bluuebunny/crossref_metadata_2025 huggingface dataset:
Nitridation of a Si(100) surface by 100–1000 eV N+2 ion beams
Authors: Kusunoki I. , Takaoka T. , Igari Y. , Ohtsuka K.
Published: July 2002

The nitridation mechanism of silicon at room temperature under exposure to 100–1000 eV N+2 ion beams has been studied in situ in an ultrahigh vacuum apparatus using x-ray photoelectron spectroscopy. The increase of the nitrogen content in a surface layer as a function of the ion dose was described by a simple formula which was derived by assuming random occupation of the reaction sites in the penetration zone of the nitrogen atoms. A change of the binding energy and the width of the N1s x-ray photoelectron spectrum during the reaction was observed and discussed with the component ratio N/Sireacted. The Si2p x-ray photoelectron spectra were deconvoluted into five components of Si(0), Si(1), Si(2), Si(3), and Si(4) by curve fitting, where Si(n) represents the component of Si bonded to n nitrogen atoms. Their populations were dependent on the ion dose and the ion energy. The nitride layers formed in the Si surface with low energy beams of 100–200 eV had near-stoichiometric composition of Si3N4. With beams of energy higher than 300 eV, however, they were nonstoichiometric compounds SiNy (y<1.3) which were mixtures of those components. The influence of the beam energy was observed by the chemical shifts of the N1s and Si2p peaks at the saturation of the N content.
Accelerated equilibration of polymer melts by time-coarse-graining
Authors: Forrest Bruce M. , Suter Ulrich W.
Published: July 2002

An effective-potential approach is presented for improving sampling efficiency in simulations of atomistically detailed models of dense long-chain liquids. The motion of atoms on short-time scales is described as rapid fluctuations about the slowly moving mean conformations of the chain molecules. The distribution of these fluctuations is approximated by that of isotropic elastic motion. The interactions between nonbonded pairs of atoms are preaveraged over this distribution and a much softer, effective interaction is obtained, allowing a correspondingly faster exploration of configuration space. The approximate sampling scheme is then tested on two model systems of united-atom liquid hydrocarbons—a melt of twenty C24 chains and one of ten C71 chains. In the C24 melt, where a comparison with fully equilibrated samples from rigorous algorithms is possible, the preaveraging is shown to produce an improvement in sampling efficiency of up to an order of magnitude at the expense of only a moderate loss in accuracy. The method is then applied to the C71 melt, where currently available, rigorous sampling algorithms fail to produce thorough equilibration. Also in this case, the preaveraging method proves to significantly enhance the sampling speed while producing satisfactory distributions of observables.
Understanding barriers to internal rotation in substituted toluenes and their cations
Authors: Lu Kueih-Tzu , Weinhold Frank , Weisshaar James C.
Published: July 2002

In substituted toluenes, the potential energy barrier to internal methyl rotation and the preferred methyl conformation depend on the position of the fluorine, amino, or methyl substituents and also on the electronic state, either S0, S1, or ground state cation. We present a unified picture of the electronic factors controlling these effects. In S0 and cation, ab initio electronic structure calculations of modest scale produce rotor potentials in good agreement with experiment. The methyl group provides a sensitive probe of local ring geometry. When the geometry of the ring in the vicinity of the rotor has good local C2v symmetry, the barrier is invariably small. In S0 ortho-substituted toluenes, we use natural steric analysis to show that repulsive steric interactions between the halogen lone pair and the methyl CH bonds dominate over attractive donor–acceptor interactions to favor the pseudo-trans conformation. When steric interactions are unimportant, the key determinant of rotor barrier height is the difference in π-bond order between the two ring CC bonds nearest methyl.

The barrier height is proportional to the calculated bond order difference, with slope of 950 cm−1 per bond. Attractive donor–acceptor interactions favor the conformation that places the rotor CH bond cis to the ring CC bond of higher order, analogous to the localized case of 2-methylpropene. In toluene cations, π-ionization creates a pattern of long and short ring CC bonds. Simple molecular orbital theory readily explains the coarse bond-order patterns calculated for ortho- and meta-substituted toluene cations. A localized picture of π-bonding from natural resonance theory explains more subtle details of the distribution of CC bond orders about the ring. When π-ionization places the methyl group between ring CC bonds of quite different order, a substantial barrier results. This explains the strong preference of m-fluorotoluene+ for the pseudo-cis conformation and contributes to the preference of o-fluorotoluene+ for the pseudo-trans conformation. Finally, we speculate that a similar molecular orbital argument applied to S1 might explain the observed characteristic changes in barrier height on S1–S0 excitation of ortho- and meta-substituted toluenes.
A density functional theory study of the alkali metal atom–carbon monoxide interactions: Singularity of the Li atom
Authors: Pullumbi P. , Bouteiller Y. , Perchard J. P.
Published: July 2002

The LiCO molecule has been previously characterized experimentally in rare gas matrices by a large red shift of the CO stretching mode and a Li–C vibration around 600 cm−1 indicating a relatively strong Li–C bonding. Up to now, post Hartree–Fock ab initio methods had described this complex as unstable in its electronic ground state and predicted a linear metastable structure owing to strong induction forces between this complex and the surrounding polarizable rare gas atoms. Electronic and vibrational calculations at the harmonic approximation on this molecule using approximate density functional theory give results which are better in line with the experimental results. Two distinct bounded structures are found to be stable in the electronic ground state for the LiCO complex. The credibility of these calculations is also assessed in two ways; the first one is the comparison between experimental and calculated vibrational analysis on the νCO modes of the Li(CO)n (n=2,3) complexes. The second one is the study of the eventual complexation of Na and K atoms with the CO molecule and the comparison with experimental available data.
Electron nuclear dynamics of H++H2 collisions at Elab=30 eV
Authors: Morales Jorge , Diz Agustin , Deumens E. , Öhrn Yngve
Published: July 2002

Proton collisions with hydrogen molecules at 30 eV in the laboratory frame is a simple ion-molecule system exhibiting a number of distinct processes such as inelastic scattering, charge transfer, rearrangement, and dissociation. The electron nuclear dynamics (END) theory which allows full electron nuclear coupling and which does not restrict the system from reaching any of the possible product channels, is applied to this sytem to produce transition probabilities, differential, and integral (vibrationally resolved) cross sections. Comparisons with experiment demonstrate that END, even in its simplest implementation, with a single determinantal state for the electrons and with classical nuclei, yields results that are competitive with other theoretical approaches.
Calculation of the third virial coefficient for water
Authors: Kusalik P. G. , Liden F. , Svishchev I. M.
Published: July 2002

Although the second virial coefficient has been used frequently to test intermolecular potentials, the direct calculation of third virial coefficients has only been attempted for atomic or simple (with linear symmetry) multipolar fluids. In this paper a general approach is described for the numerical evaluation of the third virial coefficient for molecular systems. Specific results for the second and third virial coefficients for several different water models are reported and compared with experimental data; critical points are also estimated. A polarizable model is found to be superior to standard effective liquid-phase potentials in reproducing the equation of state for steam.
A combinatorial algorithm for effective generation of long maximally compact lattice chains
Authors: Ramakrishnan R. , Pekny J. F. , Caruthers J. M.
Published: July 2002

We investigate the problem of generation of maximally compact lattice chains which are useful in understanding folding of model proteins. The term, maximally compact chain, refers to a lattice self-avoiding walk that visits every lattice site. Generation of a representative sample of compact conformations is extremely difficult by conventional simulation methods such as static growth methods or dynamic Monte Carlo techniques. Growing a random walk is ineffective for generating long walks in a compact shape because a large number of walks are rejected due to overlap (attrition). In the interest of an unbiased sample, one needs to enumerate all possible compact conformations that are realizable or produce a representative sample, the former of which is intractable for long chains. In this paper a method is proposed for generation of compact chains on a lattice based on a mathematical programming approach. The method, which we refer to as the Hamiltonian path generation method, generates a random sample of lattice filling self-avoiding walks. A detailed description of a randomized generation algorithm is presented, which is effective for producing a static sample of compact lattice chains. There is a statistical evidence of fair generation of conformations from the conformational space using this scheme. This method generates a compact conformation on a 60×60×60 cubic lattice in forty minutes on a Sparc-2 workstation.
Adiabatic and strictly diabatic potential curves of He2+2
Authors: Kolbuszewski Marcin , Gu J.-P.
Published: July 2002

Adiabatic and strictly diabatic potential curves were calculated for the two lowest electronic states of He22+. The diabatization procedure leads to a repulsive diabatic curve dissociating to He+ + He+ and a bound curve dissociating to He2+ + He, crossing at the internuclear distance of 1.55 bohr. The diabatic bound state has dissociation energy of 3.15 eV. The existence of a metastable potential well in the (adiabatic) X1Σg+ state of He22+ can be interpreted in terms of a strong interaction between the diabatic curves.
Experimental and theoretical determination of the temperature dependence of deuteron and oxygen quadrupole coupling constants of liquid water
Authors: Ludwig R. , Weinhold F. , Farrar T. C.
Published: July 2002

Quadrupole coupling constants, χQ, for the deuteron and the oxygen nuclei in neat, liquid water were determined by both theoretical and experimental methods. The theoretical values of χD=264 kHz and χO=8.4 MHz obtained from ab initio calculations at the MP2/6-31+G* level in combination with a quantum cluster equilibrium model for liquids are in good agreement with results from NMR relaxation time experiments. Both theory and experiment show no observable temperature dependence of the quadrupole coupling constants. The theoretical values reported here for the oxygen quadrupole coupling constant and both the oxygen and deuterium asymmetry parameters are different from values obtained from ab initio calculations of clusters using molecular dynamics methods. This may be due to the use of pairwise additive potentials in the molecular dynamics simulations which do not take into account many-body or polarizability effects.
A comparative analysis by means of quantum molecular similarity measures of density distributions derived from conventional ab initio and density functional methods
Authors: Solà Miquel , Mestres Jordi , Carbó Ramon , Duran Miquel
Published: July 2002

A procedure based on quantum molecular similarity measures (QMSM) has been used to compare electron densities obtained from conventional ab initio and density functional methodologies at their respective optimized geometries. This method has been applied to a series of small molecules which have experimentally known properties and molecular bonds of diverse degrees of ionicity and covalency. Results show that in most cases the electron densities obtained from density functional methodologies are of a similar quality than post-Hartree–Fock generalized densities. For molecules where Hartree–Fock methodology yields erroneous results, the density functional methodology is shown to yield usually more accurate densities than those provided by the second order Mo/ller–Plesset perturbation theory.
A correlated ab initio study of the X̃ 2A1 and Ã 2E states of MgCH3
Authors: Woon David E.
Published: July 2002

The X̃ 2A1 and Ã 2E states of the MgCH3 radical have been studied with correlation consistent basis sets and the coupled cluster method RCCSD(T) in order to compare with two recent experimental efforts [M. A. Anderson and L. M. Ziurys, Astrophys. J. 452, L157 (1995); R. Rubino, J. M. Williamson, and T. A. Miller, J. Chem. Phys. 103, 5964 (1995)]. The best computed values [RCCSD(T)/cc-pCVTZ] for the X̃ 2A1 state are (experimental results in parentheses): Ae=160.433 GHz, Be=10.948 GHz (B0=11.008 GHz), and μe=1.011 D. The Mg–CH3 bond is weak, 26.3 kcal/mol. Values for the Ã 2E state are Ae=154.648 GHz (A0=149.666 GHz), Be=10.87 GHz (B0=10.932 GHz), and μe=1.022 D. The excitation energy (Te) for the Ã 2E←X̃ 2A1 transition is 19 999 cm−1 (T00=20 030 cm−1). A brief discussion of bonding trends in Mg-containing radials is included.
Theory of gas–gas phase transition in rare-gas binary mixtures
Authors: Strigari Lidia , Rovere Mauro , D’Aguanno Bruno
Published: July 2002

The lines of critical points of rare-gas binary mixtures are evaluated within the framework of a statistical mechanics theory of thermodynamic stability, recently developed by Chen and Forstmann. The phase instabilities are related to the local compositional and density order in the fluid through a density functional expansion of the grand canonical potential. The structural inputs are determined by using the thermodynamically self-consistent integral equation scheme known as hypernetted chain/mean spherical approximation (HMSA). A good agreement is obtained with neutron scattering data of He–Xe and Ne–Xe mixtures near the phase transition. Although the accuracy in reproducing the experimental critical point lines is only semi-quantitative, the qualitative behaviors are properly described. For the first time, a detailed microscopic interpretation of the so called behavior of the ‘‘second type’’ of the critical line is given.
Equilibrium conformations and dynamic relaxation of double-tethered chain molecules at an impenetrable interface
Authors: Gulati Harpreet S. , Hall Carol K. , Jones Ronald L. , Spontak Richard J.
Published: July 2002

In this work, the static conformational properties and dynamic relaxation behavior of monolayers of chain molecules grafted at both ends (polymer ‘‘loops’’) to an impenetrable plane are investigated utilizing off-lattice discontinuous molecular dynamics (DMD) simulations. The conformational properties observed from DMD simulation are compared to the results of a previous on-lattice bond fluctuation (BF) study. This provides a unique opportunity to decouple the effects of excluded volume, chain flexibility and interparticle interactions, since these effects are treated differently in the two simulation methods. Static equilibrium properties of the looped chain layers determined from the DMD and BF models are in qualitative agreement with each other and with self-consistent field (SCF) predictions for brushes of N/2 repeat units. By allowing the chain anchors to move laterally along the interfacial plane, the effect of annealing on layer properties is also investigated. The characteristic relaxation time of the annealed looped chains, discerned from radius of gyration autocorrelation functions, obeys a scaling relationship of the form τ∼N3σ4/3, where σ is the surface density of the tethered chain ends. The lateral self-diffusion coefficient of the chains during annealing is also measured, with lateral diffusivity scaling as Nασβ. The exponents α and β undergo a smooth transition from α≊−4/3, β≊−4/3 at low densities to α≊−2, β≊−2 at high densities. Both relaxation times and lateral diffusivity exhibit different density scaling than brushes with N/2 repeat units.
A generalized Warburg impedance for a nonvanishing relaxation process
Authors: Ramos-Barrado J. R. , Galán Montenegro P. , Cambón C. Criado
Published: July 2002

For an electrochemical system with a nonvanishing relaxation time we obtain a generalized Warburg impedance from a hyperbolic diffusion equation. The generalized Warburg impedance determined in this way presents a slight positive curvature in the complex plane with smaller imaginary value at high frequencies.
The absolute cross sections of photoabsorption, photodissociation, and photoionization of the group VIB metal hexacarbonyls at 300–1600 Å
Authors: Qi Fei , Yang Xin , Yang Shihe , Liu Fuyi , Sheng Liusi , Gao Hui , Zhang Yunwu , Yu Shuqin
Published: July 2002

The absolute photoabsorption cross sections σa and photoionization quantum yields η of Cr(CO)6, Mo(CO)6, and W(CO)6 are measured in the wavelength range of ∼300–1600 Å. We have also estimated the photoionization cross sections σi and the neutral photodissociation cross sections σd from σa and η. Main absorption features are tentatively assigned with the help of the MO diagram and the photoelectron spectroscopic data of M(CO)6(M=Cr,Mo,W). Large optical oscillator strength distributions and neutral photodissociation cross sections are obtained at the photon energies far beyond the first ionization potential. Our studies revealed the valence excitation nature of the superexcited states in the extreme ultraviolet region. It is suggested that the neutral photodissociation in this region shares a similarity with that in the UV region below the first ionization potential because they both involve a repulsive ejection of the first CO ligand.
Using cluster studies to approach the electronic structure of bulk water: Reassessing the vacuum level, conduction band edge, and band gap of water
Authors: Coe James V. , Earhart Alan D. , Cohen Michael H. , Hoffman Gerald J. , Sarkas Harry W. , Bowen Kit H.
Published: July 2002

Aqueous cluster studies have lead to a reassessment of the electronic properties of bulk water, such as band gap, conduction band edge, and vacuum level. Using results from experimental hydrated electron cluster studies, the location of the conduction band edge relative to the vacuum level (often called the V0 value) in water has been determined to be −0.12 eV⩽V0⩽0.0 eV, which is an order of magnitude smaller than most experimental values in the literature. With V0=−0.12 eV and making use of the calculated solvation energy of OH in water, the band gap of water is determined to be 6.9 eV. Again, this is smaller than many literature estimates. In the course of this work, it is shown that due to water’s ability to reorganize about charge (1) photoemission thresholds of water or anionic defects in water do not determine the vacuum level, and (2) there is almost no probability of accessing the bottom of the conduction band of water with a vertical/optical process from water’s valence band. The results are presented in an energy diagram for bulk water which shows the utility of exploring the conduction band of water as a function of solvent polarization.
Equation of state of supercooled water simulated using the extended simple point charge intermolecular potential
Authors: Harrington Stephen , Poole Peter H. , Sciortino Francesco , Stanley H. Eugene
Published: July 2002

We carry out extensive molecular dynamics simulations in order to evaluate the thermodynamic equation of state of the extended simple point charge model of water (customarily described by the acronym SPC/E) over a wide range of temperature and density, with emphasis on the supercooled region. We thereby determine the location of the temperature of maximum density (TMD) line and the liquid spinodal line. In particular, we find that the experimental TMD line lies between the TMD lines of the SPC/E and ST2 models of water, so perhaps the behavior of these two models of simulated water “bracket” the behavior of real water. As temperature decreases, we find (i) that maxima appear in isotherms of the isothermal compressibility as a function of density, (ii) that isotherms of the internal energy as a function of volume display negative curvature and (iii) that the pressure of the liquid–vapor spinodal decreases. We compare the results to corresponding behavior found from simulations of the ST2 model of water and find that the behavior of SPC/E, when shifted to higher values of temperature and pressure (ΔP≈50 MPa and ΔT≈80 K), approximates that of ST2. We discuss the implications of our results for the hypothesis that a critical point occurs in the phase diagram of supercooled water. Finally, we argue that the results of our simulations are not inconsistent with the possibility that C′ exists for SPC/E water.
Surface composition and structure of GaN epilayers on sapphire
Authors: Ahn J. , Sung M. M. , Rabalais J. W. , Koleske D. D. , Wickenden A. E.
Published: July 2002

The surface composition and structure of GaN films grown on sapphire substrates by organometallic vapor-phase epitaxy (OMVPE) have been determined through the use of time-of-flight scattering and recoiling spectrometry (TOF-SARS), classical ion trajectory simulations, and low-energy electron diffraction (LEED). TOF-SARS spectra of scattered and recoiled ions plus fast neutrals were collected using 4 keV Ar+ primary ions. The scattering results were simulated by means of the three-dimensional scattering and recoiling imaging code (SARIC). This data leads to the conclusions that both N-terminated {001̄}-(1×1) and Ga-terminated {0001}-(1×1) surfaces occur, however no evidence was obtained for mixed terminations. No relaxation or reconstruction was detected on either surface, although both surfaces exhibited two structural domains. The {0001̄} surfaces are well-ordered and contained hydrogen atoms bound to the N atoms of the outermost layer. The {0001} surfaces are highly reactive towards adsorption of carbon and oxygen from residual gases, however unlike the {0001̄} surfaces, they adsorb very little hydrogen. These Ga-terminated surfaces are stabilized and obtain more ordered structures as a result of the contamination.
Chain correlations in linear and ring polymer blends
Authors: Garas George E. , Kosmas Marios K.
Published: July 2002

Our recent studies on linear–polymer blends are extended to include the linear/ring and ring/ring polymer systems. By means of the free energies of mixing, ring–polymer blends are found to be the most deviating from the Flory–Huggins theory presenting χ-parameters (interaction parameters) that depend more strongly on concentrations and molecular weights. The free energies are found to be smaller in the case of the ring–polymer blends, i.e., these blends are predicted to be the most stable, in accord with cloud-point experiments. The related neutron scattering χeff parameters are found to present higher concentration dependence than the χ-parameters of these systems. By means of their mean-square radii of gyration, both ring and linear chains are found to contract when a binary polymer system approaches the phase separation limit. Close to the spinodal, the contraction is relatively stronger for the chains of the smaller concentrations and lengths, while the three systems obtain an ideal behavior at the large molecular weight, N→∞, limit.
Far-infrared spectra and associated dynamics in acetonitrile–water mixtures measured with femtosecond THz pulse spectroscopy
Authors: Venables D. S. , Schmuttenmaer C. A.
Published: July 2002

We report the frequency-dependent absorption coefficient and index of refraction in the far-infrared region of the spectrum for mixtures of acetonitrile and water. The mixtures do not behave ideally, and deviate from ideality most noticeably for mixtures that are between 25% and 65% acetonitrile by volume. Two implementations of the Debye model for describing the dielectric relaxation behavior of mixtures are compared, and we show that these mixtures are better treated as uniform solutions rather than as two-component systems. We find an enhanced structure in the mixtures, relative to ideal mixtures, but we do not find direct evidence for microheterogeneity. The Debye time constant for the primary relaxation process for the mixtures is up to 25% longer than that for an ideal mixture.
Molecular similarity based on information entropies and distances
Authors: Hồ Minhhuy , Smith Vedene H. , Weaver Donald F. , Gatti Carlo , Sagar Robin P. , Esquivel Rodolfo O.
Published: July 2002

A similarity index based on the concept of an information distance is evaluated for a series of diatomic molecules. The results show that the momentum space information distance is a good indicator of the extent of charge separation present in a molecule. The Shannon entropy in position space is shown to decrease with increasing charge separation and as such is presented as a measure of the structure or extent of electron localization in a system. The concept of a local Shannon entropy over an atomic basin is introduced and it is shown how this quantity may be used to examine the local behavior of electron densities.
Calculation of solid-fluid phase equilibria for systems of chain molecules
Authors: Polson James M. , Frenkel Daan
Published: July 2002

We study the first order solid-fluid phase transition of a system of semi-flexible Lennard-Jones chains using molecular dynamics simulations. Thermodynamic integration methods are used to calculate the free energy of the solid and fluid phases. The solid phase free energy per chain can be calculated to an accuracy of ±0.03kBT with relative ease. The Gibbs-Duhem integration technique is used to trace out the complete melting curve, starting with a single point on the curve obtained from the free energy calculations. For the short chains studied here, we find that increasing the chain length stabilizes the solid phase; i.e., it raises the melting temperature at fixed pressure, and lowers the density at the transition at fixed temperature. Gibbs-Duhem integration was used also to investigate the effects of chain stiffness on the transition. We find that increasing the stiffness also acts to stabilize the solid phase. At fixed temperature, the transition is shifted to lower pressure and lower density with increasing chain stiffness. Further, we find that the density gap between solid and fluid broadens with increasing chain stiffness.
Radial compression and controlled cutting of carbon nanotubes
Authors: Lordi Vincenzo , Yao Nan
Published: July 2002

Utilizing high-resolution transmission electron microscopy, in tandem with molecular dynamics simulations, we conduct a fundamental study of the mechanical force–structure relationship of carbon nanotubes. We study the response of tubes to asymmetrical radial compressive forces, which are directly related to observed structure distortions in HRTEM (high-resolution transmission electron microscopy). We show that the magnitude of such forces can be readily estimated and is related to the shape of the distortion, characterized by percent compression and the radius of the tube, as well as the number of layers for very narrow tubes. The elasticity and resilience of the walls depend on the tube radius and the number of layers. We further suggest that even very large forces produce reversible, elastic distortions, indicating that radial mechanical forces should not be a feasible method to cut a nanotube without causing severe structural damage.
Numerical study of scattering from chains composed of two nonsegregated labeled blocks in an athermal solvent: Effects of compositional heterogeneity
Authors: Molina Luis A. , Freire Juan J.
Published: July 2002

Numerical calculations and Monte Carlo simulations have been performed to explore the dynamic scattering behavior of solutions of copolymer chains composed by two nonsegregated blocks in an athermal solvent. The experimental investigation of this property for real copolymers has shown the presence of a variety of modes, with a complex variation of their locations and intensities, which we try to partially understand with the present simulation work. For this purpose, we use chain samples with a small amount of polydispersity in the block lengths. We have introduced opposite scattering contrast factors for the A and B units. In this way, zero-averaged contrast conditions have been set for the overall scattering, avoiding the presence of the collective mode that otherwise would manifest for any nondilute system of polymer chains. Static and dynamic scattering functions have been obtained for the different samples. For the dynamic scattering functions, three modes are observed in agreement with existing experiments. The first two modes, predominant for low values of the scattering variable, q, are assigned to correspond to the compositional heterogeneity and the first internal relaxation of the chains, as predicted by the theory. The variation of the mode intensities and positions with polymer concentration is analyzed. It semiquantitatively agrees with the existing experimental data.
Critical behavior of uniaxial–biaxial nematic phase transition
Authors: Mukherjee Prabir K.
Published: July 2002

A model considering both rodlike molecules and platelike molecules is presented in this paper. The model is treated in the framework of a Landau mean-field theory and renormalization group theory. Mean-field theory predicts the transition between the isotropic, rod nematic, plate nematic, and biaxial nematic phases. The renormalization group analysis shows that the transition exhibits a multiphase bicritical point for n (dimensionality of the order parameter) ⩽2. However, for n>2, the transition exhibits a tetracritical point and an intermediate biaxial nematic phase can exist. The critical exponents are calculated to first order in ε (=4−d).
The exchange energy functional in a weak magnetic field
Authors: Harris Robert A. , Salsbury Freddie R.
Published: July 2002

A density functional theory for the Kohn–Sham exchange energy of a bounded, closed shell system in a weak, uniform, magnetic field is presented. The form obtained vanishes when the electron density is radial and, unlike the unscreened exchange energy of a locally uniform electron gas, does not diverge due to the Coulomb interaction. The role of the exchange-correlation functional in the context of magnetic response theory is also examined.
Segment connectivity, chain-length breathing, segmental stretch, and constraint release in reptation models. I. Theory and single-step strain predictions
Authors: Hua Chi C. , Schieber Jay D.
Published: July 2002

A self-consistent reptation theory that accounts for chain-tube interactions, segment connectivity, chain-length breathing, segmental stretch, and constraint release is proposed. Simulation results are compared semiquantitatively to experimental observations in single-step strain flows. Since stochastic simulation techniques are used, no approximations, such as independent alignment or consistent averaging are needed to obtain results. The simulation results show excellent agreement with experimental trends in shear and normal stress relaxations, including the second normal stress difference, well into the nonlinear regime. For most of these experiments, the original Doi and Edwards theory, which incorporates independent alignment or consistent averaging, is not satisfactory. In the following companion paper, we show how the model is capable of describing double-step-strain flows for all stress components. Subsequent papers show excellent agreement for the inception of steady shear flow, and steady shear flow.
Calculation of conformational transitions and barriers in solvated systems: Application to the alanine dipeptide in water
Authors: Apostolakis Joannis , Ferrara Philippe , Caflisch Amedeo
Published: July 2002

Optimal free energy paths (OFEPs) for conformational transitions are parallel to the mean force at every nonstationary point of the free energy landscape. In contrast to adiabatic paths, which are parallel to the force, OFEPs include the effect of entropy and are relevant even for systems with diffusive degrees of freedom. In this study the OFEPs are computed for the alanine dipeptide in solution. The potential of mean force is calculated and an effective potential is derived that is used to obtain the paths with a minimization based algorithm. The comparison of the calculated paths with the adiabatic paths in vacuo shows the influence of the environment on conformational transitions. The dynamics of the alanine dipeptide in water are more complex, since there are more minima and the barriers are lower. Two simpler methods for the calculation of reaction pathways in solution are evaluated by comparing their results with the OFEPs. In the first method the mean electrostatic field of the water is approximated by an analytical continuum model. The obtained paths show qualitative agreement with the OFEPs and the height of the barriers are similar. Targeted molecular dynamics (TMD), the second approach, constrains the distance to a target conformation to accelerate the transition. In the general case, however, it is difficult to assess the physical significance of the obtained paths. Changing the initial conditions by assigning different velocities leads to different solutions for the conformational transition. Furthermore, it is shown that by performing the simulations with different reaction coordinates or in opposite directions different pathways are preferred. This result can be explained by the structure of the free energy landscape around the initial conformations. In a first approximation the physical significance of different pathways is assumed to depend mainly on the free energy at the highest saddle point. In the literature the total energy of the system has often been used to estimate the position and the height of the energy barriers in the path. By comparing the total energy with the calculated free energy it is shown that the former largely overestimates the height of the barriers. Furthermore, the positions of the maxima of the total energy do not coincide with the free energy barriers. Simple approximations to the free energy lead to good quantitative agreement.
Monolayers of ortho-H2, para-H2, para-D2 and normal-H2 adsorbed on NaCl(001) single crystal surfaces
Authors: Heidberg J. , Voßberg A. , Hustedt M. , Thomas M. , Briquez S. , Picaud S. , Girardet C.
Published: July 2002

Monolayers of pure ortho-H2, para-H2, para-D2 and normal-H2 adsorbed on NaCl(001) single crystal surfaces were investigated at temperatures T⩾10 K and pressures between 1×10−10 and 1×10−5 mbar by polarization Fourier transform infrared attenuated total internal reflection spectroscopy (PIRS-ATR). Single induced polarized infrared absorptions of the fundamental vibrational modes of pure (M=|1|) ortho-H2 and para-H2, as well as pure (M=|1|) para-D2 and, slightly shifted, in the mixture of normal-H2 were observed, allowing definite assignments of each. With respect to the gas phase values, the wave numbers were measured to be redshifted by 32.0 and 28.0 cm−1 for pure (M=|1|) ortho-H2 and para-H2. In close agreement with potential calculations, these absorptions were assigned to H2 molecules adsorbed on the cation sites of the large NaCl(001) terraces, while neither the absorptions associated with H2 adsorption above defects nor the absorptions of (M=0) H2 molecules above the anion sites, detected on microcrystallite surfaces at lower temperature, are observed under the conditions set. The present experiments corroborate the fact that the NaCl surface discriminates between the (M=|1|) ortho-, (M=0) ortho- and para- species. The lateral interactions have no influence on the stable sites and the discrimination, but they contribute by an additional shift of the peak frequencies of about 15%. Moreover, with polarized light, induced transition dipole moments perpendicular to the surface for ortho-H2, para-H2 and para-D2 were observed, while none parallel to the surface was detected. These results are in accordance with the calculations showing that the s-component of the polarization disappears due to free rotation of the H2 (D2) molecules in a plane parallel to the surface, while the p-component gives a maximum intensity. The integrated cross sections σ̄⊥ were measured and found to have the same order of magnitude: (6±3)×10−22 m/molecule for (M=|1|) ortho-H2 and (4±2)×10−22 m/molecule for para-H2 within the error limits; the theoretical value is 4.5×10−22 m/molecule for both species. Applying the extended Langmuir equation, the difference between the desorption energies (Edesortho−Edespara) of (M=|1|) ortho-H2 and para-H2 adsorbed on the (001) terraces of NaCl(film) was determined to be 0.6±0.2 kJ/mol, in agreement with the calculated value (0.8 kJ/mol). Assuming that the sticking coefficients of (M=|1|) ortho-H2 and para-H2 are not dependent on temperature between 17 and 30 K, the ratio sortho/spara of 0.06±0.03 was obtained.
Correlation problems and a correlation-free torsion-rotation Hamiltonian for a molecule with an internal rotor
Authors: Duan Yun-Bo , Wang Li , Mukhopadhyay Indranath , Takagi Kojiro
Published: July 2002

To develop a correlation-free reduced form of torsion-rotation Hamiltonian that is suitable for analyzing observed spectra for methanol and its isotopic species, the origin of various classes of correlation problems often encountered in fitting the molecular constants to experimental data are analyzed for a molecule with an internal rotation. It is shown that the correlation problems can be completely eliminated by considering either the data set or the reduction of the Hamiltonian together with practical considerations, where an appropriate definition of order of magnitude plays an important role. For a molecule with C3v(M) symmetry such as methanol and its isotopic species, it is found that some terms which are included in the traditional or a reduced Hamiltonian given so far should not be used simultaneously. For example, the traditional terms k5Pa2(1−cos 3γ) and k6Pa{Pγ,1−cos 3γ} should not be used simultaneously as adjustable ones and one of three traditional terms k6Pa{Pγ,1−cos 3γ}, k7{Pγ2,1−cos 3γ}, and (V6/2)(1−cos 6γ) must be constrained. A proposed correlation-free Hamiltonian is tested for its usefulness by analyzing observed data for O-18 methanol. Total 424 microwave lines with vt⩽2, J⩽20, and K⩽9 are globally fitted to an root-mean square (RMS) deviation of 0.234 MHz including the Q-branch transitions for A species with vt=1 and K=9←8 which are successfully fitted to the experimental accuracy.
Monte Carlo simulations of copolymers at homopolymer interfaces: Interfacial structure as a function of the copolymer density
Authors: Werner A. , Schmid F. , Müller M.
Published: July 2002

By means of extensive Monte Carlo simulations of the bond fluctuation model, we study the effect of adding AB diblock copolymers on the properties of an interface between demixed homopolymer phases. The parameters are chosen such that the homopolymers are strongly segregated, and the whole range of copolymer concentrations in the two phase coexistence region is scanned. We compare the “mushroom” regime, in which copolymers are diluted and do not interact with each other, with the “wet brush” regime, where copolymers overlap and stretch, but are still swollen by the homopolymers. A “dry brush” regime is never entered for our choice of chain lengths. “Intrinsic” profiles are calculated using a block analysis method introduced by us in earlier work. We discuss density profiles, orientational profiles, and contact number profiles. In general, the features of the profiles are similar at all copolymer concentrations, however, the profiles in the concentrated regime are much broader than in the dilute regime. The results compare well with self-consistent field calculations.
Binary nucleation kinetics. IV. Directional properties and cluster concentrations at the saddle point
Authors: Wilemski Gerald
Published: July 2002

Using a new approach, Stauffer’s expression for the rate of steady state binary nucleation and Trinkaus’s expression for the steady state cluster concentrations f are derived directly from the diffusion equation that governs the evolution of f in composition space. The behavior of Φ (≡f/N, where N is the equilibrium cluster concentration) is explored since this function provides a characterization of the nucleating binary system that, to lowest order, is independent of the actual composition of the mother phase. The angle ω that describes the direction of ∇Φ at the saddle point differs, in general, from the angle φ found by Stauffer for the direction of the nucleation current at the saddle point. These two angles are related by the formula: tan φ=r tan ω, where r is the ratio of impingement frequencies defined by Stauffer. In general, at the saddle point, ∇Φ also fails to lie in the direction of steepest descent on the free energy surface.
Phase behavior of grafted chain molecules: Influence of head size and chain length
Authors: Stadler C. , Schmid F.
Published: July 2002

Constant pressure Monte Carlo simulations of a coarse-grained off-lattice model for monolayers of amphiphilic molecules at the air–water interface are presented. Our study focuses on phase transitions within a monolayer rather than on self-aggregation. We thus model the molecules as stiff chains of Lennard-Jones spheres with one slightly larger repulsive end bead (head) grafted to a planar surface. Depending on the size of the head, the temperature and the pressure, we find a variety of phases, which differ in tilt order (including tilt direction), and in positional order. In particular, we observe a modulated phase with a striped superstructure. The modulation results from the competition between two length scales, the head size, and the tail diameter. As this mechanism is fairly general, it may conceivably also be relevant in experimental monolayers. We argue that the superstructure would be very difficult to detect in a scattering experiment, which perhaps accounts for the fact that it has not been reported so far. Finally the effect of varying the chain length on the phase diagram is discussed. Except at high pressures and temperatures, the phase boundaries in systems with longer chains are shifted to higher temperatures.
Theoretical studies of CH4(H2O)20, (H2O)21, (H2O)20, and fused dodecahedral and tetrakaidecahedral structures: How do natural gas hydrates form?
Authors: Khan Arshad
Published: July 2002

Ab initio geometry optimizations (HF/6-31G*) followed by single point energy calculations (MP2/6-31G*) suggest that the CH4(H2O)20 cluster with a CH4 molecule within the (H2O)20 dodecahedral cavity has a stabilization energy (SE) of around 7 kcal/mol relative to separated CH4 and (H2O)20 molecules. The cavity of a 20 mer fused cubic or edge-shared prismic structure is too small to enclose a methane molecule. Even though the (H2O)21 cluster with a water molecule within the dodecahedral cavity is significantly more stable (by around 28 kcal/mol) than CH4(H2O)20, the dodecahedral cage is too distorted in (H2O)21 to form a fused hydrate structure. In CH4(H2O)20, on the other hand, the dodecahedral cage remains almost undistorted and hence, can form a fused hydrate structure. The present study also suggests that during a fused structure formation, each pentagonal ring sharing between two dodecahedral structures or a dodecahedral and a tetrakaidecahedral structures results in stabilization by around 20–23 kcal/mol.
Inelastic neutron scattering spectra of the longitudinal acoustic modes of the normal alkanes from pentane to pentacosane
Authors: Braden D. A. , Parker S. F. , Tomkinson J. , Hudson B. S.
Published: July 2002

The inelastic neutron scattering spectra of the polycrystalline n-alkanes, 5<n<25, have been obtained at ∼25 K. At this temperature the n-alkane molecules are extended in their all-trans configuration. The spectral region from 200 to 600 cm−1 is reported with emphasis on the higher frequency longitudinal acoustic modes (LAM). These modes are identified by their nominal translational symmetry eigenvector, κ. A complete assignment of experimental frequencies out to κ≈0.7 is presented for the first time. This assignment is consistent with the known optical data and ab initio Hartree–Fock and DFT calculations. On the basis of these theoretical results, the lower frequency, higher κ LAM modes are also assigned. The LAM sequences exhibit bandhead behavior, i.e., a maximum frequency value, vmax, at some value of κ. The maximum frequency is consistently found at κmax=0.36±0.01 [cf. polyethylene (PE), κmax=0.34] for all chain lengths but the value of the maximum frequency, vmax, increases roughly linearly with n for n>7. At about n=21 it reaches the value found for PE, 525 cm−1. This variation in frequency with chain length for the same nominal value of κ and also the full set of calculated DFT LAM results is compared with the expectations of a simple zig–zag bead bending model.
Evolution of the electronic structure and properties of neutral and charged aluminum clusters: A comprehensive analysis
Authors: Rao B. K. , Jena P.
Published: July 2002

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the global equilibrium geometries and electronic structure of neutral, cationic, and anionic aluminum clusters containing up to 15 atoms. The total energies of these clusters are then used to study the evolution of their binding energy, relative stability, fragmentation channels, ionization potential, and vertical and adiabatic electron affinities as a function of size. The geometries are found to undergo a structural change from two dimensional to three dimensional when the cluster contains 6 atoms. An interior atom emerges only when clusters contain 11 or more atoms. The geometrical changes are accompanied by corresponding changes in the coordination number and the electronic structure. The latter is reflected in the relative concentration of the s and p electrons of the highest occupied molecular orbital. Aluminum behaves as a monovalent atom in clusters containing less than seven atoms and as a trivalent atom in clusters containing seven or more atoms. The binding energy evolves monotonically with size, but Al7, Al7+, Al7−, Al11−, and Al13− exhibit greater stability than their neighbors. Although the neutral clusters do not conform to the jellium model, the enhanced stability of these charged clusters is demonstrated to be due to the electronic shell closure. The fragmentation proceeds preferably by the ejection of a single atom irrespective of the charge state of the parent clusters. While odd-atom clusters carry a magnetic moment of 1μB as expected, clusters containing even number of atoms carry 2μB for n⩽10 and 0 μB for n>10. The calculated results agree very well with all available experimental data on magnetic properties, ionization potentials, electron affinities, and fragmentation channels. The existence of isomers of Al13 cluster provides a unique perspective on the anomaly in the intensity distribution of the mass spectra. The unusual stability of Al7 in neutral, cationic, and anionic form compared to its neighboring clusters is argued to be due to its likely existence in a mixed-valence state.