Short-range Anisotropy Research Articles

Complex High-Internal Phase Emulsions that can Form Interfacial Films with Tunable Morphologies.

High-internal phase emulsions (HIPEs) were considered as an important functional material and have been the focus of intense development effort, but their fundamental attributes have hardly been altered at either the microcosmic or macroscopic level, which severely limits their practical applications in various areas. In this work, we report a general strategy for creating complex HIPEs that can form interfacial films at liquid interfaces. Double HIPEs and Janus HIPEs are both realized for the first time. They feature complex microscopic patterns with short-range anisotropy and exhibit non-Newtonian pseudoplastic flow behavior. By taking advantage of their response to a high-pH subphase, interfacial films can be successfully obtained, which are tunable in thickness and morphologies under compression. Complex HIPEs can greatly expand the applications of liquid materials, and the interfacial films of droplets represent an important step toward producing 2D soft materials with a unique functionality that can be broadly applied to biological processes.

A non-empirical intermolecular force-field for trinitrobenzene and its application in crystal structure prediction.

An anisotropic atom-atom distributed intermolecular force-field (DIFF) for rigid trinitrobenzene (TNB) is developed using distributed multipole moments, dipolar polarizabilities, and dispersion coefficients derived from the charge density of the isolated molecule. The short-range parameters of the force-field are fitted to first- and second-order symmetry-adapted perturbation theory dimer interaction energy calculations using the distributed density-overlap model to guide the parameterization of the short-range anisotropy. The second-order calculations are used for fitting the damping coefficients of the long-range dispersion and polarization and also for relaxing the isotropic short-range coefficients in the final model, DIFF-srL2(rel). We assess the accuracy of the unrelaxed model, DIFF-srL2(norel), and its equivalent without short-range anisotropy, DIFF-srL0(norel), as these models are easier to derive. The model potentials are contrasted with empirical models for the repulsion-dispersion fitted to organic crystal structures with multipoles of iterated stockholder atoms (ISAs), FIT(ISA,L4), and with Gaussian Distributed Analysis (GDMA) multipoles, FIT(GDMA,L4), commonly used in modeling organic crystals. The potentials are tested for their ability to model the solid state of TNB. The non-empirical models provide more reasonable relative lattice energies of the three polymorphs of TNB and propose more sensible hypothetical structures than the empirical force-field (FIT). The DIFF-srL2(rel) model successfully has the most stable structure as one of the many structures that match the coordination sphere of form III. The neglect of the conformational flexibility of the nitro-groups is a significant approximation. This methodology provides a step toward force-fields capable of representing all phases of a molecule in molecular dynamics simulations.

Two interacting ultracold molecules in a one-dimensional harmonic trap

We investigate the properties of two interacting ultracold polar molecules described as distinguishable quantum rigid rotors, trapped in a one-dimensional harmonic potential. The molecules interact via a multichannel two-body contact potential, incorporating the short-range anisotropy of intermolecular interactions including dipole-dipole interaction. The impact of external electric and magnetic fields resulting in Stark and Zeeman shifts of molecular rovibrational states is also investigated. Energy spectra and eigenstates are calculated by means of the exact diagonalization. The importance and interplay of the molecular rotational structure, anisotropic interactions, spin-rotation coupling, electric and magnetic fields, and harmonic trapping potential are examined in detail, and compared to the system of two harmonically trapped distinguishable atoms. Presented model and results may provide microscopic parameters for molecular many-body Hamiltonians, and may be useful for the development of bottom-up molecule-by-molecule assembled molecular quantum simulators.

Ab Initio Atom-Atom Potentials Using CamCASP: Theory and Application to Many-Body Models for the Pyridine Dimer.

Creating accurate, analytic atom-atom potentials for small organic molecules from first principles can be a time-consuming and computationally intensive task, particularly if we also require them to include explicit polarization terms, which are essential in many systems. We describe how the CamCASP suite of programs can be used to generate such potentials using some of the most accurate electronic structure methods currently applicable. We derive the long-range terms from monomer properties and determine the short-range anisotropy parameters by a novel and robust method based on the iterated stockholder atom approach. Using these techniques, we develop distributed multipole models for the electrostatic, polarization, and dispersion interactions in the pyridine dimer and develop a series of many-body potentials for the pyridine system. Even the simplest of these potentials exhibits root mean square errors of only about 0.6 kJ mol(-1) for the low-energy pyridine dimers, significantly surpassing the best empirical potentials. Our best model is shown to support eight stable minima, four of which have not been reported before in the literature. Further, the functional form can be made systematically more elaborate so as to improve the accuracy without a significant increase in the human-time spent in their generation. We investigate the effects of anisotropy, rank of multipoles, and choice of polarizability and dispersion models.

Solidlike behavior and anisotropy in rigid frictionless bead assemblies

We investigate the structure and mechanical behavior of assemblies of frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by numerical simulation. Three different loading paths are explored: triaxial compression, triaxial extension, and simple shear. Generalizing a recent result, we show that the material, despite rather strong finite sample size effects, is able to sustain a finite deviator stress in the macroscopic limit, along all three paths, without dilatancy. The shape of the yield surface in principal stress space differs somewhat from the Mohr-Coulomb prediction, and is more adequately described by the Lade-Duncan or Matsuoka-Nakai criteria. We study geometric characteristics and force networks under varying stress levels within the supported range. Although the scalar state variables stay equal to the values observed in systems under isotropic pressure, the material, once subjected to a deviator stress, possesses some fabric and force distribution anisotropies. Each kind of anisotropy can be described, in good approximation, by a single parameter. Within the supported stress range, along each one of the three investigated stress paths, among those three quantities: deviator stress to mean stress ratio, fabric anisotropy parameter, force anisotropy parameter, any one determines the values of the two others. The pair correlation function also exhibits short range anisotropy, up to a distance between bead surfaces of the order of 10% of the diameter. The tensor of elastic moduli is shown to possess a nearly singular, uniaxial structure related to stress anisotropy. Possible stress-strain relations in monotonic loading paths are also discussed.

Rovibrational energy transfer in ortho-H2+para-H2 collisions

We present the results of a full-dimensional quantum mechanical study of the rovibrational energy transfer in the collision between ortho-H2 and para-H2 in the energy range of 0.1-1.0 eV. The multiconfiguration time-dependent Hartree algorithm has been used to propagate the wave packets on the global potential energy surface by Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)] and on a modification of this surface where the short range anisotropy is reduced. State-to-state attributes such as probabilities or integral cross sections are obtained using the formalism of Tannor and Weeks [J. Chem. Phys. 98, 3884 (1993)] by Fourier transforming the correlation functions. The effect of initial rotation of the diatoms on the inelastic and de-excitation processes is investigated.

Experimental and theoretical study of proton spin-lattice relaxation in H2-Ar gas mixtures: critical examination of the XC(fit) potential energy surface.

Proton nuclear magnetic resonance spin-lattice relaxation time measurements have been carried out at 500 MHz proton Larmor frequency on two hydrogen-argon gas mixtures with 1.90% and 3.93% hydrogen at four different temperatures in the range 225 K < T < 337 K and at two different number densities. The results for different hydrogen mole percentages have been extrapolated to infinite dilution to obtain the contributions to the overall relaxation times arising from the hydrogen-argon interaction. The extrapolated relaxation times fall in the reciprocal regime in which relaxation times are inversely proportional to the density. Relaxation times have also been calculated using quantum mechanical close-coupled computations based on the H2-Ar XC(fit) potential energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)]. Significant differences found between the experimental and theoretical results indicate that the short-range anisotropy of the XC(fit) potential surface is too weak. The reciprocal regime is shown to have a much higher sensitivity to changes in the anisotropic component of the intermolecular potential energy surface.

SDS-resistant Active and Thermostable Dimers Are Obtained from the Dissociation of Homotetrameric β-Glycosidase from Hyperthermophilic Sulfolobus solfataricus in SDS: STABILIZING ROLE OF THE A-C INTERMONOMERIC INTERFACE

beta-Glycosidases are fundamental, widely conserved enzymes. Those from hyperthermophiles exhibit unusual stabilities toward various perturbants. Previous work with homotetrameric beta-glycosidase from hyperthermophilic Sulfolobus solfataricus (M(r) 226,760) has shown that addition of 0.05-0.1% SDS was associated with minimal secondary structure perturbations and increased activity. This work addresses the effects of SDS on beta-glycosidase quaternary structure. In 0.1-1% SDS, the enzyme was dimeric, as determined by Ferguson analysis of transverse-gradient polyacrylamide gels. The catalytic activity of the beta-glycosidase dimer in SDS was determined by in-gel assay. A minor decrease of thermal stability in SDS was observed after exposure to temperatures up to 80 degrees C for 1 h. An analysis of beta-glycosidase crystal structure showed different changes in solvent-accessible surface area on going from the tetramer to the two possible dimers (A-C and A-D). Energy minimization and molecular dynamics calculations showed that the A-C dimer, exhibiting the lowest exposed surface area, was more stabilized by a network of polar interactions. The charge distribution around the A-C interface was characterized by a local short range anisotropy, resulting in an unfavorable interaction with SDS. This paper provides a detailed description of an SDS-resistant inter-monomeric interface, which may help understand similar interfaces involved in important biological processes.

Electron spin resonance aboveTCin layered manganites

We have performed electron spin resonance (ESR) and dc magnetization measurements on single crystals of La{sub 2(1-x)}Sr{sub 1+2x}Mn{sub 2}O{sub 7} up to 800 K with special emphasis on the x=0.4 composition. The ESR linewidth shows behavior similar to that observed in the three-dimensional perovskites and above -500 K can be described by a universal expression {Delta}H{sub pp}(T)=[C/T{sub x}(T)]{Delta}H{sub pp}({sup {infinity}}). The linewidth and the resonance field become anisotropic below -500 K. The anisotropy in the resonance field is proportional to the magnetization M, and we concluded that it is intrinsic to the system. We show that demagnetization effects can explain only part of the anisotropy. The remainder arises from short-range uniaxial terms in the Hamiltonian that are associated with the crystal field and Dzialozhinsky-Moriya interactions. The anisotropy in the linewidth is attributed to the easy-plane ferromagnetic ordering, which also arises from the short-range anisotropy.

The interaction between He and H+ 2: anisotropy, bond length dependence and hydrogen bonding

An extensive high level ab initio study of the ground electronic state of HeH+ 2 has been used to generate an improved intermolecular potential for He + H+ 2 that includes a more complete account of the long range forces, the short range anisotropy, and the dependence of the potential on H+ 2 bond length r. In good agreement with earlier work, the potential is found to have a minimum for collinear geometry, with a well 7.799 kcal mol−1 (2728 cm−1) deep with respect to separated He + H+ 2(r = r e = 1.9982 a 0) at an intermolecular distance R = 2.963 a 0(1.5684 Å) and a bond length r = 2.0742 a 0 (1.098 Å).

The hysteretic event in the computation of magnetization

Simulations of magnetic and magnetostrictive behavior based on micromagnetic theory exhibit hysteresis. These systems have a highly nonlinear character involving both short range anisotropy and elastic fields and dispersive demagnetization fields. Hysteresis occurs even in the absence of an imposed dynamical mechanism, for example, a Landau-Lifshitz-Gilbert dissipative equation for the magnetic moment, and is symptomatic of the way the system navigates a path through local minima of its energy space. It is not sensitive to the particular method: We implement continuation based on the conjugate gradient method, although the same results were obtained by, for example, a Newton’s method. The phenomenon is robust: Computational experiments confirm that the shape of the loop is invariant over several decades of mesh refinement. Our experience has led us to hold that optimization procedures have the propensity to become marooned at local extrema when applied to nonconvex situations and that this presents a fundamental challenge to analysis. Understanding and controlling such phenomena present the opportunity to develop predictive tools and diagnostics. For example, since the energy picture is mesh-independent, computing on a fairly coarse grid suffices to establish its character.

Close-coupling studies of rotationally inelastic HF–HF collisions at hyperthermal energies

Earlier close-coupling studies on the HF–HF system [A. E. DePristo and M. H. Alexander, J. Chem. Phys. 66, 1334 (1977)] have been extended to larger channel bases, allowing the determination of converged integral cross sections for excitation out of the lower rotational levels of the bimolecular system. The calculations were confined to collision energies appropriate to supersonic beam experiments (E =0.5–1.5 eV). Two potential surfaces were used, both taken from our earlier fit to ab initio points [M. H. Alexander and A. E. DePristo, J. Chem. Phys. 65, 5009 (1976)]. In the first surface the symmetry of the only anisotropic term included corresponds to the standard dipole–dipole interaction; to which were added, in the second surface, a primarily repulsive anisotropy as well as the long-range dipole–quadrupole interaction. The largest cross sections (40–60 Å2) are associated with R–R processes of the type j1 j2→j1±1, j2∓1 which are dipole-allowed in first order. The magnitudes of these cross sections are little affected by the presence of the shorter-range anisotropic terms, since much of the inelasticity occurs at large impact parameter. Cross sections for processes which are dipole-allowed only in second or higher order are considerably smaller (1–10 Å2), have classical dynamical thresholds at high energy, and are substantially lowered when the additional anisotropic terms are added to the potential, which has the effect of redirecting inelastic flux into the dipole forbidden channels. The cross sections for first order dipole–quadrupole transitions are also small, even in cases of near resonance. By contrast we find sizeable cross sections (7–15 Å2) for transitions which are coupled only by the short range anisotropy, which implies that rotational energy transfer between polar molecules cannot be fully described by models which rely solely on the standard long-range multipole expansion of the potential.

Energy transfer in NH3–He collisions

The electron gas intermolecular potential for NH3–He developed previously is shown to be inadequate for a quantitative description of energy transfer in this system by comparing with available experimental data—pressure broadening, double resonance, and beam scattering. A new potential is constructed by combining the Hartree–Fock results of Davis, Boggs, and Mehrotra with semiempirical long-range induction and dispersion terms. The resulting potential is shown to be in harmony with beam scattering and pressure broadening measurements. It is found that the quadrupole-induced–dipole interaction, which has been invoked previously to account for pressure broadening in this system, is not, in fact, as effective as the short-range anisotropy. Although results are in general accord with double resonance data, a few unresolved discrepancies remain. It is suggested that these are due to improper consideration of the dependence of cross sections on degenerate magnetic quantum levels in the experimental analysis. Using this new intermolecular potential state-to-state rate constants are presented for temperatures to 300 K.

Anisotropic ordering in binary alloys

The problem of ordering in a binary alloy with anisotropic interaction between atoms is analyzed in the Kirkwood approximation. It is shown that the emerging oriented superstructure does not depend on the isotropic component of ordering energy. Transition to the ordered state destroys the short-range anisotropy. The constant of induced magnetic anisotropy is calculated for a ferromagnetic alloy, while its dependence on the alloy composition and the degree of far-range order as well as on the anneal temperature and the test temperature is established.

Anisotropic intermolecular potentials from an analysis of spectra of H2- and D2-inert gas complexes

A method is developed for analyzing the discrete infrared absorption spectra of the van der Waals complexes formed between H2 or D2 and Xe, Kr, Ar, or Ne. Its application involves automatic nonlinear least squares fits of trial spectra calculated from realistic three-dimensional intermolecular potential models, to the experimental data. The secular determinant method used for calculating the energy levels of the anisotropic trial potentials proves to be highly reliable and relatively inexpensive. As a result, the present techniques should be readily applicable to cases where the anisotropic part of the potential is much stronger than it is here. The potential form used here is a Lennard-Jones (m, 6) function with independent long- and short-range anisotropy coefficients, and independent parameters characterizing the effect of the stretching of the hydrogen bond on the attractive and repulsive parts of the potential. The final fits concurrently used all uniquely assigned and nonoverlapping lines of both isotopes (H2 and D2) of a given complex, and all parameters (except ``m,'' which was varied manually) were allowed to vary simultaneously. The resulting potential surfaces are compared with those determined from molecular beam and relaxation time measurements.

Critical Behavior of Magnets with Dipolar Interactions. III. Antiferromagnets

Exact renormalization group recursion formulas, derived in Paper I for $\ensuremath{\epsilon}=4\ensuremath{-}d$ small, are applied to antiferromagnets. It is shown, that in contrast to the ferromagnetic case, the main parameters characterizing the dipolar interactions become irrelevant for antiferromagnets, so that the critical exponents maintain their short-range values. However, the relative decay of the dipolar parameters is slow (the appropriate exponents being of order ${\ensuremath{\epsilon}}^{2}$), and thus the possibility of observing their existence experimentally is discussed briefly. In addition, the dipolar anisotropies, deriving from the lattice structure, produce weak instabilities which are even harder to detect than in the ferromagnetic case. Ferromagnetic short-range anisotropy is considered briefly. The Appendix contains a calculation of the renormalization group eigenvalues of the operators $\ensuremath{\Sigma}{S}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{R}}}^{4}$ and $\ensuremath{\Sigma}{({S}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{R}}}^{a})}^{4}$. The latter is shown to be relevant.

Role of the Anisotropy of the Potential in Quenching of Glory Extrema in Atom–Diatomic Molecule Scattering

Damping of the glory undulations in the velocity dependence of the total cross sections for atom–molecule scattering was first observed by Gislason and Kwei and (correctly) explained by them qualitatively in terms of inelastic and reactive processes concurrent with anisotropic elastic scattering. Subsequent experiments by Helbing and Rothe confirmed the quenching phenomenon and extended the range of systems exhibiting such behavior. The present theory deals mainly with nonreactive atom–diatomic collisions, considering the coupled equations in the total angular-momentum representation and taking full cognizance of the anisotropic distortion of the potential. The resulting equations yield a lower bound on the degree of quenching, but should be fairly generally applicable except for extremely reactive systems of the “stripping” type. The extent of the damping and its velocity dependence is expected to be increased for systems of larger reduced mass with deeper van der Waals potential wells (ε), larger-size parameters (rm), and greater short-range anisotropy coefficients. The equations suggest a simple method to retrieve the short-range anisotropy coefficient of the potential from quenching data.