102,028 research outputs found
Liquid crystal properties of the n-alkyl-cyanobiphenyl series from atomistic simulations with Ab initio derived force fields
No full textLengthy molecular dynamics (MD) simulations were performed at constant atmospheric pressure and different
temperatures for the series of the 4-n-alkyl-4¢-cyanobiphenyls (nCB) with n ) 6, 7, and 8. The accurate
atomistic force field (Bizzarri, M.; Cacelli, I.; Prampolini, G; Tani, A. J. Phys. Chem. A 2004, 108, 10336),
successfully employed to reproduce thermodynamic and transport properties of the 5CB molecule, has here
been extended to higher homologues. Nematic and isotropic phases were found for all members of the series,
and also, a smectic phase was (tentatively) identified for 8CB at 1 atm and 300 K. Transition temperatures
reproduce the experimental values within (10 K. Also, structural properties as second and fourth rank
orientational order parameters are in good agreement with the corresponding experimental quantities. This
means that the well-known odd-even effect, observed for many properties along the nCB series, is well
reproduced, despite the narrow range of oscillations, e.g., in clearing temperatures. A detailed analysis of the
correlation between molecular properties and odd-even effects is presented
Parameterization and validation of intramolecular force fields derived from DFT calculations
No full textThe energy and its first and second geometrical derivatives obtained by DFT calculations for a number of conformations of a single molecule are used to parametrize intramolecular force fields, suitable for computer simulations. A systematic procedure is proposed to adequately treat either fully atomistic or more simplified force fields, as within the united atom approach or other coarse grained models. The proposed method is tested and validated by performing molecular dynamics simulations on several different molecules, comparing the results with literature force fields and relevant experimental data. Particular emphasis is given to the united atom approach for flexible molecules characterized by "soft" torsional potentials which are known to retain a high degree of chemical specificity
Torsional barriers and correlations between dihedrals in p-polyphenyls
No full textThe torsional energy curves for biphenyl, p-terphenyl, and p-quaterphenyl are calculated using the B3LYP density functional with a triple-zeta polarized basis set. In agreement with recent accurate literature data, barriers of similar height are found at 0degrees and 90degrees for biphenyl. For the higher members, the torsional energy curves show an increasing tendency to lower the barrier of the coplanar conformations. The correlation effects between different dihedrals are reasonably small and discussed extensively. In addition, torsional potential functions at different levels of accuracy, suitable for computer simulations, are proposed for all the members of the series up to p-quinquephenyl
DFT conformational study of banana-shaped molecules
No full textThe conformational energy of the 1,3-phenylene bis[4-4-(methoxy) benzoyloxy] benzoate and 4-chloro-1,3-phenylene bis[4-4'(methoxy) benzoyloxy] benzoate molecules has been studied using the B3LYP density functional, with a double polarized basis set. These molecules can be seen as a model for the aromatic core of recently discovered banana-shaped mesogens. The relationship between the bent molecular shape and the conformations assumed by the phenyl ester dihedrals is studied together with the effect of substitutions on the central ring. In particular, the inclusion of a chlorine atom in the central ring results in an increase of the bending angle, consistently with recent experimental findings. A discussion of the chirality of some of the resulting energy minima conformations is also given
Accurate yet feasible post-Hartree-Fock computation of magnetic interactions in large biradicals through a combined variational/perturbative approach: setup and validation
No full textSubdiffusive dynamics of a liquid crystal in the isotropic phase
No full textThe isotropic phase dynamics of a system of 4-n-hexyl-4'-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275-330 K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the beta-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1 ns). The time-temperature superposition principle and the factorization property predicted by the idealized version of MCT hold, again for both kinds of dynamics. The temperature dependence of diffusion coefficient and orientational relaxation time is well described by a power law. Critical temperatures T(c) are 244 +/- 6 and 258 +/- 6 K, respectively, the latter is some 10 K below the corresponding experimental values. The different values of T(c) we obtained indicate that ODOF freezes earlier than TDOF. This appears due to the strongly anisotropic environment that surrounds a 6CB molecule, even in the isotropic phase. The lifetime of these "cages," estimated by time dependent conditional probability functions, is strongly temperature dependent, ranging from some hundreds of picoseconds at 320 K to a few nanoseconds at 275 K
Force-field modeling through quantum mechanical calculations: molecular dynamics simulations of a nematogenic molecule in its condensed phases
No full textInteraction energy of the 4-n-pentyloxy-4'-cyanobiphenyl (5OCB) dimer is Computed at MP2 level, for many geometrical arrangements using the Fragmentation Reconstruction Method (FRM). DFT calculations are performed for a number of geometries of the monomer. The resulting database is used to parameterize an atomistic intra- and inter-molecular force-field suitable for classical bulk simulations. Several structural and dynamical properties in 5OCB isotropic and liquid crystalline phases are computed from molecular dynamics simulation mainly in the NPT ensemble. Lengthy runs (more than 70 us) and large sample sizes (up to 806 molecules) were used to determine the nematic to isotropic transition temperature up to a precision of few K. Good agreement was found in most of the investigated properties, thus validating the accuracy of the proposed model potential. only derived by quantum mechanical calculations
Atomistic simulation of a nematogen using a force field derived from quantum chemical calculations
No full textBulk phase atomistic computer simulations of 4-n-pentyl-4¢-cyanobiphenyl (5CB) were performed with a
specific force field obtained from ab initio and DFT calculations. The intermolecular potential was previously
derived through the fragmentation reconstruction method (FRM), developed in our group. The description of
some intramolecular interactions, like the torsional potential between the phenyl rings and at the aryl-alkyl
linkage, is achieved through accurate DFT studies. Lengthy (=40 ns) molecular dynamics (MD) simulations
were then carried out at constant atmospheric pressure and different temperatures. The system was stable in
the experimental crystalline structure up to 285 K, where the early stage of the melting process appears with
the loss of positional order. At higher temperatures (between 290 and 305 K) a kinetically stable, orientationally
ordered phase is obtained. This nematic phase was reached starting with three initial configurations, differing
in their orientational order parameter. The calculated values of thermodynamic and structural properties of
each phase were in fairly good agreement with the relevant experimental data
Anomalous Diffusion and Cage Effects in the Isotropic Phase of a Liquid Crystal
No full textThe translational motion of 4-n-hexyl-4'-cyanobiphenyl (6CB) in its isotropic phase has been studied by atomistic molecular dynamics simulation from 280 to 330 K. The mean square displacement shows evidence of a subdiffusive dynamics, with a plateau that becomes very apparent at the lowest temperatures. A three-time self-intermediate scattering function reveals that this plateau is connected with a homogeneous dynamics that, at longer times, becomes heterogeneous and finally exponential. These features are shared by, for example, a high-density system of hard spheres, which supports the universal character of the translational dynamics of liquids in their supercooled condition. As predicted by the idealized version of the mode-coupling theory (MCT), the diffusion coefficient dependence upon temperature is well described by a power law, with a critical temperature very close to that obtained by experimental measurements on orientational relaxation. This agreement might indicate a complete freezing of both rotational and translational intradomain dynamics. The time-temperature superposition principle also holds. The shape of the cage that surrounds a 6CB molecule has been reconstructed, and this analysis suggests a preferential side-by-side arrangement of molecules, which locally tend to align their long axes even in the isotropic phase
Modeling a Liquid Crystal Dynamics by Atomistic Simulation with an Ab Initio Derived Force Field
No full textAtomistic molecular dynamics (MD) simulations of 4-n-pentyl 4'-cyano-biphenyl (5CB) have been performed, adopting a specific ab initio derived force field.(1) Two state points in the nematic phase and three in the isotropic phase, as determined in a previous work,(2) have been considered. At each state point, at least 10 ns have been produced, allowing us to accurately calculate single-molecule properties. In the isotropic phase, the values of the translational diffusion coefficient, and even more so the activation energy for the process, agree well with experimental data. Qualitatively, also the dynamic anisotropy of the nematic phase is correctly accounted for. Rotational diffusion coefficients, which describe spinning and tumbling motions, fall well within the range of experimental values. The reorientational dynamics of our model 5CB covers diverse time regimes. The longest one is strongly temperature dependent and characterized by a relaxation time in accord with experimental dielectric relaxation data. Shear viscosity and Landau-de Gennes relaxation times, typically collective variables, reproduce the experimental results very well in the isotropic phase. In the nematic phase, despite a large statistical uncertainty due to the extremely slow relaxation of the correlation functions involved, our simulation yields the correct relative order of the three experimental Miesowicz viscosities
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