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The static and dynamic properties of three liquid alkali metals were studied using molecular dynamics: lithium (at 470 K), rubidium (over a temperature range from 350 to 1873 K) and cesium (just above the melting point, at 308 K). The interatomic potential is based for Li on an OPW pseudopotential and for Rb and Cs on an empty-core pseudopotential. From the raw computer data, both single-particle and collective dynamic correlation functions were extracted. Interpretation of these correlation functions in terms of hydrodynamic, memory-function and (simple) mode-coupling models allows one to derive both thermodynamic and elastic properties. The dynamic structure factors have been compared for all three metals with recent neutron-scattering data. It is found that — except for the higher temperature states of Rb (i.e., at the onset of the metal-non-metal transition) — agreement between theory and experiment is good. Concerning elastic and thermodynamic properties, differences of 10–15% with respect to the experimental data are encountered.

A few years ago, Winn and Logan demonstrated that, within a single-site theory, the electronic density-of-states (DOS) for disordered materials may be determined from a complex-valued Ornstein-Zernike (OZ)-type integral equation, involving the pair distribution function, g ( r ), of the system and the transfer matrix element, V ( r ). Winn and Logan derived by graph theoretical methods an additional closure relation and succeeded to solve these equations analytically for a model system, where g ( r ) is a step function. A numerical method has been implemented which allows solution of this equation for arbitrary g ( r ) and V ( r ). The influence of these two quantities has been studied on the electronic DOS for hard-core systems: (i) concerning the influence of g ( r ), the contact value turns out to be the most important parameter; (ii) concerning the influence of V ( r ), the DOS is very sensitive both to range and shape of this function. Further, it is found — as asssumed by Winn and Logan — that a DOS which is determined via this OZ-type equation is automatically normalized.

The triplet correlation function, g (3) ( r , r′ ) , was calculated for two liquid metals, rubidium and germanium, both near their respective melting points (373 K and 1250 K). The results have been obtained (i) from a molecular dynamics study and (ii) using a numerical technique, which is based on a formal density expansion of the triplet correlation function. Results of both methods are found to be in good agreement. In a closer analysis of g (3) ( r , r′ ) in terms of angle distributions, it was found that in the case of germanium a tetragonal coordination is preferred for triangle configurations with sides near to nearest neighbour distance, whereas for rubidium rather close-packed structures are encountered.

We present results of monomer-resolved Monte Carlo simulations for a system of amphiphilic dendrimers of second generation. Our investigations validate a coarse-grained level description based on the zero-density limit effective pair-interactions for low and intermediate densities, which predicted the formation of stable, finite aggregates in the fluid phase. Indeed, we find that these systems form a homogeneous fluid for low densities, which on increasing the density spontaneously transforms into a fluid of clusters of dendrimers. Although these clusters are roughly spherical in nature for intermediate densities, also more complex structures are detected for the highest densities considered., Comment: 23 pages, 11 figures

Using molecular dynamics simulations we study the slow dynamics of a colloidal fluid annealed within a matrix of obstacles quenched from an equilibrated colloidal fluid. We choose all particles to be of the same size and to interact as hard spheres, thus retaining all features of the porous confinement while limiting the control parameters to the packing fraction of the matrix, {\Phi}m, and that of the fluid, {\Phi}f. We conduct detailed investigations on several dynamic properties, including the tagged-particle and collective intermediate scattering functions, the mean-squared displacement, and the van Hove function. We show the confining obstacles to profoundly impact the relaxation pattern of various quantifiers pertinent to the fluid. Varying the type of quantifier (tagged-particle or collective) as well as {\Phi}m and {\Phi}f, we unveil both discontinuous and continuous arrest scenarios. Furthermore, we discover subdiffusive behavior and demonstrate its close connection to the matrix structure. Our findings partly confirm the various predictions of a recent extension of mode-coupling theory to the quenched-annealed protocol., Comment: 16 pages, 20 figures, minor revisions

The interaction between rotor blades and non-rotating stator blades is the most significant blade excitation mechanism in turbomachines. It is well documented in various numerical and experimental investigations for turbine cascades. Like turbine blades, also compressor blades are excited as well by potential fields of the following stator, the downstream flowfield of the stator of the previous stage or struts and incoming flow distortions. In this paper, experimental investigations of the excitation of a transonic compressor cascade due to gust generating struts upstream are presented. The experiments were performed in the test facility of non-rotating annular cascades at EPFL using a compressor cascade, which consists of 20 blades (NACA3506 profile) mounted on elastic spring suspensions for torsional motions at the midchord. For the non-rotating annular cascade, relative flow conditions similar to those present in a rotating cascade are generating by swirling the flow in front of the test test section. The struts are rotating in order to create a periodic excitation upstream of the cascade. The so generated pressure distribution on the cascade’s profiles as well as the measured vibration response of the blades are presented and compared for a pure subsonic and a transonic flow case.

We have investigated the slow dynamics of ultrasoft particles in crystalline cluster phases, where point particles interact through the generalized exponential potential u(r) = \epsilon \exp[-(r/\sigma)^n], focusing on the cluster fcc phase of this model with n=4. In an effort to elucidate how the mechanisms of mass transport depend on the microscopic dynamics and in order to mimic a realistic scenario in a related experiment we have performed molecular dynamics, Brownian dynamics, and Monte Carlo simulations. In molecular dynamics simulations the diffusion of particles proceeds through long-range jumps, guided by strong correlations in the momentum direction. In Monte Carlo and Brownian dynamics simulations jump events are short-ranged, reflecting the purely configurational nature of the dynamics. In contrast to what was found in models of glass-forming liquids, the effect of Newtonian and stochastic microscopic dynamics on the long-time relaxation cannot be accounted for by a temperature-independent rescaling of the time units. From the obvious qualitative discrepancies in the short time behavior between the three simulation methods and the non-trivial difference in jump length distributions, long time relaxation, and dynamic heterogeneity, we learn that a more complex modeling of the dynamics in realistic systems of ultrasoft colloids is required., Comment: 12 pages, 18 figures, added results of Brownian dynamics simulations

We analyze the phonon spectra of periodic structures formed by two-dimensional mixtures of dipolar colloidal particles. These mixtures display an enormous variety of complex ordered configurations [J. Fornleitner {\it et al.}, Soft Matter {\bf 4}, 480 (2008)], allowing for the systematic investigation of the ensuing phonon spectra and the control of phononic gaps. We show how the shape of the phonon bands and the number and width of the phonon gaps can be controlled by changing the susceptibility ratio, the concentration and the mass ratio between the two components., Comment: 4 pages 3 figures

We present results of a molecular-dynamics study on liquid cesium, just above the melting point; i.e., we have performed a computer experiment on exactly the same system that was studied recently in a very accurate neutron-scattering experiment by Bodensteiner et al. In this simulation the interaction of the cesium atoms is based on an Ashcroft empty-core pseudopotential; the total simulation length extends over 100 000 time steps, i.e., 8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ s real time. Both static and dynamic structure factors are in very good agreement with experimental results: as in the experiment, we also get\char22{}within very good accuracy\char22{}a positive dispersion relation. However, concerning propagating sound modes beyond the main peak in S(q) (q\ensuremath{\ge}${\mathit{q}}_{\mathit{p}}$\ensuremath{\sim}1.4 \AA{}) we cannot find sufficient evidence for their existence. Analysis of the transverse current correlation function shows that the corresponding decay mechanism is built up by two relaxation processes (a fast binary one and a slow collective one); starting from \ensuremath{\sim}1 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$ the latter one is completely extinguished. The correlation functions are fitted to hydrodynamic and memory-function models that contain generalized thermodynamic and elastic quantities as parameters. In several cases\char22{}if statistics allows\char22{}the true physical values can be recovered; then agreement with experimental values is quite satisfactory (differences of 10\char21{}15 % are observed).

Two-dimensional dipolar mixtures consisting of spherical particles with unequal susceptibilities are shown to order to an enormous variety of crystalline structures, whose geometry can be tuned by the stoichiometry of the compound, the susceptibility ratio, as well as the density. Our results are based on the application of genetic algorithms, which allow for an efficient and unbiased search over the parameter space. Structures that are practically degenerate energetically are discovered at various parameter combinations. Implications on the ability to tune the phononic spectra of such composite materials are discussed.

International audience; We have investigated the structural properties of a fluid in which particles, interacting via soft potentials, are imbibed into a disordered porous structure built up by soft particles. Using a Gaussian potential for all interactions involved, we determine via Monte Carlo simulations and integral-equation theory the fluid-fluid, fluid-matrix, connected, and blocked structure factors. Within the explored range of state parameters, the fluid-fluid structure factors display a distinct pre-peak, which we identify as a fingerprint of the structure of the matrix. We show that this feature at low wave-vectors arises from contributions of blocked correlations to the fluid-fluid structure factor. We argue that a similar feature may be found also in systems in which particles interact via harshly repulsive potentials. On the other hand, the variation of the main peak of the fluid-fluid structure factor resembles the behaviour of the equilibrium Gaussian core model fluid. In particular, the height of the main peak changes non-monotonically with increasing fluid density at fixed matrix density. Finally, we analyse the effect of a mismatch between the temperature of the fluid and the temperature of the matrix on the structural properties of the system.

The analysis of ethyl glucuronide (EtG) in hair is a powerful tool for chronic alcohol abuse control because of the typical wide detection window of the hair matrix and due to the possibility of segmentation, allowing evaluation of alcohol consumption in different periods. Additionally, EtG in hair is often the only diagnostic parameter of choice for alcohol abuse when other clinical parameters such as ALT, AST, gammaGT and CDT (asialotransferrin and disialotransferrin) are in the normal range and EtG in urine negative. In this paper, we describe the development, optimization and validation of a new method based on hair extraction with water, clean-up by solid phase extraction (SPE), derivatization with heptafluorobutyric anhydride and headspace solid-phase microextraction (HS-SPME) in combination with GC-MS/MS according to forensic guidelines. The assay linearity of EtG was confirmed over the range from 2.8 to 1000 pg/mg hair, with a coefficient of determination (r(2)) above 0.999. The LLOQ was 2.8 pg/mg and the LLOD was 0.6 pg/mg. An error profile calculated according to the "Guide to the Expression of Uncertainty in Measurement" (GUM) at 99% confidence intervals for the range 5-750 pg/mg hair did not exceed 10%. This range corresponds to more than 98% of the positive samples analysed.

Using molecular dynamics simulations we study the slow dynamics of a hard sphere fluid confined in a disordered porous matrix. The presence of both discontinuous and continuous glass transitions as well as the complex interplay between single-particle and collective dynamics are well captured by a recent extension of mode-coupling theory for fluids in porous media. The degree of universality of the mode-coupling theory predictions for related models of colloids is studied by introducing size-disparity between fluid and matrix particles, as well as softness in the interactions., 4 pages, 5 figures, minor revisions

We identify the sequences of emerging ordered equilibrium structures as a three-dimensional crystal grows in thickness, starting from a two-dimensional lattice. To this end, we consider a system of particles that interact via a Gaussian potential and are confined between two parallel plates separated by a distance D. Using optimization tools that are based on genetic algorithms, we identify the T = 0, ground state configurations of the system. Based on these results, we investigate and interpret in detail two archetypes of structural transitions occurring in the diagram of states: one of them is a sequence of square → centered rectangular → hexagonal transitions at fixed confinement D as the density grows and the other is the often-discussed buckling transition, which emerges at fixed density as the system forms a new layer with increasing thickness D. These theoretical investigations are complemented and confirmed by Monte Carlo simulations.

The authors present explicit formulae which allow an economical but very accurate evaluation of the pair-distribution functions gij(r) for the whole range of distances of an additive binary hard-sphere system within the Percus-Yevick approximation. The method is based on the fact that for such a system the Laplace-transforms gij(t) of the gij(r) are known analytically. The inversion of the Laplace-transform is performed in two different ways: the first one-preferably applied from the contact up to distances of a few diameters-calculates the inverse by means of an exact analytical procedure, whereas the other one-very useful for intermediate and large distances-uses an asymptotic method, truncating a rapidly converging series expansion. In the overlap region of the two methods they obtain perfect agreement (i.e. up to machine precision), including the first few terms of this expansion. Both formulae are presented as general as possible to allow for an easy extension to other binary hard-core systems.

6 pages.-- PACS nrs.: 64.70.Dv; 82.30.Nr; 61.20.Ja; 82.70.Dd.-- Special issue of Computer Physics Communications based on the Conference on Computational Physics 2007 - CCP 2007., In this paper, we present a short review as well as novel results on a recently established counterintuitive phenomenon of cluster aggregation of particles that interact via purely repulsive interactions. We demonstrate how repulsion can lead to clustering provided that the interaction allows full particle overlaps and also displays negative Fourier components. The formation of crystals with average site occupancy that scales linearly with density, and which is in general a noninteger number, is demonstrated by means of density functional theory and Monte Carlo simulations. Noninteger average occupancy of lattice sites is sustained by incessant hopping processes in the crystal, which we observe and quantitatively analyze by means of Molecular Dynamics simulations. The simultaneous presence of relaxative dynamics on the single-particle and arrested dynamics on the collective level leads to an unusual interplay between the two and to novel scenarios of dynamical arrest in the cluster crystal., C.N.L. acknowledges partial support from the DFG through the SFB-TR6, Project Section C3. B.M.M., D.G., and G.K. acknowledge financial support by the Österreichische Forschungsfond (FWF) under Project Nos. P17823-N08 and P19890-N16, and the Hochschuljubiläumsstiftung der Stadt Wien under Project No. 1080/2002. Further, B.M.M. thanks the Österreichische Forschungsgemeinschaft for financial support. This work was partly carried out under the HPC-EUROPA Project (RII3-CT-2003-506079), with the support of the European Community—Research Infrastructure Action under the FP6 "Structuring the European Research Area" Programme. A.J.M. acknowledges financial support from SoftComp (NMP3-CT-2004-502235) and DIPC-Spain.

Up to now additive hard spheres have been used as reference systems in perturbation theories of binary liquid systems; this is due to the fact that only for this case are analytic expressions for the direct correlation functions available within the Percus–Yevick approximation. Here for the first time a Weeks–Chandler–Andersen perturbation theory for binary systems, using nonadditive hard spheres as a reference system for the repulsive parts of the potentials, is presented. The nonadditivity of realistic systems−as binary metal alloys−can therefore properly be taken into account. Treating nonadditive systems with additive reference systems by introducing additional (sometimes unphysical) parameters is no longer necessary, and our parameters−i.e., the three hard‐sphere diameters−are determined uniquely. Including in a subsequent step the attractive forces by means of the optimized random phase approximation, it is found that agreement with other liquid state theories as well as with experimental scattering data is very good, even for extremely nonadditive systems.

We present a Weeks-Chandler-Andersen (WCA) perturbative approach to the structure of liquid binary systems using a non-additive hard-sphere (NAHS) reference system. Consequently former doubts about the uniqueness of how to determine parameters when using an additive reference system are ruled out. We present results for strongly non-additive systems as some specially chosen metal alloys and a metal-molten salt solution; comparison with experimental data shows good agreement.

We present results of a perturbative approach to the structure and thermodynamics of soft spheres using a hard-sphere Yukawa (HSY) reference system. For the determination of the parameters we use instead of the Weeks-Chandler-Andersen (WCA) the Lado criteria, since the latter ones lead to a thermodynamically self-consistent (TSC) description of the system.

We demonstrate the accuracy of the hypernetted chain closure and of the mean-field approximation for the calculation of the fluid-state properties of systems interacting by means of bounded and positive-definite pair potentials with oscillating Fourier transforms. Subsequently, we prove the validity of a bilinear, random-phase density functional for arbitrary inhomogeneous phases of the same systems. On the basis of this functional, we calculate analytically the freezing parameters of the latter. We demonstrate explicitly that the stable crystals feature a lattice constant that is independent of density and whose value is dictated by the position of the negative minimum of the Fourier transform of the pair potential. This property is equivalent with the existence of clusters, whose population scales proportionally to the density. We establish that regardless of the form of the interaction potential and of the location on the freezing line, all cluster crystals have a universal Lindemann ratio L = 0.189 at freezing. We further make an explicit link between the aforementioned density functional and the harmonic theory of crystals. This allows us to establish an equivalence between the emergence of clusters and the existence of negative Fourier components of the interaction potential. Finally, we make a connection between the class of models at hand and the system of infinite-dimensional hard spheres, when the limits of interaction steepness and space dimension are both taken to infinity in a particularly described fashion., 19 pages, 5 figures, submitted to J. Chem. Phys; new version: minor changes in structure of paper

Recent theoretical studies have predicted a new clustering mechanism for soft matter particles that interact via a certain kind of purely repulsive, bounded potentials. At sufficiently high densities, clusters of overlapping particles are formed in the fluid, which upon further compression crystallize into cubic lattices with density-independent lattice constants. In this work we show that amphiphilic dendrimers are suitable colloids for the experimental realization of this phenomenon. Thereby, we pave the way for the synthesis of such macromolecules, which form the basis for a novel class of materials with unusual properties., Comment: 4 pages, 4 figures, 1 table

We generalize the inverse patchy colloid model that was originally developed for heterogeneously charged particles with two identical polar patches and an oppositely charged equator to a model that can have a considerably richer surface pattern. Based on a Debye-Hueckel framework, we propose a coarse-grained description of the effective pair interactions that is applicable to particles with an arbitrary patch decoration. We demonstrate the versatility of this approach by applying it to models with (i) two differently charged and/or sized patches, and (ii) three, possibly different patches.

We investigate the structural and thermodynamic properties of a new class of patchy colloids, referred to as inverse patchy colloids (IPCs) in their fluid phase via both theoretical methods and simulations. IPCs are nano- or micro- meter sized particles with differently charged surface regions. We extend conventional integral equation schemes to this particular class of systems: our approach is based on the so-called multi-density Ornstein-Zernike equation, supplemented by the associative Percus-Yevick approximation (APY). To validate the accuracy of our framework, we compare the obtained results with data extracted from $NpT$ and $NVT$ Monte Carlo simulations. In addition, other theoretical approaches are used to calculate the properties of the system: the reference hypernetted-chain (RHNC) method and the Barker-Henderson thermodynamic perturbation theory. Both APY and RHNC frameworks provide accurate predictions for the pair distribution functions: APY results are in slightly better agreement with MC data, in particular at lower temperatures where the RHNC solution does not converge., 41 pages, 9 figures

We have investigated the phase behaviour of a symmetrical binary fluid mixture for the situation where the chemical potentials $\mu_1$ and $\mu_2$ of the two species differ. Attention is focused on the set of interparticle interaction strengths for which, when $\mu_1=\mu_2$, the phase diagram exhibits both a liquid-vapor critical point and a tricritical point. The corresponding phase behaviour for the case $\mu_1\ne\mu_2$ is investigated via integral-equation theory calculations within the mean spherical approximation (MSA), and grand canonical Monte Carlo (GCMC) simulations. We find that two possible subtypes of phase behaviour can occur, these being distinguished by the relationship between the critical lines in the full phase diagram in the space of temperature, density, and concentration. We present the detailed form of the phase diagram for both subtypes and compare with the results from GCMC simulations, finding good overall agreement. The scenario via which one subtype evolves into the other, is also studied, revealing interesting features., Comment: 22 pages, 13 figures

We have considered a system where the interaction, v(r) = v_IS(r) + xi^2 v_MF(r), is given as a linear combination of two potentials, each of which being characterized with a well-defined critical behavior: for v_IS(r) we have chosen the potential of the restricted primitive model which is known to belong to the Ising 3D (IS) universality class, while for v_MF(r) we have considered a long-range interaction in the Kac-limit, displaying mean field (MF) behavior. We study the performance of two theoretical approaches and of computer simulations in the critical region for this particular system and give a detailed comparison between theories and simulation of the critical region and the location of the critical point. Both, theory and simulation give evidence that the system belongs to the MF universality class for any positive value of xi and that it shows only non-classical behavior for xi=0. While in this limiting case theoretical approaches are known to fail, we find good agreement for the critical properties between the theoretical approaches and the simulations for xi^2 larger than 0.05., 9 pages, 11 figures, 3 tables

We present results from density functional theory and computer simulations that unambiguously predict the occurrence of first-order freezing transitions for a large class of ultrasoft model systems into cluster crystals. The clusters consist of fully overlapping particles and arise without the existence of attractive forces. The number of particles participating in a cluster scales linearly with density, therefore the crystals feature density-independent lattice constants. Clustering is accompanied by polymorphic bcc-fcc transitions, with fcc being the stable phase at high densities.

A numerical method has been developed to study the effects of structural mis- tuning on the aeroelastic behavior of turbomachinery cascades. The approach used is the combination of a modal reduction technique, where the structural properties of each blade are represented by only a few eigenmodes, with a lin- earized Euler method for the aerodynamic calculations. The method is validated and applied to two test cases, comprising of a high pressure turbine rotor and a transonic compressor rotor. Both are representative of modern turbomachinery designs. The results of the validation confirm the ability of the present method to accurately capture the dominant effects that influence the aeroelastic behavior of the cascades. Further case studies are performed to assess the influence of alternating and random mistuning on the resonant response amplitudes and on the aeroelastic stability.

We present results from density functional theory and computer simulations that unambiguously predict the occurrence of first-order freezing transitions for a large class of ultrasoft model systems into cluster crystals. The clusters consist of fully overlapping particles and arise without the existence of attractive forces. The number of particles participating in a cluster scales linearly with density, therefore the crystals feature density-independent lattice constants. Clustering is accompanied by polymorphic bcc-fcc transitions, with fcc being the stable phase at high densities.

We have identified a fourth archetype of phase diagram in binary symmetrical mixtures, which is encountered when the ratio of the interaction between the unlike and the like particles is sufficiently small. This type of phase diagram is characterized by the fact that the lambda line (i.e., the line of the second-order demixing transition) intersects the first-order liquid-vapor curve at densities smaller than the liquid-vapor critical density.

We propose genetic algorithms as a new tool that is able to predict all possible solid candidate structures into which a simple fluid can freeze. In contrast to the conventional approach where the equilibrium structures of the solid phases are chosen from a preselected set of candidates, genetic algorithms perform a parameter-free, unbiased, and unrestricted search in the entire search space, i.e., among all possible candidate structures. We apply the algorithm to recalculate the zero-temperature phase diagrams of neutral star polymers and of charged microgels over a large density range. The power of genetic algorithms and their advantages over conventional approaches is demonstrated by the fact that new and unexpected equilibrium structures for the solid phases are discovered. Improvements of the algorithm that lead to a more rapid convergence are proposed and the role of various parameters of the method is critically assessed.

We present a theoretical analysis of the structural properties and phase behavior of spherical, loosely cross-linked ionic microgels that possess a low monomer concentration. The analysis is based on the recently derived effective interaction potential between such particles fA. R. Denton, Phys. Rev. E 67, 011804 s2003dg. By employing standard tools from the theory of the liquid state, we quantitatively analyze the pair correlations in the fluid and find anomalous behavior above the overlap concentration, similar to the cases of star-branched neutral and charged polymers. We also employ an evolutionary algorithm in order to predict the crystalline phases of the system without any a priori assumptions regarding their symmetry class. A very rich phase diagram is obtained, featuring two reentrant melting transitions and a number of unusual crystal structures. At high densities, both the Hansen‐Verlet freezing criterion fJ.-P. Hansen and L. Verlet, Phys. Rev. 184, 151 s1969dg and the Lindemann melting criterion fF. A. Lindemann, Phys. Z. 11, 609 s1910dg lose their validity. The topology of the phase diagram is altered when the steric interactions between the polymer segments become strong enough, in which case the lower-density reentrant melting disappears and the region of stability of the fluid is split into two disconnected domains, separated by intervening fcc and bcc regions. © 2005 American Institute of Physics . fDOI: 10.1063/1.1850451g

Taking advantage of the availability of the analytic solution of the mean spherical approximation for a mixture of charged hard spheres with an arbitrary number of components we show that the polydisperse fluid mixture of charged hard spheres belongs to the class of truncatable free energy models, i.e., to those systems where the thermodynamic properties can be represented by a finite number of (generalized) moments of the distribution function that characterizes the mixture. Thus, the formally infinitely many equations that determine the parameters of the two coexisting phases can be mapped onto a system of coupled nonlinear equations in these moments. We present the formalism and demonstrate the power of this approach for two systems; we calculate the full phase diagram in terms of cloud and shadow curves as well as binodals and discuss the distribution functions of the coexisting daughter phases and their charge distributions.

We employ effective interaction potentials between spherical polyelectrolyte microgels in order to investigate theoretically the structure, thermodynamics, and phase behavior of ionic microgel solutions. Combining a genetic algorithm with accurate free energy calculations we are able to perform an unrestricted search of candidate crystal structures. Hexagonal, body-centered orthogonal, and trigonal crystals are found to be stable at high concentrations and charges of the microgels, accompanied by reentrant melting behavior and fluid-fcc-bcc transitions below the overlap concentration.

We have investigated the phase diagram of the Girifalco model of C60 fullerene in the framework provided by the MHNC and the SCOZA liquid state theories, and by a Perturbation Theory (PT), for the free energy of the solid phase. We present an extended assessment of such theories as set against a recent Monte Carlo study of the same model [D. Costa et al, J. Chem. Phys. 118:304 (2003)]. We have compared the theoretical predictions with the corresponding simulation results for several thermodynamic properties. Then we have determined the phase diagram of the model, by using either the SCOZA, or the MHNC, or the PT predictions for one of the coexisting phases, and the simulation data for the other phase, in order to separately ascertain the accuracy of each theory. It turns out that the overall appearance of the phase portrait is reproduced fairly well by all theories, with remarkable accuracy as for the melting line and the solid-vapor equilibrium. The MHNC and SCOZA results for the liquid-vapor coexistence, as well as for the corresponding critical points, are quite accurate. All results are discussed in terms of the basic assumptions underlying each theory. We have selected the MHNC for the fluid and the first-order PT for the solid phase, as the most accurate tools to investigate the phase behavior of the model in terms of purely theoretical approaches. The overall results appear as a robust benchmark for further theoretical investigations on higher order C(n>60) fullerenes, as well as on other fullerene-related materials, whose description can be based on a modelization similar to that adopted in this work., RevTeX4, 15 pages, 7 figures; submitted to Phys. Rev. E

Continuing our investigation into the Hierarchical Reference Theory of fluids for thermodynamic states of infinite isothermal compressibility kappa[T] we now turn to the available numerical evidence to elucidate the character of the partial differential equation: Of the three scenarios identified previously, only the assumption of the equations turning stiff when building up the divergence of kappa[T] allows for a satisfactory interpretation of the data. In addition to the asymptotic regime where the arguments of part I (cond-mat/0308467) directly apply, a similar mechanism is identified that gives rise to transient stiffness at intermediate cutoff for low enough temperature. Heuristic arguments point to a connection between the form of the Fourier transform of the perturbational part of the interaction potential and the cutoff where finite difference approximations of the differential equation cease to be applicable, and they highlight the rather special standing of the hard-core Yukawa potential as regards the severity of the computational difficulties., Comment: J. Stat. Phys., in press. Minor changes to match published version

The paper compares three numerical strategies to predict the aerodynamic rotor excitation sources of "Low Engine Order" (LEO) in a high-pressure turbine stage. Main focus is laid on methods to compute the stator exit flow. The aim is to evaluate computationally cheap approaches to avoid modelling the whole circumference of the stator. A single passage viscous strategy, a single passage inviscid linear blade movement strategy and a viscous multi passage sector strategy are compared and evaluated. The assessment of the prediction quality is made by comparison of the computed stator exit flow to experimental data. The main result is that only the global behaviour of the stator exit flow is estimated right, both level and amplitude of Mach number and pressure are computed with poor agreement to experiments. Future evaluations of the resulting rotor excitation pressure are needed to estimate the level of necessary agreement to give acceptable predictions of the low engine order forced response.

Load distribution is an essential factor to parallel efficiency of numerical simulations that are based on spatial grids, especially on clusters of symmetric multiprocessors (SMPs). This paper presents a method of mapping spatial grid nodes to processors that combines two load balancing methodologies, graph partitioning and graph matching, to achieve maximum parallel efficiency on SMP clusters. The method has been successfully applied to load distribution in a parallel Computational Fluid Dynamics (CFD) simulation code named TRACE. Test runs on the SR8000 prove the effectiveness of the method.

We develop a theory of the pole topology of the Laplace transform of the structure functions of continuous N component systems based on the Wiener-Hopf technique. We classify systems according to the spectrum of the NxN matrix Q(t), with elements Q(ij)(t)=delta(ij)-2pi square root [rho(i)rho(j)]integrale(-tr)q(ij)(r)dr, associated with their factor functions q(ij)(r). For the simplest nontrivial class of systems--namely, that with only two eigenvalues of Q(t) different from one--a full and explicit analysis of the pole topology is possible. We illustrate the theory with exactly solvable examples, such as the Percus-Yevick equation for arbitrary mixtures of hard spheres (HS) and polydisperse HS and the mean spherical model for binary mixtures of adhesive spheres.

Usually, a liquid metal is treated as a one-component liquid where the particles interact via a binary effective potential, which is determined within the pseudopotential formalism. However, this quite successful method for a liquid metal cannot be extended to calculate the structure of a plasma, since for such a system this kind of pseudopotential cannot be set up. In previous contributions, we have proposed a method which allows the calculation of the radial distribution functions (RDF’s) in an electron–ion mixture on the basis of the density functional (DF) theory; it is called the quantal hypernetted (QHNC) approximation: the QHNC equations are derived from exact expressions for the electron–ion and ion–ion RDF’s in an electron–ion mixture. Up to now, we have applied this approach to liquid metallic hydrogen, lithium, sodium, potassium and aluminum, obtaining ion–ion structure factors in excellent agreement with experiments. Recently, we have extended the formalism and have performed a first-principles molecular dynamics simulation based on the QHNC theory for alkali metals near the triple point: in this study those small deviations which were still observed between experimental results and QHNC data for the structure factor disappeared completely. In the present study we first show that the QHNC method can provide an accurate description of liquid metals in a wide range of densities and temperatures: we calculate the structure factors of compressed liquid rubidium, which have been studied experimentally at high pressures from 0.2 to 6.1 GPa by Tsuji; furthermore we calculate the structure factors of expanded rubidium states which have been studied by Franz et al. Secondly, we show that the QHNC method can be extended to treat a plasma: in such a system, both the ionic valency and the electron–ion interaction may vary over a wide range as temperature and density are changed. Our method is in particular suited to treat a plasma, since it is able to calculate these quantities in a self-consistent manner using the atomic number of the system as the only input data. In order to treat plasmas, the electron–electron correlation must be determined for arbitrary temperatures. In this work, we show that the QHNC method applied to the electron gas is in fact able to provide the electron correlation at arbitrary temperature. Using then this electron–electron correlation, we study how a liquid metal becomes a plasma for the case of rubidium by increasing the temperature at a fixed density. We can consider a liquid metal or a plasma as a mixture of electrons and ions interacting

Compared to the simple one-component case, the phase behaviour of binary liquid mixtures shows an incredibly rich variety of phenomena. In this contribution we restrict ourselves to so-called binary symmetric mixtures, i.e. where like-particle interactions are equal (Φ 11(r) = Φ22(r)), whereas the interactions between unlike fluid particles differ from those of likes ones (Φ 11(r)≠Φ 12(r)). Using both the simple mean spherical approximation and the more sophisticated self-consistent Ornstein-Zernike approximation, we have calculated the structural and thermodynamic properties of such a system and determine phase diagrams, paying particular attention to the critical behaviour (critical and tricritical points, critical end points). We then study the thermodynamic properties of the same binary mixture when it is in thermal equilibrium with a disordered porous matrix which we have realized by a frozen configuration of equally sized particles. We observe — in qualitative agreement with experiment — that already a minute matrix density is able to lead to drastic changes in the phase behaviour of the fluid. We systematically investigate the influence of the external system parameters (due to the matrix properties and the fluid—matrix interactions) and of the internal system parameters (due to the fluid properties) on the phase diagram.

This paper investigates the flutter mechanism of a high loaded transonic compressor rotor by applying different numerical methods. For predictions of the stability in the frequency domain, harmonic fluid forces are calculated with a linearized Euler method and with a nonlinear time domain Navier-Stokes solver for forced motion. To include nonlinear effects in the stability prediction, the time domain method is applied in a fluid-structure coupled mode of operation. The results of this 3D viscous aeroelastic computations in the time domain for the whole annulus of the rotor are compared to the frequency domain solutions.

We have exposed a two-dimensional nanodrop of particles interacting via an ultrasoft (i.e., bounded), purely repulsive potential to a combined thermo- and barostat. While increasing the pressure steadily via a suitable pressure increment the temperature of the system is kept at a fixed target temperature. Once the hexagonal crystal composed of clusters of overlapping particles has formed, we investigate the system's reaction on the non-equilibrium conditions. Recording the trajectories of the particles in molecular dynamics simulations, we can identify how particle hopping and cluster merging events are realized. We find that the number of particles involved in these processes is of comparable size and that under-populated clusters (with ∼70% of the average cluster size) are prone for merging processes. Theoretical predictions about the density-dependence of the average cluster size and of the nearest cluster-distance are confirmed within good accuracy.

The original published Supporting Information omitted the references; these now have been added.

We use monomer-resolved numerical simulations to study the positional and orientational structure of a dense dendrimer solution, focusing on the effects of dendrimers' prolate shape and deformability on the short-range order. Our results provide unambiguous evidence that the nearest-neighbor shell of a tagged particle consists of a mixture of crossed, side-by-side, side-to-end, and end-to-end pair configurations, imposing antinematic rather than nematic order observed in undeformable rodlike particles. This packing pattern persists even at densities where particle overlap becomes sizable. We demonstrate that the antinematic arrangement is compatible with the A15 crystal lattice reported in several dendrimer compounds., 6 pages, 3 figures

Combining renormalization group theoretical ideas with the integral equation approach to fluid structure and thermodynamics, the Hierarchical Reference Theory is known to be successful even in the vicinity of the critical point and for sub-critical temperatures. We here present a software package independent of earlier programs for the application of this theory to simple fluids composed of particles interacting via spherically symmetrical pair potentials, restricting ourselves to hard sphere reference systems. Using the hard-core Yukawa potential with z=1.8/sigma for illustration, we discuss our implementation and the results it yields, paying special attention to the core condition and emphasizing the decoupling assumption's role., RevTeX, 16 pages, 2 figures. Minor changes, published version

Based on an analytic solution of the mean spherical model for a binary hard sphere Yukawa mixture, we have examined the pair distribution functions g ij (r), focusing, in particular, on two aspects: ~i! We present two complementary methods to compute the g ij (r) accurately and efficiently over the entire r range. ~ii! The poles of the Laplace transforms of the pair distribution functions in the left half of the complex plane close to the origin determine the universal asymptotic behavior of the g ij (r). Although the meaning of the role of the subsequent poles—which typically are arranged in two branches—is not yet completely clear, there are strong indications that the distribution pattern of the poles is related to the thermodynamic state of the system. A series of model systems still plays, despite its simplic- ity, an important role in the theory of classical fluids: the pair interactions of the particles are characterized, throughout, by a hard core part, at contact adding an attractive ~or repulsive! tail. These systems are commonly referred to—in order of increasing complexity—as hard spheres ~HS's !, adhesive HS's, charged HS's, and HS's with a Yukawa tail ~HSY!. Characteristic of these systems is the fact that their structural and thermodynamic properties can be obtained from the so- lution of the Ornstein-Zernike ~OZ! equation—along with a suitable closure relation—to a large extent analytically. These systems were studied in the late 1960s and 1970s, but, still, considerable effort is being dedicated to research activi- ties of these systems; of course, meanwhile, the ideas that are pursued have changed to a higher level of complexity. There are several reasons why these analytically solvable model systems are still that attractive, and in the following a few of them we list: ~i! The interatomic potentials of some colloidal suspensions can be modeled quite accurately by such hard core interactions @1#. ~ii! It is certainly more con- venient to start the development of new theoretical methods or basic investigations from simple model systems, where at least for the structural and thermodynamic properties of the uniform fluid analytic expressions are available; three recent examples of such investigations are the extension of multi- component concepts to the polydisperse case @2,3# and the development of the fundamental measure theory @4# or of the self-consistent OZ approximation @5#. These developments were started, throughout, for such analytically solvable mod- els. ~iii! Despite the simplicity of their interactions, such sys- tems show phenomena which are qualitatively similar to sys- tems with more realistic potentials. The present paper is dedicated to a detailed analysis of the structural properties of a binary HSY mixture; we address aspects of the above mentioned items ~ii! and ~iii!. Among the series of model systems mentioned before, the HSY in- teraction represents undoubtedly the most general potential, since it covers—via special limiting prescriptions—the re- maining three cases. The properties of a HSY mixture are given within the mean spherical model ~MSM! to a large extent by analytical expressions only. While the solution of the MSM in terms of the direct correlation functions and the calculation of the thermodynamic properties have been dis- cussed ~see Ref. @6# and subsequent papers!, in the present paper we focus on a closer analysis of the pair distribution functions ~PDF's ! g ij (r) of a binary HSY mixture: first, we present two complementary semianalytic methods to com- pute the pair distribution functions; based on these represen- tations of g ij (r), in a subsequent step we investigate the asymptotic behavior of these functions and the distribution of the poles of the gij (t), the Laplace transforms of the @rg ij (r)#, in the left half ~LH! of the complex t plane. Additional motivation for our investigations on HSY sys- tems comes from a more methodological aspect. HS and HSY systems play an important role as reference systems in thermodynamic perturbation theories. In this context, the HSY system can be used in at least three ways: ~i! as a reference system for potentials with long range interactions, ~ii! as an improved parametrization of a HS system ~as for instance in the framework of the generalized mean spherical model @7#!, and ~iii! as an exact benchmark for numerical solutions of integral equations. In all cases an accurate and efficient calculation of the PDF's in the entire r range is required, avoiding, at the same time, time-consuming nu- merical Fourier transforms and numerical inaccuracies intro- duced by discontinuities of the correlation functions at con- tact. This aspect is covered by the first goal of this paper: we present a generalization of two methods @known in literature as the shell representation ~SR! and the asymptotic represen- tation ~AR!; for a brief historic review, see Ref. @8## to the case of a HSY system, which together form a reliable tool for the calculation of the PDF's. The methods are complemen- tary in the sense that they allow a computation of PDF's in the entire r range: the SR is more suitable for distances from contact up to intermediate r values, while in more distant regions ~where the SR becomes conceptually tedious and numerically inaccurate! the AR becomes more appropriate. The overlap region is located around five times the respec- tive HS diameter. Both approaches are based on the availability of analytic expressions for gij (t) which can be extracted from an ana

We use the virial theorem to derive a closed analytic form of the Helmholtz free energy for a polydisperse system of sticky hard spheres (SHS) within the mean spherical model (MSM). To this end we calculate the free energy of the MSM for an N-component mixture of SHS via the virial route and apply to it-after imposing a Lorentz-Berthelot type rule on the interactions-the stochastic (i.e., polydisperse) limit. The resulting excess free energy of this polydisperse system is of the truncatable moment free energy format. We also discuss the compressibility and the energy routes.

The phase behavior of a binary symmetric fluid in thermal equilibrium with a porous matrix has been studied with the optimized random phase approximation and grand canonical Monte Carlo simulations. Depending on the matrix properties and the matrix-fluid and fluid-fluid interactions we find three types of phase diagram characterized by a tricritical point, a tricritical point with a triple point, or a critical end point. Small changes in the properties of the matrix or in the interactions are demonstrated to lead to drastic modifications of the phase diagram of the fluid, in qualitative agreement with observations in experimental studies. We show, in particular, that the change between the different types of phase diagram is triggered not only by the fluid-fluid interactions (internal parameters) but also by the properties of the matrix and of the matrix-fluid potentials (external parameters).