The Experts below are selected from a list of 32229 Experts worldwide ranked by ideXlab platform
Benoît Debbaut - One of the best experts on this subject based on the ideXlab platform.
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fourier transform rheology experiments and finite element simulations on linear polystyrene solutions
Journal of Rheology, 2003Co-Authors: Thorsten Neidhofer, Manfred Wilhelm, Benoît DebbautAbstract:Large amplitude oscillatory shear strain was applied to anionically synthesized linear polystyrene solutions in dioctylphthalate. The resulting torque was analyzed in Fourier space with respect to frequency, magnitude, and phase (Fourier-transform rheology). The concentration of the solutions was varied to achieve different degrees of entanglement. In addition, numerical simulations were performed using the Giesekus constitutive equation fitted on the basis of linear viscoelastic data. We found a good qualitative agreement between experiments and predictions; a quantitative agreement was reached for intermediate strain amplitudes. Some deviations were observed at very low strain amplitudes. We present a descriptive relation for the relative magnitude of the third harmonic as a Function of strain amplitude using a modified Damping Function. From this relation we obtained a universal parameter that describes the scaling behavior law for the increasing non-linearity (e.g., measured by the relative intensity of the third harmonic with respect to the response at the excitation frequency) as a Function of the strain amplitude. We found that the scaling exponent for the investigated linear polymer systems was independent of various factors. In addition, we analyzed the strain dependence of the relative phase of the higher harmonics. For vanishing strain amplitudes we define a property Φ30 that should reflect the contribution of the different relaxation modes to the viscoelastic response, and thus, a potential correlation to polymer topology.Large amplitude oscillatory shear strain was applied to anionically synthesized linear polystyrene solutions in dioctylphthalate. The resulting torque was analyzed in Fourier space with respect to frequency, magnitude, and phase (Fourier-transform rheology). The concentration of the solutions was varied to achieve different degrees of entanglement. In addition, numerical simulations were performed using the Giesekus constitutive equation fitted on the basis of linear viscoelastic data. We found a good qualitative agreement between experiments and predictions; a quantitative agreement was reached for intermediate strain amplitudes. Some deviations were observed at very low strain amplitudes. We present a descriptive relation for the relative magnitude of the third harmonic as a Function of strain amplitude using a modified Damping Function. From this relation we obtained a universal parameter that describes the scaling behavior law for the increasing non-linearity (e.g., measured by the relative intensity ...
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fourier transform rheology experiments and finite element simulations on linear polystyrene solutions
Journal of Rheology, 2003Co-Authors: Thorsten Neidhofer, Manfred Wilhelm, Benoît DebbautAbstract:Large amplitude oscillatory shear strain was applied to anionically synthesized linear polystyrene solutions in dioctylphthalate. The resulting torque was analyzed in Fourier space with respect to frequency, magnitude, and phase (Fourier-transform rheology). The concentration of the solutions was varied to achieve different degrees of entanglement. In addition, numerical simulations were performed using the Giesekus constitutive equation fitted on the basis of linear viscoelastic data. We found a good qualitative agreement between experiments and predictions; a quantitative agreement was reached for intermediate strain amplitudes. Some deviations were observed at very low strain amplitudes. We present a descriptive relation for the relative magnitude of the third harmonic as a Function of strain amplitude using a modified Damping Function. From this relation we obtained a universal parameter that describes the scaling behavior law for the increasing non-linearity (e.g., measured by the relative intensity ...
Hiromi Nakai - One of the best experts on this subject based on the ideXlab platform.
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density Functional method including weak interactions dispersion coefficients based on the local response approximation
Journal of Chemical Physics, 2009Co-Authors: Takeshi Sato, Hiromi NakaiAbstract:A new method to calculate the atom-atom dispersion coefficients in a molecule is proposed for the use in density Functional theory with dispersion (DFT-D) correction. The method is based on the local response approximation due to Dobson and Dinte [Phys. Rev. Lett. 76, 1780 (1996)], with modified dielectric model recently proposed by Vydrov and van Voorhis [J. Chem. Phys. 130, 104105 (2009)]. The local response model is used to calculate the distributed multipole polarizabilities of atoms in a molecule, from which the dispersion coefficients are obtained by an explicit frequency integral of the Casimir–Polder type. Thus obtained atomic polarizabilities are also used in the Damping Function for the short-range singularity. Unlike empirical DFT-D methods, the local response dispersion (LRD) method is able to calculate the dispersion energy from the ground-state electron density only. It is applicable to any geometry, free from physical constants such as van der Waals radii or atomic polarizabilities, and com...
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density Functional method including weak interactions dispersion coefficients based on the local response approximation
Journal of Chemical Physics, 2009Co-Authors: Takeshi Sato, Hiromi NakaiAbstract:A new method to calculate the atom-atom dispersion coefficients in a molecule is proposed for the use in density Functional theory with dispersion (DFT-D) correction. The method is based on the local response approximation due to Dobson and Dinte [Phys. Rev. Lett. 76, 1780 (1996)], with modified dielectric model recently proposed by Vydrov and van Voorhis [J. Chem. Phys. 130, 104105 (2009)]. The local response model is used to calculate the distributed multipole polarizabilities of atoms in a molecule, from which the dispersion coefficients are obtained by an explicit frequency integral of the Casimir-Polder type. Thus obtained atomic polarizabilities are also used in the Damping Function for the short-range singularity. Unlike empirical DFT-D methods, the local response dispersion (LRD) method is able to calculate the dispersion energy from the ground-state electron density only. It is applicable to any geometry, free from physical constants such as van der Waals radii or atomic polarizabilities, and computationally very efficient. The LRD method combined with the long-range corrected DFT Functional (LC-BOP) is applied to calculations of S22 weakly bound complex set [Phys. Chem. Chem. Phys. 8, 1985 (2006)]. Binding energies obtained by the LC-BOP+LRD agree remarkably well with ab initio references.
Thorsten Neidhofer - One of the best experts on this subject based on the ideXlab platform.
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fourier transform rheology experiments and finite element simulations on linear polystyrene solutions
Journal of Rheology, 2003Co-Authors: Thorsten Neidhofer, Manfred Wilhelm, Benoît DebbautAbstract:Large amplitude oscillatory shear strain was applied to anionically synthesized linear polystyrene solutions in dioctylphthalate. The resulting torque was analyzed in Fourier space with respect to frequency, magnitude, and phase (Fourier-transform rheology). The concentration of the solutions was varied to achieve different degrees of entanglement. In addition, numerical simulations were performed using the Giesekus constitutive equation fitted on the basis of linear viscoelastic data. We found a good qualitative agreement between experiments and predictions; a quantitative agreement was reached for intermediate strain amplitudes. Some deviations were observed at very low strain amplitudes. We present a descriptive relation for the relative magnitude of the third harmonic as a Function of strain amplitude using a modified Damping Function. From this relation we obtained a universal parameter that describes the scaling behavior law for the increasing non-linearity (e.g., measured by the relative intensity of the third harmonic with respect to the response at the excitation frequency) as a Function of the strain amplitude. We found that the scaling exponent for the investigated linear polymer systems was independent of various factors. In addition, we analyzed the strain dependence of the relative phase of the higher harmonics. For vanishing strain amplitudes we define a property Φ30 that should reflect the contribution of the different relaxation modes to the viscoelastic response, and thus, a potential correlation to polymer topology.Large amplitude oscillatory shear strain was applied to anionically synthesized linear polystyrene solutions in dioctylphthalate. The resulting torque was analyzed in Fourier space with respect to frequency, magnitude, and phase (Fourier-transform rheology). The concentration of the solutions was varied to achieve different degrees of entanglement. In addition, numerical simulations were performed using the Giesekus constitutive equation fitted on the basis of linear viscoelastic data. We found a good qualitative agreement between experiments and predictions; a quantitative agreement was reached for intermediate strain amplitudes. Some deviations were observed at very low strain amplitudes. We present a descriptive relation for the relative magnitude of the third harmonic as a Function of strain amplitude using a modified Damping Function. From this relation we obtained a universal parameter that describes the scaling behavior law for the increasing non-linearity (e.g., measured by the relative intensity ...
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fourier transform rheology experiments and finite element simulations on linear polystyrene solutions
Journal of Rheology, 2003Co-Authors: Thorsten Neidhofer, Manfred Wilhelm, Benoît DebbautAbstract:Large amplitude oscillatory shear strain was applied to anionically synthesized linear polystyrene solutions in dioctylphthalate. The resulting torque was analyzed in Fourier space with respect to frequency, magnitude, and phase (Fourier-transform rheology). The concentration of the solutions was varied to achieve different degrees of entanglement. In addition, numerical simulations were performed using the Giesekus constitutive equation fitted on the basis of linear viscoelastic data. We found a good qualitative agreement between experiments and predictions; a quantitative agreement was reached for intermediate strain amplitudes. Some deviations were observed at very low strain amplitudes. We present a descriptive relation for the relative magnitude of the third harmonic as a Function of strain amplitude using a modified Damping Function. From this relation we obtained a universal parameter that describes the scaling behavior law for the increasing non-linearity (e.g., measured by the relative intensity ...
Stefan Grimme - One of the best experts on this subject based on the ideXlab platform.
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extension of the d3 dispersion coefficient model
Journal of Chemical Physics, 2017Co-Authors: Eike Caldeweyher, Christoph Bannwarth, Stefan GrimmeAbstract:A new model, termed D4, for the efficient computation of molecular dipole-dipole dispersion coefficients is presented. As in the related, well established D3 scheme, these are obtained as a sum of atom-in-molecule dispersion coefficients over atom pairs. Both models make use of dynamic polarizabilities obtained from first-principles time-dependent density Functional theory calculations for atoms in different chemical environments employing fractional atomic coordination numbers for interpolation. Different from the D3 model, the coefficients are obtained on-the-fly by numerical Casimir-Polder integration of the dynamic, atomic polarizabilities α(iω). Most importantly, electronic density information is now incorporated via atomic partial charges computed at a semi-empirical quantum mechanical tight-binding level, which is used to scale the polarizabilities. Extended statistical measures show that errors for dispersion coefficients with the proposed D4 method are significantly lower than with D3 and other, computationally more involved schemes. Alongside, accurate isotropic charge and hybridization dependent, atom-in-molecule static polarizabilities are obtained with an unprecedented efficiency. Damping Function parameters are provided for three standard density Functionals, i.e., TPSS, PBE0, and B3LYP, allowing evaluation of the new DFT-D4 model for common non-covalent interaction energy benchmark sets.
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effect of the Damping Function in dispersion corrected density Functional theory
Journal of Computational Chemistry, 2011Co-Authors: Stefan Grimme, Stephan Ehrlich, Lars GoerigkAbstract:It is shown by an extensive benchmark on molecular energy data that the mathematical form of the Damping Function in DFT-D methods has only a minor impact on the quality of the results. For 12 different Functionals, a standard "zero-Damping" formula and rational Damping to finite values for small interatomic distances according to Becke and Johnson (BJ-Damping) has been tested. The same (DFT-D3) scheme for the computation of the dispersion coefficients is used. The BJ-Damping requires one fit parameter more for each Functional (three instead of two) but has the advantage of avoiding repulsive interatomic forces at shorter distances. With BJ-Damping better results for nonbonded distances and more clear effects of intramolecular dispersion in four representative molecular structures are found. For the noncovalently-bonded structures in the S22 set, both schemes lead to very similar intermolecular distances. For noncovalent interaction energies BJ-Damping performs slightly better but both variants can be recommended in general. The exception to this is Hartree-Fock that can be recommended only in the BJ-variant and which is then close to the accuracy of corrected GGAs for non-covalent interactions. According to the thermodynamic benchmarks BJ-Damping is more accurate especially for medium-range electron correlation problems and only small and practically insignificant double-counting effects are observed. It seems to provide a physically correct short-range behavior of correlation/dispersion even with unmodified standard Functionals. In any case, the differences between the two methods are much smaller than the overall dispersion effect and often also smaller than the influence of the underlying density Functional.
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effect of the Damping Function in dispersion corrected density Functional theory
Journal of Computational Chemistry, 2011Co-Authors: Stefan Grimme, Stephan Ehrlich, Lars GoerigkAbstract:It is shown by an extensive benchmark on molecular energy data that the mathematical form of the Damping Function in DFT-D methods has only a minor impact on the quality of the results. For 12 different Functionals, a standard “zero-Damping” formula and rational Damping to finite values for small interatomic distances according to Becke and Johnson (BJ-Damping) has been tested. The same (DFT-D3) scheme for the computation of the dispersion coefficients is used. The BJ-Damping requires one fit parameter more for each Functional (three instead of two) but has the advantage of avoiding repulsive interatomic forces at shorter distances. With BJ-Damping better results for nonbonded distances and more clear effects of intramolecular dispersion in four representative molecular structures are found. For the noncovalently-bonded structures in the S22 set, both schemes lead to very similar intermolecular distances. For noncovalent interaction energies BJ-Damping performs slightly better but both variants can be recommended in general. The exception to this is Hartree-Fock that can be recommended only in the BJ-variant and which is then close to the accuracy of corrected GGAs for non-covalent interactions. According to the thermodynamic benchmarks BJ-Damping is more accurate especially for medium-range electron correlation problems and only small and practically insignificant double-counting effects are observed. It seems to provide a physically correct short-range behavior of correlation/dispersion even with unmodified standard Functionals. In any case, the differences between the two methods are much smaller than the overall dispersion effect and often also smaller than the influence of the underlying density Functional. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011
Paul Geerlings - One of the best experts on this subject based on the ideXlab platform.
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evaluating interaction energies of weakly bonded systems using the buckingham hirshfeld method
Journal of Chemical Physics, 2014Co-Authors: Alisa Krishtal, Christian Van Alsenoy, Paul GeerlingsAbstract:We present the finalized Buckingham-Hirshfeld method (BHD-DFT) for the evaluation of interaction energies of non-bonded dimers with Density Functional Theory (DFT). In the method, dispersion energies are evaluated from static multipole polarizabilities, obtained on-the-fly from Coupled Perturbed Kohn-Sham calculations and partitioned into diatomic contributions using the iterative Hirshfeld partitioning method. The dispersion energy expression is distributed over four atoms and has therefore a higher delocalized character compared to the standard pairwise expressions. Additionally, full multipolar polarizability tensors are used as opposed to effective polarizabilities, allowing to retain the anisotropic character at no additional computational cost. A density dependent Damping Function for the BLYP, PBE, BP86, B3LYP, and PBE0 Functionals has been implemented, containing two global parameters which were fitted to interaction energies and geometries of a selected number of dimers using a bi-variate RMS fit. The method is benchmarked against the S22 and S66 data sets for equilibrium geometries and the S22x5 and S66x8 data sets for interaction energies around the equilibrium geometry. Best results are achieved using the B3LYP Functional with mean average deviation values of 0.30 and 0.24 kcal/mol for the S22 and S66 data sets, respectively. This situates the BHD-DFT method among the best performing dispersion inclusive DFT methods. Effect of counterpoise correction on DFT energies is discussed.
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evaluating london dispersion interactions in dft a nonlocal anisotropic buckingham hirshfeld model
Journal of Chemical Theory and Computation, 2012Co-Authors: Alisa Krishtal, Dirk Geldof, Kenno Vanommeslaeghe, Christian Van Alsenoy, Paul GeerlingsAbstract:In this work, we present a novel model, referred to as BH-DFT-D, for the evaluation of London dispersion, with the purpose to correct the performance of local DFT exchange-correlation Functionals for the description of van der Waals interactions. The new BH-DFT-D model combines the equations originally derived by Buckingham [Buckingham, A. D. Adv. Chem. Phys1967, 12, 107] with the definition of distributed multipole polarizability tensors within the Hirshfeld method [Hirshfeld, F.L. Theor. Chim. Acta1977, 44, 129], resulting in nonlocal, fully anisotropic expressions. Since no Damping Function has been introduced yet into the model, it is suitable in its present form for the evaluation of dispersion interactions in van der Waals dimers with no or negligible overlap. The new method is tested for an extended collection of van der Waals dimers against high-level data, where it is found to reproduce interaction energies at the BH-B3LYP-D/aug-cc-pVTZ level with a mean average error (MAE) of 0.20 kcal/mol. Next...
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the use of atomic intrinsic polarizabilities in the evaluation of the dispersion energy
Journal of Chemical Physics, 2007Co-Authors: Andras Olasz, Alisa Krishtal, Kenno Vanommeslaeghe, Christian Van Alsenoy, Tamas Veszpremi, Paul GeerlingsAbstract:The recent approach presented by Becke and Johnson [J. Chem. Phys. 122, 154104 (2005); 123, 024101 (2005); 123, 154101 (2005); 124, 174104 (2006); 124, 014104 (2006)] for the evaluation of dispersion interactions based on the properties of the exchange-hole dipole moment is combined with a Hirshfeld-type partitioning for the molecular polarizabilities into atomic contributions, recently presented by some of the present authors [A. Krishtal et al., J. Chem. Phys. 125, 034312 (2006)]. The results on a series of nine dimers, involving neon, methane, ethene, acetylene, benzene, and CO(2), taken at their equilibrium geometry, indicate that when the C(6), C(8), and C(10) terms are taken into account, the resulting dispersion energies can be obtained deviating 3% or 8% from high level literature data [E. R. Johnson and A. D. Becke, J. Chem. Phys. 124, 174104 (2006)], without the use of a Damping Function, the only outlier being the parallel face-to-face benzene dimer.
