bluuebunny/crossref_metadata_2025 huggingface dataset:
Bilayer ice and alternate liquid phases of confined water
Authors: Zangi Ronen , Mark Alan E.
Published: July 2003

We report results from molecular dynamics simulations of the freezing and melting, at ambient temperature (T=300 K), of a bilayer of liquid water induced by either changing the distance between two confining parallel walls at constant lateral pressure or by lateral compression at constant plate separation. Both transitions are found to be first order. The system studied consisted of 1200 water molecules that were described by the TIP5P model. The in-plane symmetry of the oxygen atoms in the ice bilayer was found to be rhombic with a distorted in-registry arrangement. Above and below the stability region of the ice bilayer we observed two bilayer phases of liquid water that differ in the local ordering at the level of the second shell of nearest neighbors and in the density profile normal to the plane, yielding two liquid phases with different densities. These results suggest the intriguing possibility of a liquid–liquid transition of water, confined to a bilayer, at regions where the ice bilayer is unstable with respect to either of the liquid phases. In addition, we find that under the same conditions, water confined to 3–8 layers remains in the liquid phase (albeit stratification of the transverse density profile) with values of the lateral diffusion coefficient comparable to that of the bulk.
Periodic trends in the bonding and vibrational coupling: Pyridine interacting with transition metals and noble metals studied by surface-enhanced Raman spectroscopy and density-functional theory
Authors: Wu De-Yin , Ren Bin , Xu Xin , Liu Guo-Kun , Yang Zhi-Lin , Tian Zhong-Qun
Published: July 2003

Pyridine adsorbed on the group-VIIIA transition metal and the coinage metal (IB) electrode surfaces is employed as a model system to explore the metal-dependent nature of surface bonding. The periodic trends of the surface-enhanced Raman spectra are interpreted with the density-functional theory calculation results of bond geometries, bond energies, vibrational frequencies, and the force constants. On the basis of the normal-mode analysis the vibrational frequency shifts of the pyridine internal modes with the total symmetry are discussed in detail from the coupling between these internal modes and the N–M bonds. We show that the spectral properties of the ring breath mode (ν1) and the ring asymmetric deformation mode (ν6a) are sensitive to the interaction between metal and pyridine molecule. Although the frequency shift of the ν1 mode is generally regarded as an indicator of the bonding strength, we find that the frequency shift of the ν6a mode has a nearly linear dependence on the force constant of the N–M bond; and thus conclude that ν6a is a better indicator than ν1 for the strength of pyridine binding with the metals.
A scaling function of nucleation barrier based on the diffuse interface theory
Authors: Kusaka Isamu
Published: July 2003

We present a phenomenological approach to determine the scaling function for the free energy barrier of fluid phase nucleation. The essential ingredients of our approach are (1) thermodynamic consistency imposed at the phase coexistence and at the mean-field spinodal, (2) the assumed quadratic functional form of the scaling function, and (3) the diffuse interface theory applied at the low supersaturation limit. The accuracy of the scaling function was tested for simple fluids, i.e., square-well fluids and truncated and shifted Lennard-Jones fluids. Despite the simplicity of the approach, the resulting scaling function is moderately successful in reproducing free energy barrier calculated by density functional theory for both vapor phase condensation and bubble nucleation, though the agreement is worse for the latter case.
Transport phenomena and microscopic structure in partially miscible binary fluids: A simulation study of the symmetrical Lennard-Jones mixture
Authors: Das Subir K. , Horbach Jürgen , Binder Kurt
Published: July 2003

Static and dynamic structure factors and various transport coefficients are computed for a Lennard-Jones model of a binary fluid (A,B) with a symmetrical miscibility gap, varying both the temperature and relative concentration of the mixture. The model is first equilibrated by a semi-grandcanonical Monte Carlo method, choosing the temperature and chemical potential difference Δμ between the two species as the given independent variables. Varying for Δμ=0 the temperature and particle number N over a wide range, the location of the coexistence curve in the thermodynamic limit is estimated. Well-equilibrated configurations from these Monte Carlo runs are used as initial states for microcanonical molecular dynamics runs, in order to study the microscopic structure and the behavior of transport coefficients as well as dynamic correlation functions along the coexistence curve. Dynamic structure factors Sαβ(q,t) [and the corresponding static functions Sαβ(q)] are recorded (α,β∈A,B), q being the wave number and t the time, as well as the mean square displacements of the particles (to obtain the self-diffusion constants DA, DB) and transport coefficients describing collective transport, such as the interdiffusion constant and the shear viscosity. The minority species is found to diffuse a bit faster than the majority species. Despite the presence of strong concentration fluctuations in the system the Stokes–Einstein relation is a reasonable approximation.
Reorganization energy of intermolecular electron transfer in solvents near isotropic/nematic transition
Authors: Lilichenko Mark , Matyushov Dmitry V.
Published: July 2003

Monte Carlo simulations of the fluid of dipolar spherocylinders have been carried out to produce structure factors of longitudinal polarization fluctuations in the region of orientational isotropic/nematic phase transition. These data are used to calculate the solvent reorganization energy of intermolecular electron transfer. The reorganization energy shows substantial anisotropy in the nematic phase in respect to the angle between the direction of electron transfer and nematic director. This anisotropy is sufficient to change the rate of electron transfer by orders of magnitude by reorienting the donor–acceptor complex relative to the nematic director.
Polyelectrolyte solutions with added salt: A simulation study
Authors: Liu S. , Ghosh K. , Muthukumar M.
Published: July 2003

Using Langevin dynamics simulations, we have investigated the distribution of counterions around a flexible polyelectrolyte chain as a function of polymer concentration (Cp), salt concentration (Cs), and valency of the counterion from the added salt. In the present simulations, the aqueous solutions are at room temperatures and polymer concentrations are either below or comparable to overlap concentrations. The net polymer charge and the radius of gyration (Rg) of a labeled chain are found to decrease with an increase in either Cp or Cs. We present details of the distribution of monovalent and divalent counterions inside the counterion worm surrounding a polymer chain, when a salt-free solution of polyelectrolytes with monovalent counterions is challenged by a salt with divalent counterions. The simulation results for the dependence of Rg on chain length (N), Cp and Cs are compared with the theory of Muthukumar [J. Chem. Phys. 86, 7230 (1987); 105, 5183 (1996)] which contains two parameters, viz., degree of ionization (α) and strength (w) of excluded volume interaction. Using the values of α and w as determined by simulations, there is a very good agreement between theoretical predictions and simulations for monovalent counterions. For the case of divalent counterions there is evidence for significant bridging between polymer segments mediated by counterions. This bridging leads to an enhanced shrinkage of polymer size beyond expected from averaged electrostatic screening. A mean-field counting of the bridging effect as an effective two-body attraction leads to a good agreement between theory and simulations.
Quantum instanton approximation for thermal rate constants of chemical reactions
Authors: Miller William H. , Zhao Yi , Ceotto Michele , Yang Sandy
Published: July 2003

A quantum mechanical theory for chemical reaction rates is presented which is modeled after the [semiclassical (SC)] instanton approximation. It incorporates the desirable aspects of the instanton picture, which involves only properties of the (SC approximation to the) Boltzmann operator, but corrects its quantitative deficiencies by replacing the SC approximation for the Boltzmann operator by the quantum Boltzmann operator, exp(−βĤ). Since a calculation of the quantum Boltzmann operator is feasible for quite complex molecular systems (by Monte Carlo path integral methods), having an accurate rate theory that involves only the Boltzmann operator could be quite useful. The application of this quantum instanton approximation to several one- and two-dimensional model problems illustrates its potential; e.g., it is able to describe thermal rate constants accurately (∼10–20% error) from high to low temperatures deep in the tunneling regime, and applies equally well to asymmetric and symmetric potentials.
Multilayer formulation of the multiconfiguration time-dependent Hartree theory
Authors: Wang Haobin , Thoss Michael
Published: July 2003

A multilayer (ML) formulation of the multiconfiguration time-dependent Hartree (MCTDH) theory is presented. In this new approach, the single-particle (SP) functions in the original MCTDH method are further expressed employing a time-dependent multiconfigurational expansion. The Dirac–Frenkel variational principle is then applied to optimally determine the equations of motion. Following this strategy, the SP groups are built in several layers, where each top layer SP can contain many more Cartesian degrees of freedom than in the previous formulation of the MCTDH method. As a result, the ML-MCTDH method has the capability of treating substantially more physical degrees of freedom than the original MCTDH method, and thus significantly enhances the ability of carrying out quantum dynamical simulations for complex molecular systems. The efficiency of the new formulation is demonstrated by converged quantum dynamical simulations for systems with a few hundred to a thousand degrees of freedom.
Quantum initial value representations using approximate Bohmian trajectories
Authors: Bittner Eric R.
Published: July 2003

Quantum trajectories, originating from the de Broglie–Bohm hydrodynamic description of quantum mechanics, are used to construct time-correlation functions in an initial value representation. The formulation is fully quantum mechanical and the resulting equations for the correlation functions are similar in form to their semiclassical analogs but do not require the computation of the stability or monodromy matrix or conjugate points. We then move to a local trajectory description by evolving the cumulants of the wave function along each individual path. The resulting equations of motion are an infinite hierarchy, which we truncate at a given order. We show that time-correlation functions computed using these approximate quantum trajectories can be used to accurately compute the eigenvalue spectrum for various potential systems.
Forward–backward semiclassical dynamics for condensed phase time correlation functions
Authors: Wright Nicholas J. , Makri Nancy
Published: July 2003

The forward–backward semiclassical dynamics (FBSD) scheme for obtaining time correlation functions shows much promise as a method for including quantum mechanical effects into the calculation of dynamical properties of condensed phase systems. By combining this scheme with a discretized path integral representation of the Boltzmann operator one is able to calculate correlation functions at finite temperature. In this work we develop constant temperature molecular dynamics techniques for sampling the phase space and path integral variables. The resulting methodology is applied to the calculation of the velocity autocorrelation function of liquid argon. At the chosen state point the FBSD results are in good agreement with classical trajectory predictions, but the existence of a non-negligible imaginary part of the correlation function illustrates the importance of proper density quantization even under nearly classical conditions.
Calculation of electric properties using regular approximations to relativistic effects: The polarizabilities of RuO4, OsO4, and HsO4 (Z=108)
Authors: Filatov Michael , Cremer Dieter
Published: July 2003

Analytic expressions for the derivatives of the total molecular energy with respect to external electric field are derived within the regular approximation to the full four-component relativistic Hamiltonian and presented in matrix form suitable for implementation in standard quantum-chemical codes. Results of benchmark calculations using the infinite-order regular approximation with modified metric method are presented and discussed. The static electric dipole polarizabilities of group VIII metal tetroxides MO4 for M=Ru, Os, Hs (Z=108) are studied with the help of second-order Møller–Plesset perturbation theory using the infinite-order regular approximation with modified metric Hamiltonian. The polarizabilities obtained vary in the sequence RuO4>OsO4>HsO4, which is different from those obtained in other studies. However, it is in line with calculated T21←A11 excitation energies of the group VIII tetroxides, which provide a measure for the magnitude of their polarizabilities.
Linear scaling for the local energy in quantum Monte Carlo
Authors: Manten Sebastian , Lüchow Arne
Published: July 2003

The scaling of the diffusion quantum Monte Carlo method can be greatly improved when localized orbitals and short-range correlation functions are employed as recently suggested by the authors. The local diffusion quantum Monte Carlo method is described in detail with a careful analysis of errors. The new method achieves near linear scaling in the calculation of the local energy. Results demonstrating the improved performance are presented.
NO
2
:
Global potential energy surfaces of the ground (1 2A1) and the first excited (1 2B2) electronic states
Authors: Kurkal Vandana , Fleurat-Lessard Paul , Schinke Reinhard
Published: July 2003

Global ab initio potential energy surfaces for the ground (1 2A1) and the first excited (1 2B2) electronic states of NO2 are presented. The calculations are performed at a multireference configuration interaction level with the correlation consistent quadruple zeta atomic basis set of Dunning (cc-pVQZ). A total of about 5000 ab initio points were calculated. The dissociation energy corresponding to NO2→NO+O and other features of the surfaces are found to be in close agreement with experimental values. The adiabatic ground and the first excited electronic states form a conical intersection and have been diabatized to form two coupled diabatic surfaces. Quantum dynamics calculations are performed to obtain the fundamental frequencies and the energies of low lying bound states. Comparison of bound state energies with experimental values show good agreement.
Dielectric properties of Pr2O3 high-k films grown by metalorganic chemical vapor deposition on silicon
Authors: Lo Nigro Raffaella , Raineri Vito , Bongiorno Corrado , Toro Roberta , Malandrino Graziella , Fragalà Ignazio L.
Published: July 2003

Praseodymium oxide (Pr2O3) thin films have been deposited on Si(100) substrates by metalorganic chemical vapor deposition using praseodymium tris-2,2,6,6-tetramethyl-3,5-heptandionate as source material. Film structural, morphological, and compositional characterizations have been carried out. Dielectric properties have been studied as well by capacitance–voltage and current–voltage measurements on metal-oxide-semiconductor capacitors of several areas. The Pr2O3 films have shown a dielectric constant ε=23–25 and a leakage current density of 8.8×10−8 A/cm2 at +1 V.
Molecular transport in nanopores
Authors: Bhatia Suresh K. , Nicholson David
Published: July 2003

Simulation of the transport of methane in cylindrical silica mesopores have been performed using equilibrium and nonequilibrium molecular dynamics (NEMD) as well as dual control volume grand canonical molecular dynamics methods. It is demonstrated that all three techniques yield the same transport coefficient even in the presence of viscous flow. A modified locally averaged density model for viscous flow, combined with consideration of wall slip through a frictional condition, gives a convincing interpretation of the variation of the transport coefficient over a wide range of densities, and for various pore sizes and temperatures. Wall friction coefficients extracted from NEMD simulations are found to be consistent with momentum transfer arguments, and the approach is shown to be more meaningful than the classical slip length concept.
Distinguishing the H3 and T4 silicon adatom model on 6H–SiC(0001) √3×√3R30° reconstruction by dynamic rocking beam approach
Authors: Xie X. N. , Yakolev Nikolai , Loh Kian Ping
Published: July 2003

Silicon adatoms can occupy either the H3 or T4 site, corresponding to the hollow or on-top site of the hexagonal unit cell of the 6H–SiC(0001)−√3×√3R30° superstructure. Distinguishing these two possibilities is impossible with the one-beam calculation method in surface electron diffraction. We provide the experimental evidence to differentiate between these two possibilities using a dynamic, multiple rocking beam approach and demonstrate the sensitivity of this approach to the lateral displacement of atoms on the surface. Our study shows that the rocking curve based on the T4 model provides a more convincing fit to the experiment compared with the H3 model, with a metric distance as low as 7%. We also identify A-type termination to be the most likely bulk-truncated substrate face among the three possible truncated faces for the 6H–SiC polytype. Coverage dependence of the silicon adatoms on the profile of the rocking curve is also investigated.
Study of He flow properties to test He dimer potentials
Authors: Pedemonte L. , Bracco G.
Published: July 2003

The parameters which characterize the energy distribution of a supersonic helium beam are measured at different source parameters. The data are compared with the results of calculations based on three different He–He interatomic potentials in order to test their ability to describe the helium free jet expansion. This article follows a previous paper where calculations were performed at source temperatures between 20 and 80 K using the the Lennard-Jones and the Tang–Toennies–Yiu potential [K. T. Tang, J. P. Toennies, and C. L. Yiu, Phys. Rev. Lett. 74, 1546 (1995)] but no satisfactory agreement was found. Here, calculations are presented also for a potential curve recently proposed by Hurly and Moldover [J. J. Hurly and M. R. Moldover, J. Res. Natl. Inst. Stand. Technol. 105, 667 (2000)]. The source temperature range where calculations are performed is extended between 6 and 300 K in order to compare the predictions of the three potentials with other measurements present in literature. Possible experimental limitations to the beam performances are discussed and in particular the skimmer interference is taken into account. As the considered potentials only partially describe the experiments, a phenomenological viscosity cross section is proposed which represents in a satisfactory way the He flow properties over the whole range of source temperatures.
Effect of cyclic chain architecture on properties of dilute solutions of polyethylene from molecular dynamics simulations
Authors: Jang Seung Soon , Çağin Tahir , Goddard William A.
Published: July 2003

We have used molecular dynamics methods to investigate the effects of cyclic chain architecture on the properties of dilute solutions. In order to include solvent effects in estimating these properties, we use a van der Waals scaling factor determined for each solvent by matching to the theta condition. We predict that the theta temperature (θ) of cyclic PE (c-PE) is ∼10% lower than for the linear case (l-PE). This can be compared to the experimental results for polystyrene (PS), where θ for cyclic PS is 2% lower. For conditions corresponding to n-pentane solvent, we predict that 〈Rg2〉cyclic/〈Rg2〉linear is 0.59 for all temperatures above 350 K. The deviation from the ratio of 0.50–0.53 expected from analytic theory is due to the competition between chain stiffness and excluded volume effects. To calculate the intrinsic viscosity of c-PE and l-PE we extended the Bloomfield–Zimm type theory to include chain stiffness corrections. We find that for the theta temperature, the ratio of viscosities for c-PE and l-PE is 0.71, which is 7% higher than the value of 0.66 from the freely jointed chain model. This difference is caused by the larger value of 〈Rg2〉cyclic/〈Rg2〉linear from the simulations.
Solution reaction path Hamiltonian based on reference interaction site model self-consistent field method: Application to Menshutkin-type reactions
Authors: Ohmiya Kazuhiro , Kato Shigeki
Published: July 2003

A reaction path Hamiltonian model is developed for describing chemical reaction dynamics in solution based on reference interaction site model self-consistent field (RISM-SCF) method. A systematic procedure is devised to determine all the quantities involved in the present Hamiltonian by the RISM-SCF electronic structure calculations for reaction systems combined with molecular dynamics simulations for bulk solvents. The present model is applied to the Menshutkin-type reactions between NH3 and methylhalides CH3X (X=Cl, Br, and I) in aqueous solution. The barrier height becomes lower with the increase of the size of leaving halogen atom, and the calculated activation energy for the CH3I reaction is in a good agreement with the experiment. The magnitude of the time dependent as well as the static solvent friction is CH3Cl<CH3Br<CH3I in the increasing order. The frequency dependent solute–solvent coupling term is found to be almost a linear function of the reaction coordinate. The transmission coefficient is estimated to be 0.84, 0.81, and 0.60, respectively, for the reactions of CH3Cl,CH3Br, and CH3I by trajectory calculations with the present reaction path Hamiltonian model. The calculated transmission coefficients are compared with those by the Grote-Hynes and Kramers’ theory.
Correlation function-based finite-difference time-domain method for simulating ultrashort pulse propagation. I. Formalism
Authors: Gruetzmacher Julie A.
Published: July 2003

A finite-difference time-domain formalism for simulating coherent linear pulse propagation is presented that incorporates a medium response described by any two-time energy gap correlation function. Two algorithms, for real and complex correlation functions, are developed to evaluate the electric polarization through explicit treatment of the density matrix for a two-level system. The coherence relaxation terms in the resulting finite-differenced Maxwell–Liouville equations depend on integrals over the energy gap fluctuation correlation function. The algorithms are used to simulate ultrashort mid-infrared pulse propagation through optically dense samples of HDO in liquid D2O as a demonstration of their performance and flexibility. These algorithms represent a first step toward the goal of incorporating complicated material responses into the full-field simulation of nonlinear pulse propagation and nonlinear optical spectroscopy.
Variational formulation for the electrostatic potential in dielectric continua
Authors: Attard Phil
Published: July 2003

The thermodynamic potential for fixed charges in dielectric continua is given for arbitrary values of the polarization density. Minimization of the functional gives the equilibrium polarization density (equivalently image charges) and the electrostatic potential throughout the media. The functional, which in general involves volume integrals, for the case of piecewise uniform dielectrica is reduced to surface integrals of the constrained surface charge density at the dielectric discontinuities. A linear integral equation for the induced surface charge, which can be solved by iteration, is given. The conjugate thermodynamic force for the constrained surface charge is also given. The latter formulation is suitable for Car–Parrinello or Lagrangian molecular dynamics in complex geometries.
The crystal structure of methane phase III
Authors: Neumann Marcus A. , Press Werner , Nöldeke Christian , Asmussen Bernd , Prager Michael , Ibberson Richard M.
Published: July 2003

Methane is the simplest organic molecule, and like many supposedly simple molecular materials it has a rich phase diagram. While crystal structures could be determined for two of the solid phases, that of the low temperature phase III remained unsolved. Using high-resolution neutron powder diffraction and a direct-space Monte Carlo simulated annealing approach, this fundamental structure has now finally been solved. It is orthorhombic with space group Cmca, and 16 molecules in the unit cell. The structure is closely related to that of phase II, yet is no subgroup of it.
Explicitly correlated divide-and-conquer-type electronic structure calculations based on two-electron reduced density matrices
Authors: Janesko Benjamin G. , Yaron David
Published: July 2003

A method for constructing a description of a large N-electron system from explicitly correlated calculations on O(N) overlapping subsystems is developed. One- and two-electron reduced density matrices for the system are constructed using the reduced density matrices obtained from explicitly correlated calculations on the subsystems. This “locally correlated reduced density matrix” method is closely related to divide-and-conquer treatments of mean-field and density-functional theory, and to wave-function-based O(N) treatments of correlation. Calculations on toy systems indicate that the method recovers a complete description of local correlation effects, is size extensive, and can be formally linear scaling.
Assessment of density functional methods for nuclear magnetic resonance shielding calculations
Authors: Magyarfalvi Gábor , Pulay Peter
Published: July 2003

Several different methods for calculating nuclear magnetic resonance shieldings in density functional theory are compared. All methods were implemented using gauge-including atomic orbitals to eliminate gauge dependence. The calculated shielding tensors are compared to accurate coupled cluster results with perturbative triples correction and to experimental data. We have implemented a simple method based on a uniform shift of virtual orbital energies and determined the optimum level shift in small molecules. Of the methods tested, the uniform level shift and procedure of Wilson, Amos, and Handy show the best performance. Malkin’s correction also gave good results. The optimized exchange functional of Cohen and Handy, in conjunction with the Lee–Yang–Parr (LYP) correlation functional, gives a marked improvement over the Becke exchange plus LYP.
Intermolecular potential and the equation of state of solid C60
Authors: Hasegawa M. , Nishidate K. , Katayama M. , Inaoka T.
Published: July 2003

First-principles total-energy calculations are performed for the low-temperature ordered phase of solid C60 in the wide range of lattice parameter. The intermolecular potential obtained from these calculations is successfully modeled in terms of the interaction between carbon atoms on different C60 molecules and the multipole Coulombic interaction between molecules. This model enables us to calculate structural and thermodynamic properties of both the orientationally ordered and disordered phases in a consistent manner. We find that the equation of state at finite temperature obtained for this potential is not in good agreement with experiments and the intermolecular potential is much shallower than that expected from the experimental heat of sublimation. These discrepancies are interpreted as arising from the limited capability of the density-functional calculations to appropriately incorporate the effect of electron correlations at large separation, which is responsible for the long-range behavior of the van der Waals interaction between molecules. In order to circumvent this difficulty we take an empirical approach to estimate the additional van der Waals interaction, which is not taken into account in the current density-functional calculations.
Density functional study of the insertion mechanism for ethylene-styrene copolymerization with constrained geometry catalysts
Authors: Yang Sung Hoon , Jo Won Ho , Noh Seok Kyun
Published: July 2003

The ethylene-styrene copolymerization with two constrained geometry catalysts (CGCs), CGC1 and CGC2 was investigated by the density functional theory. Structures and energetics of reactants, π complexes, transition states, and products during insertion of ethylene and/or styrene monomers into the catalytic reactive site of the CGCs are analyzed using the software DMOL3. The general gradient approximation correction was applied to the total energy calculation after being calculated at the local density approximation level. The relativistic effective core potential is used for titanium while all other electrons are treated explicitly. Structures at stationary points on the energy profile were generated by minimizing the energy with respect to all the degrees of freedom except for the reaction coordinate which is defined as the distance between the Cα carbon and the nearest carbon of inserting monomer. The CGC2 with an electron-rich ligand shows a weaker coordination with monomers, resulting in longer Ti–Cp, Ti–Ph, and Ti–Cγ distances and higher energy states. The electron-rich styrene monomer forms a stronger π complex than ethylene, whereas the insertion of styrene through the complex into the product suffers a higher energy barrier than the insertion of ethylene. All of the transition states show a typical four-membered ring structure and are stabilized by an α- or phenyl-agostic interaction. The γ- or phenyl-agostic interaction stabilizes the product. From analysis of the relative energy, it is found that both CGCs yield E–S copolymers with sequences of ethylene blocks separated by styrene. More interestingly, CGC1 shows higher activity for all insertion processes than CGC2, while CGC2 is more effective than CGC1 in producing E–S copolymers with higher composition of styrene.
Nuclear spin–spin coupling density functions and the Fermi hole
Authors: Soncini A. , Lazzeretti P.
Published: July 2003

Nuclear spin–spin coupling density functions yield a three-dimensional picture of the interaction between two nuclear dipole moments mediated by electron spin density. A physical interpretation of the Fermi contact coupling density maps can be readily arrived at on account of the Fermi correlation between same-spin electrons as the mechanism whereby the spin polarization induced about one nucleus is transmitted to another nucleus coupled to it. It is shown that the Fermi hole density function, evaluated by an opportune choice of the reference electron, is characterized by morphological aspects very similar to those appearing in the plots of one- and two-bonds Fermi contact density functions. A comparison has been made for hydrogen fluoride, water, ammonia, and methane molecules at the Hartree–Fock level of theory. The results confirm the role of the Fermi correlation as the fundamental vehicle propagating nuclear-spin/electron-spin contact interaction, i.e., the process mainly responsible for nuclear spin–spin coupling. The plots of Fermi hole density show that the geminal H–H coupling would not be possible without the essential contribution of the spin density in the vicinity of the heavier nucleus. The combined use of Fermi contact density functions and Fermi hole distributions yields a very promising approach to the study of nuclear magnetic resonance coupling constants, and provides a sound physical basis for their interpretation.
Pressure dependence of the high-frequency light scattering susceptibility of ortho-terphenyl: A mode coupling analysis
Authors: Patkowski A. , Matos Lopes M. , Fischer E. W.
Published: July 2003

The high frequency part of the depolarized light scattering susceptibility has been measured for ortho-terphenyl (OTP) in a broad temperature and pressure range and analyzed using the mode-coupling theory (MCT). We found that the time–temperature–pressure superposition does not work for OTP in the frequency range of the susceptibility minimum and both a and b exponents of the MCT are temperature and pressure dependent. The pressure induced shift of the position of the minimum is much weaker than that of the α-peak resulting in dTc/dP=8.5 K/kbar while dTg/dP=26 K/kbar. Thus any universal scaling of the entire susceptibility including both the α-peak and the MCT minimum is not possible. These results are in contradiction to the previously reported conclusions obtained from the analysis of incoherent neutron scattering data [A. Tölle et al., Phys. Rev. Lett. 80, 2374 (1998)].
Adsorption of apolar molecules at the water liquid–vapor interface: A Monte Carlo simulations study of the water-n-octane system
Authors: Jedlovszky Pál , Varga Imre , Gilányi Tibor
Published: July 2003

The adsorption of n-octane at the water liquid–vapor interface has been investigated by Monte Carlo computer simulation. For this purpose, simulation of five different water–apolar interfacial systems have been performed, in which the number of n-octane molecules has been varied. The results clearly show that the apolar n-octane molecules are adsorbed from the vapor phase at the interface. The adsorption is driven by the weak attraction due to the dispersion forces acting between the water molecules and the methyl and methylene groups of the octanes. This weak attraction is, however, amplified by the fact that it is added up for the CH2 and CH3 groups belonging to the same molecule. Consistently, the n-octane molecules located closest to the aqueous phase are found to prefer all-trans conformation and parallel alignment with the plane of the interface. On the other hand, entropic effects become more important among the molecules of the outer part of the adsorption layer. Hence, the preferred orientation of these molecules is perpendicular to the interface, as they can be extended toward the less dense region of the apolar phase; and gauche dihedrals appear more frequently here than among the molecules located next to the aqueous phase.
Global geometry optimization of silicon clusters described by three empirical potentials
Authors: Yoo S. , Zeng X. C.
Published: July 2003

The “basic-hopping” global optimization technique developed by Wales and Doye is employed to study the global minima of silicon clusters Sin(3⩽n⩽30) with three empirical potentials: the Stillinger–Weber (SW), the modified Stillinger–Weber (MSW), and the Gong potentials. For the small-sized SW and Gong clusters (3⩽n⩽15), it is found that the global minima obtained based on the basin-hopping method are identical to those reported by using the genetic algorithm [Iwamatsu, J. Chem. Phys. 112, 10976 (2000)], as well as with those by using molecular dynamics and the steepest-descent quench (SDQ) method [Feuston, Kalia, and Vashishta, Phys. Rev. B 37, 6297 (1988)]. However, for the mid-sized SW clusters (16⩽n⩽20), the global minima obtained differ from those based on the SDQ method, e.g., the appearance of the endohedral atom with fivefold coordination starting at n=17, as opposed to n=19. For larger SW clusters (20⩽n⩽30), it is found that the “bulklike” endohedral atom with tetrahedral coordination starts at n=20. In particular, the overall structural features of SW Si21, Si23, Si25, and Si28 are nearly identical to the MSW counterparts. With the SW Si21 as the starting structure, a geometric optimization at the B3LYP/6-31G(d) level of density-functional theory yields an isomer similar to the ground-state- isomer of Si21 reported by Pederson et al. [Phys. Rev. B 54, 2863 (1996)].
Temperature dependence of local segmental motion in polystyrene and its variation with molecular weight
Authors: Roland C. M. , Casalini R.
Published: July 2003

Dielectric measurements are reported for the α-relaxation in polystyrene (PS) of varying molecular weights. Although the segmental relaxation dispersion was essentially invariant to Mw, the Tg-normalized temperature dependence (fragility) increases systematically with molecular weight. The latter result corroborates dynamic mechanical and light scattering studies, but is at odds with the reported (T−Tg) superpositioning of the shift factors for the recoverable creep compliance of PS. The failure of the dielectric relaxations time to superimpose when expressed as a function of T−Tg is consistent with the analysis of equation of state data for PS. We find that volume is not the dominant control variable, and in fact, temperature exerts a stronger influence on the relaxation times.
Characterization of the CCCl radical in the X̃ 2A′ state by Fourier-transform microwave spectroscopy and ab initio calculations
Authors: Sumiyoshi Yoshihiro , Ueno Taketoshi , Endo Yasuki
Published: July 2003

Pure rotational transitions of a new carbon-chain radical, CCCl, a chlorine derivative of CCH, have been observed for the first time by Fourier-transform microwave spectroscopy. The radical has been produced in a supersonic free jet by a pulsed discharge in CCl4 diluted to 0.3% with Ne. Transitions with spin splittings were observed for two isotopomers, CC35Cl and CC37Cl, in the regions at 12.6 GHz for N=1–0 and 25.2 GHz for N=2–1, respectively. The radical shows a spectral pattern for a molecule with Σ2 symmetry as is the case for CCH. Hyperfine splittings due to the Cl nucleus were also clearly resolved. The molecular constants have been precisely determined for the two isotopomers. Ab initio calculations at the MRCI level with the cc-pVTZ basis set have revealed that the first excited electronic state corresponding to the Π2 state at linear geometry is very close to the ground electronic state, and the two states are more strongly interacting with each other than the case of CCH. Based on the results of the ab initio calculations and the determined hyperfine constants, it was found that a conical intersection exists due to a strong vibronic coupling in the vicinity of the ground state, and as a result the CCCl radical has a bent structure in the ground state.
The bond-forming reaction between CF22+ and H2O/D2O: A computational and experimental study
Authors: Lambert Natalie , Kaltsoyannis Nikolas , Price Stephen D.
Published: July 2003

Ground-state stationary points on the potential energy surface of the reaction CF22++H2O→OCF++HF+H+ were calculated using the density-functional theory hybrid method B3LYP and the ab initio coupled cluster singles and doubles with perturbative triples [CCSD(T)] algorithm. The calculations reveal a reaction mechanism involving two transition states. The first transition state involves the migration of one hydrogen within the primary collision complex and the second corresponds to the loss of a proton. The neutral HF molecular product is formed in its stable ground Σ1 state. Comparison of activation energies for the reactions of CF22+ with H2O and with D2O, calculated from Becke three parameter Lee–Yang–Parr (B3LYP) zero-point energies, slightly favor the H2O pathway by 0.04 and 0.07 eV for the first and second activations, respectively. Rate constant calculations using Rice–Ramsperger–Kassel–Marcus/quasiequilibrium theory also kinetically favor the H2O pathway in comparison with the D2O pathway. However, the magnitudes of the calculated rate constants are so large (1012–1014 s−1) that the differences between the rates of reaction of CF22+ with H2O and with D2O should not be distinguished by a crossed-beam time-of-flight mass spectrometer experiment. Indeed, the ion yields reported in this paper from new collision experiments between CF22+ and D2O showed no isotope effect when compared with previous data from collisions of CF22+ with H2O.
Peptide structure determination by two-dimensional infrared spectroscopy in the presence of homogeneous and inhomogeneous broadening
Authors: Bredenbeck Jens , Hamm Peter
Published: July 2003

The potential information content of two-dimensional infrared (2D-IR) spectroscopy of the amide-I band as a structure analysis method of small peptides is explored in a computational study, applying it to a cyclic penta peptide as an example. In the presence of realistic homogeneous and inhomogeneous broadening, the structure resolution power in the case of a nonisotope labeled molecule would be vanishingly small. However, 2D-IR spectroscopy can reveal the structure of the peptide uniquely if using a sufficiently large set of isotope labeled compounds. Design strategies for isotope labeling are developed. In the case of the cyclic penta peptide studied here, at least three single C13 labeled compounds would be needed to determine the structure. While double C13 labeling does not offer any advantage compared to single C13 labeling, mixed C13O16–C12O18 or C13O16–C13O18 double labeling does. It is furthermore explored to what extent a structure can still be determined even under nonideal conditions, i.e., if systematic errors in the molecular models are allowed or if the molecule is allowed to coexists in different conformations simultaneously.
Role of cyclic sets of transition dipoles in the pump–probe polarization anisotropy: Application to square symmetric molecules and perpendicular chromophore pairs
Authors: Qian Wei , Jonas David M.
Published: July 2003

Theoretical models considering inhomogeneous energy splitting effects are developed to explain impulsive pump–probe polarization anisotropy experiments on degenerate states of square symmetric molecules and chromophore pairs with perpendicular transition dipoles. When detecting the pump-induced change in probe transmission through the sample, the pump–probe signal arises from lost absorption due to ground state depopulation, stimulated emission from the singly excited states, and absorption from the singly excited states to doubly excited states. Including coherence between the singly excited states, the total impulsive initial anisotropy is 2/5 in all cases. In contrast to the first and second contributions, the time dependent excited state absorption anisotropy depends on a cyclic set of four transition dipoles, not just the energetic splitting. Ignoring molecular rotation, inhomogeneous anisotropy dynamics arise only for coupled chromophore pairs and Jahn–Teller distortions of the degenerate state that produce a cyclic set of transition dipoles different from the set for uncoupled direct product states. In all systems treated, a time dependent pump–probe anisotropy requires coupling between excitations.
Amide I modes of tripeptides: Hessian matrix reconstruction and isotope effects
Authors: Ham Sihyun , Cha Sangyob , Choi Jun-Ho , Cho Minhaeng
Published: July 2003

For seven representative tripeptide conformations, we carried out ab initio geometry optimizations and vibrational analyses. By using the Hessian matrix reconstruction method developed in the present paper, both the diagonal and off-diagonal coupling force constants of a given tripeptide were calculated and the relationship between the three-dimensional conformation and local amide I mode frequency was discussed. Isotope labeling effects on both the amide I local and normal modes were elucidated. It was found that the C=O bond length of a given peptide, which represents the structural distortion induced by neighboring peptides, is linearly proportional to the local amide I mode frequency. A theoretical model for predicting the local amide I mode frequencies is discussed and compared with the ab initio calculation results.
Three-qubit nuclear magnetic resonance quantum information processing with a single-crystal solid
Authors: Leskowitz Garett M. , Ghaderi Nima , Olsen Ryan A. , Mueller Leonard J.
Published: July 2003

We report the first implementation of NMR quantum information processing in a static single-crystal solid, illustrating pseudopure state preparation and gate manipulations on a three-qubit system, U–13C2, N15-glycine (H215N13CH213COOH). We consider issues important to single-crystal NMR implementations of quantum information processing and introduce several innovations specific to such implementations. In particular, concurrent cross-polarization of both the C13 and N15 spins from proton magnetization enhances and equalizes populations on these nuclei in a single step. This multiple cross-polarization step simplifies preparation of pseudopure spin states relative to previously described multiple-pulse sequences. Also, proton decoupling, which prolongs coherence during spin-evolution and detection periods, is switched off to dephase unwanted coherences during preparation of pseudopure spin states. Such “relaxation averaging” (or T2 averaging) is an alternative to spatial averaging, which requires special hardware and large gradient strengths for small crystals.
Time delay as a tool to identify the signatures of reactive resonance: F+HD and F+H2 reactions
Authors: Chao Sheng Der , Skodje Rex T.
Published: July 2003

The formalism of scattering time delay is investigated as a tool to identify the signatures of resonance in reactive molecular collisions. The concept of state and angle resolved time delay is reviewed and applied to model problems. Several numerical pathologies of the time delay formalism are discussed that potentially may obscure a resonance signature, or may lead to a false positive result. The time delay is computed explicitly for the F+HD→HF+D and F+H2→HF+H reactions using the results of full scattering calculations on the Stark–Werner potential energy surface. The reactive resonance known to exist for the F+HD reaction is clearly apparent both in the state resolved and angle resolved time delay functions. On the other hand, the analogous resonance in the F+H2 reaction is masked in the state resolved time delay function through the influence of a nearby energetic threshold. However, the angle resolved time delay does provide a clear signature of the resonance, thus settling a controversy on the existence of a resonance for the F+H2 system.