, which can be a bottleneck in frequency-space calculations, the response of large molecular systems to time-varying electric fields is calculated in real time. This method is based on the finite field approach generalized to the dynamic case. To speed the nonlinear calculations, our approach uses Gaussian enveloped quasimonochromatic external fields. We thereby obtain the frequency-dependent second harmonic generation β(−2ω;ω,ω), the dc nonlinear rectification β(0;−ω,ω), and the electro-optic effect β(−ω;ω,0). The method is applied to nanoscale photonic nonlinear optical molecules, including p-nitroaniline and the FTC chromophore, i.e., 2-[3-Cyano-4-(2-{5-[2-(4-diethylamino-phenyl)-vinyl] - thiophen-2-yl} - vinyl)-5,5-dimethyl-5H-furan-2-ylidene]- malononitrile, and yields results in good agreement with experiment.
An improved long-range corrected hybrid exchange-correlation functional including a short-range Gaussian attenuation (LCgau-BOP)
Authors: Song Jong-Won , Tokura Seiken , Sato Takeshi , Watson Mark A. , Hirao Kimihiko
Published: October 2007

A new hybrid exchange-correlation functional is presented based on the long-range correction (LC) scheme [H. Iikura et al., J. Chem. Phys. 115, 3540 (2001); Tawada et al., J. Chem. Phys. 120, 8425 (2004)], named LCgau-BOP. The key feature is the use of a two-parameter Gaussian correction to the Coulomb attenuation, which allows a more flexible description of exact exchange at short-range interelectronic separations. The new partitioning preserves 100% exact exchange in the long range, which is known to be important for the success of the LC scheme, with an asymptotic attenuation described by a standard error function with a parameter of 0.42. The LCgau partitioning was optimized for the reproduction of atomization energies over the G2 set and reaction barrier heights over Database/3, and produced results which are superior to B3LYP, CAM-BLYP, and the best LC functionals we are aware of. The results highlight the importance of including a substantial portion of exact exchange in the short range. Using the same parameters, the new functional was tested for the reproduction of geometries, as well as valence, Rydberg and charge-transfer excitations which are known challenges for conventional density functional theory. Our conclusion is that LCgau-BOP can provide a consistently more accurate description of thermochemistries, chemical reactions, and excitation energies than other existing long-range corrected functionals.
Vibrational and thermal effects on the dipole polarizability of methane and carbon tetrachloride from vibrational structure calculations
Authors: Kongsted Jacob , Christiansen Ove
Published: October 2007

We present a theoretical study of vibrational and thermal effects on the dipole polarizability of methane and carbon tetrachloride. Using a fourth order Taylor expansion in rectilinear normal coordinates of the potential and property surfaces we solve the vibrational problem using vibrational structure theory, e.g., through vibrational self-consistent-field or vibrational configuration-interaction theory. For each vibrational state we calculate in addition the vibrational state average polarizability. Constructing the vibrational partition function by “brute force” allows for prediction of thermal effects on the dipole polarizability. The method is not restricted in any way to polarizabilities nor to the specific representation of the potential and property surfaces employed in this work. Any molecular property with a suitable normal coordinate representation may be considered. We discuss the performance of vibrational self-consistent field as compared to vibrational configuration interaction and study in detail the convergence of the former method with respect to the number of vibrational states included in the thermal averaging. Based on calculations including up to 170 000 vibrational self-consistent-field states we present thermal effects on the dipole polarizability of methane and carbon tetrachloride in the temperature ranges 0–1100 and 0–500K, respectively. The predicted thermal effect on the dipole polarizability of methane is found to be ∼0.8% which compare well with previous experimental measurements.
The relative importance of fluid and kinetic frequency shifts of an electron plasma wave
Authors: Winjum B. J. , Fahlen J. , Mori W. B.
Published: October 2007

The total nonlinear frequency shift of a plasma wave including both fluid and kinetic effects is estimated when the phase velocity of the wave is much less than the speed of light. Using a waterbag or fluid model, the nonlinear frequency shift due to harmonic generation is calculated for an arbitrary shift in the wavenumber. In the limit where the wavenumber does not shift, the result is in agreement with previously published work [R. L. Dewar and J. Lindl, Phys. Fluids 15, 820 (1972); T. P. Coffey, Phys. Fluids 14, 1402 (1971)]. This shift is compared to the kinetic shift of Morales and O’Neil [G. J. Morales and T. M. O’Neil, Phys. Rev. Lett. 28, 417 (1972)] for wave amplitudes and values of kλD of interest to Raman backscatter of a laser driver in inertial confinement fusion.
Revisiting the Frenkel-Ladd method to compute the free energy of solids: The Einstein molecule approach
Authors: Vega Carlos , Noya Eva G.
Published: October 2007

In this paper a new method to evaluate the free energy of solids is proposed. The method can be regarded as a variant of the method proposed by Frenkel and Ladd [J. Chem. Phys. 81, 3188 (1984)]. The main equations of the method can be derived in a simple way. The method can be easily implemented within a Monte Carlo program. We have applied the method to determine the free energy of hard spheres in the solid phase for several system sizes. The obtained free energies agree within the numerical uncertainty with those obtained by Polson et al. [J. Chem. Phys. 112, 5339 (2000)]. The fluid-solid equilibria has been determined for several system sizes and compared to the values published previously by Wilding and Bruce [Phys. Rev. Lett. 85, 5138 (2000)] using the phase switch methodology. It is shown that both the free energies and the coexistence pressures present a strong size dependence and that the results obtained from free energy calculations agree with those obtained using the phase switch method, which constitutes a cross-check of both methodologies. From the results of this work we estimate the coexistence pressure of the fluid-solid transition of hard spheres in the thermodynamic limit to be p*=11.54(4), which is slightly lower than the classical value of Hoover and Ree (p*=11.70) [J. Chem. Phys. 49, 3609 (1968)]. Taking into account the strong size dependence of the free energy of the solid phase, we propose to introduce finite size corrections, which allow us to estimate approximately the free energy of the solid phase in the thermodynamic limit from the known value of the free energy of the solid phase with N molecules. We have also determined the free energy of a Lennard-Jones solid by using both the methodology of this work and the finite size correction. It is shown how a relatively good estimate of the free energy of the system in the thermodynamic limit is obtained even from the free energy of a relatively small system.
ICMMM2D: An accurate method to include planar dielectric interfaces via image charge summation
Authors: Tyagi Sandeep , Arnold Axel , Holm Christian
Published: October 2007

An extension of the MMM2D method, called ICMMM2D, is presented to deal with the electrostatic interactions in partially periodic systems that are confined along one direction by two planar dielectric interfaces. The method handles all electrostatic interactions that are induced by the presence of dielectric interfaces by an image charge method. Our method accurately treats repeated image charges under the planar dielectric interfaces as well as the periodic images that are due to the periodic boundary conditions along the other two directions. The scaling of the method with the number of charges, N, is still N5∕3, and the overhead involved approximately doubles the CPU time compared to the original MMM2D method. The errors are fully under control and the error bounds can be preset up to computer accuracy.
Molecular nanopolaritonics: Cross manipulation of near-field plasmons and molecules. I. Theory and application to junction control
Authors: Neuhauser Daniel , Lopata Kenneth
Published: October 2007

Near-field interactions between plasmons and molecules are treated in a simple unified approach. The density matrix of a molecule is treated with linear-response random phase approximation and the plasmons are treated classically. The equations of motion for the combined system are linear, governed by a simple Liouvillian operator for the polariton (plasmon+molecule excitation) dynamics. The dynamics can be followed in time or directly in frequency space where a trace formula for the transmission is presented. A model system is studied, metal dots in a forklike arrangement, coupled to a two level system with a large transition-dipole moment. A Fano-type resonance [Phys. Rev. 103, 1202 (1956)] develops when the molecular response is narrower than the width of the absorption spectrum for the plasmons. We show that the direction of the dipole of the molecule determines the direction the polariton chooses. Further, the precise position of the molecule has a significant effect on the transfer.
Finite electron temperature effects on interferometric and polarimetric measurements in fusion plasmas
Authors: Mirnov V. V. , Ding W. X. , Brower D. L. , Van Zeeland M. A. , Carlstrom T. N.
Published: October 2007

Finite electron temperature effects on interferometry and polarimetry measurements for burning plasma are considered with particular focus on analytically understanding the role of weakly relativistic effects. Development of a new iterative technique, in the limit when the probing wave frequency is much higher than the electron cyclotron frequency, yields the dispersion relation to lowest (linear) order in Te∕mec2⪡1. Perturbative treatment of the wave phase and polarization is presented in a form suitable for interpretation of experimental data. Previous analysis of the problem included nonrelativistic calculations only. Herein, it is shown that relativistic effects are equally important. Theoretical results are in agreement with computations and can be used for benchmarking of ray tracing codes. The implication of finite temperature effects on future burning plasma interferometer diagnostics is discussed.
Dynamics of a one-dimensional model for the emergence of the plasma edge shear flow layer with momentum-conserving Reynolds stress
Authors: Calvo I. , Carreras B. A.
Published: October 2007

A one-dimensional version of the second-order transition model based on the sheared flow amplification by Reynolds stress and turbulence suppression by shearing is presented. The model discussed in this paper includes a form of the Reynolds stress which explicitly conserves momentum. A linear stability analysis of the critical point is performed. Then, it is shown that the dynamics of weakly unstable states is determined by a reduced equation for the shear flow. In the case in which the flow damping term is diffusive, the stationary solutions are those of the real Ginzburg-Landau equation.
Acoustic excitation of superharmonic capillary waves on a meniscus in a planar microgeometry
Authors: Xu Jie , Attinger Daniel
Published: October 2007

The effects of ultrasound on the dynamics of an air-water meniscus in a planar microgeometry are investigated experimentally. The sonicated meniscus exhibits harmonic traveling waves or standing waves, the latter corresponding to a higher ultrasound level. Standing capillary waves with subharmonic and superharmonic frequencies are also observed, and are explained in the framework of parametric resonance theory, using the Mathieu equation.
Study of the first intershell (KLL) dielectronic recombination resonances for Be-, B-, and C-like xenon
Authors: Chen W. D. , Hu W. , Fu Y. Q. , Xiao J. , Liu Y. , Meng F. C. , Shen T. M. , Chen C. Y. , Wu S. , Wei B. , Hutton R. , Zou Y.
Published: October 2007

Dielectronic recombination (DR) processes are very important processes in hot plasmas. Electron beam ion traps are excellent devices for experimentally disentangling the atomic physics processes occurring in hot plasmas. This work reports studies of the KLL DR processes for Be-, B-, and C-like xenon ions using the newly developed electron beam ion trap in Shanghai [X. Zhu et al., Nucl. Instrum. Methods Phys. Res. B 235, 509 (2005)]. X rays from the stabilization of the recombined ions were measured in a direction perpendicular to the electron beam. Angular variation coefficients of the stabilization transitions were calculated and used in the data analysis for extracting DR resonant strengths. The corresponding theoretical DR resonances were obtained, for comparison with the experimental results, based on relativistic configuration interaction approximation.
Electromagnetic geodesic acoustic modes in tokamak plasmas
Authors: Zhou Deng
Published: October 2007

The drift kinetic equation is solved for investigation of the plasma response to electromagnetic geodesic acoustic modes. The plasma flow within magnetic surfaces is considered. A perpendicular magnetic perturbation with poloidal number m=2 is created due to the m=2 parallel return current.
System-size dependence of the free energy of crystalline solids
Authors: de Miguel Enrique , Marguta Ramona G. , del Río Elvira M.
Published: October 2007

We investigate the system-size dependence of the Helmholtz free energy of crystalline solids from computer simulation. We employ a standard thermodynamic integration technique along a reversible path that links the crystalline solid with a noninteracting Einstein crystal with the same structure. The key contribution to the free energy is computed by using the so-called expanded-ensemble technique and the results are compared with those obtained from conventional integration of the derivative of the free energy along the path using Gaussian-Legendre quadrature. We find that both methods yield fully consistent results. The free energy is found to exhibit a strong dependence with system size, in agreement with the behavior found by Polson et al. [J. Chem. Phys. 112, 5339 (2000)] but at variance with the dependence reported more recently by Chang and Sandler [J. Chem. Phys. 118, 8390 (2003)]. This has been tested for the face-centered cubic (fcc) and hexagonal close-packed phases of a crystal of hard spheres at a density close to the melting point. We also investigate any possible dependence of the free energy of the solid phase with the shape of the simulation box. We find that this contribution may not be as important as previous investigations suggest. The present results seem to indicate that there is a non-negligible contribution to the free energy arising from the orientation of the closed-packed crystal layers with respect to the simulation cell. This contribution is particularly noticeable for small system sizes and is believed to be an effect of the periodic boundary conditions used in the simulations. The results presented here corroborate the stability of the fcc phase of the hard-sphere solid close to melting.
Overtone spectroscopy of H2D+ and D2H+ using laser induced reactions
Authors: Asvany Oskar , Hugo Edouard , Müller Frank , Kühnemann Frank , Schiller Stephan , Tennyson Jonathan , Schlemmer Stephan
Published: October 2007

The method of laser induced reaction is used to obtain high-resolution IR spectra of H2D+ and D2H+ in collision with n-H2 at a nominal temperature of 17K. For this purpose three cw-laser systems have been coupled to a 22-pole ion trap apparatus, two commercial diode laser systems in the ranges of 6100–6600cm−1 and 6760–7300cm−1, respectively, and a high-power optical parametric oscillator tunable in the range of 2600–3200cm−1. In total, 27 new overtone and combination transitions have been detected for H2D+ and D2H+, as well as a weak line in the ν1 vibrational band of H2D+ (220←101) at 3164.118cm−1. The line positions are compared to high accuracy ab initio calculations, showing small but mode-dependent differences, being largest for three vibrational quanta in the ν2 symmetric bending of H2D+. Within the experimental accuracy, the relative values of the ab initio predicted Einstein B coefficients are confirmed.
Ab initio molecular dynamics of hydrogen dissociation on metal surfaces using neural networks and novelty sampling
Authors: Ludwig Jeffery , Vlachos Dionisios G.
Published: October 2007

We outline a hybrid multiscale approach for the construction of ab initio potential energy surfaces (PESs) useful for performing six-dimensional (6D) classical or quantum mechanical molecular dynamics (MD) simulations of diatomic molecules reacting at single crystal surfaces. The algorithm implements concepts from the corrugation reduction procedure, which reduces energetic variation in the PES, and uses neural networks for interpolation of smoothed ab initio data. A novelty sampling scheme is implemented and used to identify configurations that are most likely to be predicted inaccurately by the neural network. This hybrid multiscale approach, which couples PES construction at the electronic structure level to MD simulations at the atomistic scale, reduces the number of density functional theory (DFT) calculations needed to specify an accurate PES. Due to the iterative nature of the novelty sampling algorithm, it is possible to obtain a quantitative measure of the convergence of the PES with respect to the number of ab initio calculations used to train the neural network. We demonstrate the algorithm by first applying it to two analytic potentials, which model the H2∕Pt(111) and H2∕Cu(111) systems. These potentials are of the corrugated London-Eyring-Polanyi-Sato form, which are based on DFT calculations, but are not globally accurate. After demonstrating the convergence of the PES using these simple potentials, we use DFT calculations directly and obtain converged semiclassical trajectories for the H2∕Pt(111) system at the PW91/generalized gradient approximation level. We obtain a converged PES for a 6D hydrogen-surface dissociation reaction using novelty sampling coupled directly to DFT. These results, in excellent agreement with experiments and previous theoretical work, are compared to previous simulations in order to explore the sensitivity of the PES (and therefore MD) to the choice of exchange and correlation functional. Despite having a lower energetic corrugation in our PES, we obtain a broader reaction probability curve than previous simulations, which is attributed to increased geometric corrugation in the PES and the effect of nonparallel dissociation pathways.