Equilibrium Molecular Dynamics Calculations Research Articles

The contribution of propagons and diffusons in heat transport through calcium-silicate-hydrates

Whether it is glass, ceramics, cement, or concrete, minimizing thermal conduction through disordered materials is a determining factor when it comes to reducing the energy consumption of cities. In this work, we explore underlying physical processes involved in thermal conduction through the disordered glue of cement, calcium-silicate-hydrates (CSH). We find that at 300 K, phonon-like propagating modes in accordance with the Boltzmann transport theory, propagons, account for more than 30% of the total thermal conductivity, while diffusons, described via the Allen-Feldman theory, contribute to the remainder. The cumulative thermal conductivity proves to be close to both equilibrium molecular dynamics calculations and experimental values. These findings help us establish different strategies, such as localization schemes (to weaken diffusons) and scattering mechanisms (to constrain propagons), for reduction of thermal conductivity of CSH without sacrificing its mechanical properties.

Phonon-mediated heat dissipation in a monatomic lattice: case study on Ni

The recently introduced analytical model for the heat current autocorrelation function of a crystal with a monatomic lattice [Evteev et al., Phil. Mag. 94 (2014) p. 731 and 94 (2014) p. 3992] is employed in conjunction with the Green–Kubo formalism to investigate in detail the results of an equilibrium molecular dynamics calculations of the temperature dependence of the lattice thermal conductivity and phonon dynamics in f.c.c. Ni. Only the contribution to the lattice thermal conductivity determined by the phonon–phonon scattering processes is considered, while the contribution due to phonon–electron scattering processes is intentionally ignored. Nonetheless, during comparison of our data with experiment an estimation of the second contribution is made. Furthermore, by comparing the results obtained for f.c.c. Ni model to those for other models of elemental crystals with the f.c.c. lattice, we give an estimation of the scaling relations of the lattice thermal conductivity with other lattice properties such as the coefficient of thermal expansion and the bulk modulus. Moreover, within the framework of linear response theory and the fluctuation-dissipation theorem, we extend our analysis in this paper into the frequency domain to predict the power spectra of equilibrium fluctuations associated with the phonon-mediated heat dissipation in a monatomic lattice. The practical importance of the analytical treatment lies in the fact that it has the potential to be used in the future to efficiently decode the generic information on the lattice thermal conductivity and phonon dynamics from a power spectrum of the acoustic excitations in a monatomic crystal measured by a spectroscopic technique in the frequency range of about 1–20 THz.

Thermal conductivity in amorphous ices from molecular dynamics

The observed unusual thermal conductivities of low-, high- and very high-density amorphous ices ($a$-ices, LDA, HDA, and VHDA) are reproduced by equilibrium molecular-dynamics calculations with the Green-Kubo approach. The fast and slow relaxation times of the heat flux correlation functions were found to be dependent on, respectively, the short- and intermediate-range structural ordering. In conjunction with the collective nature of the acoustic vibrations, it can be concluded that the short and intermediate ordering of $a$-ices is similar to collapsed ice rather than a liquid. The larger phonon linewidths in HDA and VHDA vis-\`a-vis LDA are a clear indication of hybridization between acoustic and optic phonon branches resulting in an increase in decay channels in the dense amorphous states and shorter phonon mean-free paths.

Guest and host contributions towards thermal conduction in various polymorphs of methane hydrate

The thermal conductivity for structure I, II and H hydrate of methane have been calculated from heat flux correlation functions derived from equilibrium molecular dynamics calculations employing Green–Kubo response theory. For structure I hydrate, both the calculated temperature profile and the thermal conductivity values are in semi-quantitative agreement with experimental results. Analysis of the individual components of the heat flux correlation functions reveal the detailed mechanisms to explain the peculiar thermal behaviour of clathrate hydrates.

Evaluation of Thermal Conductivity of Hyperstoichiometric UO2+x by Molecular Dynamics Simulation

The thermal conductivity of UO2+x has been investigated by an equilibrium molecular dynamics (EMD) simulation up to 2000 K using the Born–Mayer–Huggins interatomic potential with the partially ionic model. In the present EMD system with the Green–Kubo method, the thermal conductivity was determined by the auto-correlation functions of energy and charge currents and the cross-coupling term. The thermal conductivity of UO2+x decreased with an increase of x and temperature. Its temperature dependence was relatively small for large x values, which was attributed to phonon scattering by excess oxygens. In addition, the heat capacity was calculated using the phonon-level density deduced by the velocity auto-correlation function for constituent ions. The phonon velocity was also evaluated by the phonon- dispersion relationship. Using these thermal properties obtained by EMD calculations, the effect of excess oxygens on the phonon mean free path was discussed.

Calculation of flexoelectric coefficients for a nematic liquid crystal by atomistic simulation

Equilibrium molecular dynamics calculations have been performed for the liquid crystal molecule n-4-(trans-4-n-pentylcyclohexyl)benzonitrile (PCH5) using a fully atomistic model. Simulation data have been obtained for a series of temperatures in the nematic phase. The simulation data have been used to calculate the flexoelectric coefficients e(s) and e(b) using the linear response formalism of Osipov and Nemtsov [M. A. Osipov and V. B. Nemtsov, Sov. Phys. Crstallogr. 31, 125 (1986)]. The temperature and order parameter dependence of e(s) and e(b) are examined, as are separate contributions from different intermolecular interactions. Values of e(s) and e(b) calculated from simulation are consistent with those found from experiment.

Calculation of the rotational viscosity of a nematic liquid crystal

Equilibrium molecular dynamics calculations have been performed for the liquid crystal molecule n-4-( trans-4- n-pentylcyclohexyl)benzonitrile (PCH5) using a fully atomistic model. Simulation data has been obtained for a series of temperatures in the nematic phase. The rotational viscosity co-efficient, γ 1, has been calculated using the angular velocity correlation function of the nematic director, n, the mean squared diffusion of n and statistical mechanical methods based on the rotational diffusion co-efficient. We find good agreement between the first two methods and experimental values.

Transport diffusivity of methane in silicalite from equilibrium and nonequilibrium simulations

The results of equilibrium and nonequilibrium simulations designed to predict the transport diffusivity of methane in silicalite are presented. The results are compared with equilibrium molecular dynamics calculations of the self-diffusivity for the same system, as well as with the Darken model, which is often invoked to relate self-diffusivities and transport diffusivities in zeolites. Good agreement is found between simulation results and the Darken model. Systems that are expected to deviate from the Darken model are discussed. 47 refs., 9 figs.

Thermal transport coefficients including the Soret coefficient for various liquid Lennard-Jones mixtures.

Thermal-transport coefficients have been determined for binary-liquid Lennard-Jones mixtures composed of strongly different component particles by equilibrium molecular-dynamics calculations. Two very unequal types of mixtures have been considered. The first type contained systems of particles of different interaction strength and equal ``volume,'' while the second type was comprised of systems of particles of different ``volume'' and equal interaction strength. For both mixture types, the 1-2 interaction potentials of the unlike pairs of particles have been varied systematically. We have chosen the Lennard-Jones potential parameters such that the mixtures with average 1-2 interaction showed nearly the same values for the mutual-diffusion coefficient, the shear and the bulk viscosity coefficients, as well as the thermal conductivity coefficient. This facilitated a comparison of the thermal diffusion coefficients of both types of systems. It was found that the latter coefficient differed drastically in magnitude and sign. On the other hand, the coefficient appeared to be rather insensitive to alterations of the 1-2 interaction, whereas the diffusion and viscosity coefficients showed considerable dependence on them.

Transport coefficients of dense fluids composed of globular molecules. Equilibrium molecular dynamics investigations using more-center Lennard-Jones potentials

Equilibrium molecular dynamics calculations with constraints have been performed for model liquids SF6 and CF4. The computations were carried out with four- and six-center Lennard-Jones potentials and up to 2×105 integration steps. Shear, bulk viscosity and the thermal conductivity have been calculated with use of Green–Kubo relations in the formulation of ‘‘molecule variables.’’ Various thermodynamic states were investigated. For SF6, a detailed comparison with experimental data was possible. For CF4, the MD results could only be compared with experiment for one liquid state. For the latter liquid, a complementary comparison was performed using MD results obtained with a one-center Lennard-Jones potential. A limited test of the particle number dependence of the results is presented. Partial and total correlations functions are shown and discussed with respect to findings obtained for the one-center Lennard-Jones liquid.

The Dufour and Soret coefficients of isotopic mixtures from equilibrium molecular dynamics calculations

The Soret and Dufour cross correlations functions (CCFs), as determined by equilibrium molecular dynamics calculations, are compared for soft sphere and Lennard-Jones (LJ) mixtures. The isotopic mixture was investigated to avoid separate determination of the partial enthalpy of the system. For the LJ system, potential truncation and particle number effects on the thermal diffusion coefficient were additionally studied. Main findings are: (i) Soret and Dufour CCFs are numerically equivalent, but the fluctuations in the latter are more pronounced, particularly for the LJ mixture. (ii) Using a symmetrized form of the tensor occurring in the heat current, differences between the two CCFs are noticeable, when the Stoermer–Verlet integration algorithm is applied. (iii) The particle number dependence of the CCF, and hence of the thermal diffusion coefficient, is very small.

Soret coefficient of isotopic Lennard-Jones mixtures and the Ar-Kr system as determined by equilibrium molecular-dynamics calculations.

The Soret coefficient (thermal diffusion coefficient) characterizing the coupling between heat and mass transport in more-component systems has been obtained by equilibrium molecular dynamics. Two-component liquid systems of the Lennard-Jones type were considered: three isotopic mixtures and Ar-Kr. Computations 20 times longer than the ones used for a ``usual'' transport coefficient, as the thermal conductivity, were necessary to obtain this cross coefficient with modest accuracy. The analysis of the Green-Kubo integrand shows that, opposite to the thermal conductivity, the thermal diffusion depends strongly on the terms containing the partial enthalpies of the components. For the isotopic mixtures, it was found that the Soret coefficient grows with the mass ratio in a way consistent with experimental results. For the Ar-Kr system, direct comparisons with nonequilibrium molecular-dynamics data indicated good agreement. As experimental Soret coefficients for Ar-Kr are lacking, we cannot compare our results with data of the real mixture. The order of magnitude found agrees, however, with that experimentally observed for binary liquid mixtures.