1,720,969 research outputs found
Numerical study of a calamitic liquid-crystal model: Phase behavior and structure
No full textWe have studied an idealized calamitic liquid-crystal model, consisting of a linear rigid array of nine soft repulsive spheres, employing both theory and molecular dynamics simulation. The phase behavior (which includes crystalline, smectic, nematic, and isotropic phases) and structure of a collection of these rodlike particles have been determined by molecular dynamics simulation in an isothermal-isobaric ensemble. The liquid crystalline part of the phase diagram has been compared to that emerging from an Onsager-type density-functional theory. We have found a fair agreement between theory and computer simulation results, with a similar accuracy for the smectic to nematic and nematic to isotropic phase transitions
Atomistic Molecular Dynamics Simulation of Hexakis(pentyloxy)triphenylene: Structure and Translational Dynamics of Its Columnar State
No full textAtomistic molecular dynamics computer simulations have been performed on the columnar state of the discogen hexakis(pentyloxy)triphenylene (HAT5). The parameters of the empirical model potential have been taken from the AMBER and OPLS force fields. A 10 ns simulation run has been carried out in Berendsen's ensemble at pressure P = 0.1 MPa and temperature T = 375 K to study bulk and molecular structure. The bulk structure has been characterized by obtaining structural parameters that compare favorably with those deduced from X-ray diffraction experiments. C-13 and H-2 NMR data suggest two completely different pictures of the position of the pendant chains with respect to the plane of the aromatic core: one, called diablo-like, with the chains in the triphenylene plane; the other, octopus-like, with the chains out of this plane and, alternatively, above and below it. Our simulated data are consistent with an intermediate situation. In agreement with all NMR data, we find that the chains become more and more disordered, moving away from the core. We have observed a decrease of the disorder of the lateral chains in going from gas phase to condensed bulk state. Information on translational dynamics has been obtained by performing a 1 ns simulation in the microcanonical ensemble; these calculations definitely assess the one-dimensional fluid character of the columnar state. We have found that the mean square displacement has a solidlike behavior in the plane perpendicular to the column axis. The translational dynamics along the direction parallel to it is dominated by the reciprocal, fluidlike sliding motion of the columns as a whole. If this collective dynamics is removed, also the parallel diffusion coefficient turns out to be solidlike
Computer Simulation of Solid and Liquid Benzene with an Atomistic Interaction Potential Derived from ab Initio Calculations
No full textMolecular dynamics atomistic simulations of solid and liquid benzene have been performed, employing a model intermolecular potential derived from quantum mechanical calculations. The ab initio database includes approximately 200 geometries of the benzene dimer with interaction energies computed at the MP2 level of theory. The accuracy of the modeled force field results is satisfactory. The thermodynamic and structural properties, calculated in the condensed phases, are compared with experimental data and previous simulation results. Single particle and collective dynamical properties are also investigated through the calculation of translational and rotational diffusion coefficients, reorientational dynamics, and viscosities. The agreement of these data with experimental measurements confirms the reliability of the proposed force field
Orientational dynamics in the isotropic phase of a calamitic liquid-crystal model
No full textWe report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of similar or equal to 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions. Qualitatively, our simulation data are consistent with these findings. Both single-particle and collective time correlation function derivatives possess, in their respective log-log plots, a linear region at very short times, whose slope is essentially independent from the thermodynamic state. Nevertheless, the single-particle orientational correlation functions are better described by a function which is the sum of a fast exponential, an intermediate stretched-exponential and a slow exponential, while the collective orientational correlation functions are satisfactorily described by a sum of two exponentials, at higher density, or by just one exponential, at lower density
Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation
No full textWe report a molecular dynamics simulation study on an ensemble of rod-like particles, each composed of nine soft spheres held rigidly along a line. We have calculated translational mean square displacements and velocity autocorrelation functions in the fluid phases exhibited by the model, i.e., smectic A, nematic and isotropic. These quantities have then been used to compute diffusion coefficients. In addition, we have calculated viscosities in the nematic and isotropic phases. Despite its crude nature, the model is capable of providing a faithful reproduction of many features of the transport behavior observed in real liquid-crystalline materials. The simulation results have been compared with the predictions of the modified affine transformation theory, finding only a fair agreement
On the Distribution Functions of Depletion Interactions
No full textMolecular dynamics computer simulations were performed on model colloidal binary mixtures of two large and many small soft repulsive spheres. Depletion forces arise between the two large spheres, as a function of their distance, because of the nonadditivity of the volume they exclude to the small spheres. The probability distribution functions of both longitudinal and transverse component of the total force exerted by the small particles were calculated and generally turned out non-Gaussian. The distributions of the collective forces were analyzed in terms of the distribution of the force that a single small sphere exerts on a large sphere and of the number of the surrounding small spheres. The reconstructed function matches well the corresponding exact distribution. Residual correlation among small particles, combined with a relatively small number of neighbors, slows the approach to the Gaussian limit. In our fully repulsive model, the direct force between a large and a small sphere is a monotonic function of their distance. On these bases, we propose and successfully test an approach that relates the probability distribution function of the depletion forces to the large-sphere small-sphere radial distribution function. This approach can be extended to experimental data of radial distribution function, thus allowing for an estimate of depletion force fluctuations in real colloidal mixture
Computer simulation of mesogens with ab-initio interaction potentials. An application to oligophenyls
No full textModeling benzene with single-site potentials from ab initio calculations: a step toward hybrid models of complex molecules
No full textExtensive ab initio calculations at the MP2/6-31G* level have been carried out to sample the energy surface for the interactions of the benzene dimers. This database has been used to parameterize two anisotropic single-site models, meant to be used as building blocks in hybrid models of complex, liquid crystal forming molecules. A quadrupolar Gay-Berne (GBQIII) and an S-function (SF) Corner potentials have been obtained in this way. Their ability to reproduce, qualitatively at least, the phase diagram as well as energetic and structural properties of benzene has been tested with Monte Carlo simulations and compared with previous literature potentials, GBQI [S. Gupta , Mol. Phys. 65, 961 (1988)] and GBQII [T. R. Walsh, Mol. Phys. 100, 2867 (2002)]. It turned out that GBQI showed no melting transition in the temperature range explored (100-400 K), while GBQII underwent a phase transition from solid to gas, with no liquid phase. Conversely, both models parameterized on our database of ab initio interaction energies (GBQIII and SF) gave rise to a stable liquid phase. Melting has been observed between 100 and 150 K (GBQIII) and in the range 300-350 K (SF), i.e., substantially below and slightly above the experimental value at ambient pressure, 278 K. The description of the crystal structure of benzene at atmospheric pressure is also in better agreement with experimental data if the SF model is used, while positional correlations in the liquid are better described by the GBQIII potential. The S-function potential is also computationally more convenient. These results could be useful in the semirealistic modeling of more complex molecules
Structure and dynamics of mesogens using intermolecular potentials derived from ab initio calculations
No full textA method for the calculation of the two-body intermolecular potential which can be applied to large molecules is presented. Each monomer is fragmented in a number of moieties whose interaction energies are used to recover the interaction energy of the whole dimer. For these reasons this strategy has been called fragmentation reconstruction method (FRM). By a judicious choice of the fragmentation scheme it is shown that very accurate interaction energies can be obtained. The sampling of the potential energy surface of a dimer is then used to obtain intermolecular force fields at several levels of complexity, suitable to be employed in bulk phase computer simulations. Applications are presented for benzene and for some mesogenic molecules which constitute the principal interest of the authors. A number of properties ranging from phase stability, thermodynamic quantities, orientational order parameter and collective dynamics properties are computed and discussed
Hierarchical modeling of carbon nanoribbon devices for CNR-FETs engineering
No full textWe present a hierarchical approach to the modelling of CNR-FETs, which blends together first-principle density functional theory (DFT) for subband calculations, full 2D atomistic TB modelling, and effective mass (EM) 1D quantum transport modelling, improved with nonparabolic (NP) corrections. The approach is applicable to armchair semiconductor CNRs. Moving along the hierarchy of models from the most physically in-depth (DFT) to the most details-free (EM) approach, more accurate models are used to calibrate the parameters of less accurate ones. In-depth models are suitable for the simulation of very small FETs (both narrow and short ribbons), but are impractical for devices of large sizes, which however are the ones that can be fabricated with the state-of-the-art technology. For such devices, where quantum effects already play a major role, the NPEM approach is quite effective. We compare simulation results from the various approaches for FETs based on very narrow CNRs, namely (6,0) with W = 0.6 nm and (12,0) with W = 1.35 nm. We show that the NPEM model can fairly well describe the I-V characteristics in all bias conditions, including the regimes dominated by direct or band-to-band tunneling, provided first-order NP corrections are properly included. A (40,0) CNR-FET, corresponding to a more realistic W = 4.8 nm, is investigated by means of the NPEM approach, suggesting the possibility of an optimization study
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