This paper demonstrates the excellent temperature control and rapid equipartioning of the kinetic energy of the massive generalized Gaussian moment thermostat (MGGMT, one thermostat is coupled to each degree of freedom) in isothermal density functional molecular dynamics (MD) simulations on the Born–Oppenheimer potential energy surface. The MGGMT is implemented in the DMoL3 approach and, as far as we know, it is the first time in literature that the MGGMT is combined with density functional methods. The performance of the MGGMT approach is illustrated with MD simulations of the iron porphyrin–imidazole–carbon monoxide [FeP(Im)(CO)] complex and compared with constant energy MD simulations on the same system. Both MD approaches lead to similar average structures of the FeP(Im)(CO) complex. The examination of the frequency distribution functions reveals that the structural dynamics are not seriously affected by the dynamics of the parameters introduced by the MGGMT. The equipartitioning rates in the MGGMT simulations are significantly faster than in the constant energy simulation. We recommend the MGGMT approach as an very efficient equilibration technique in MD simulations and it emerges as a useful technique for, e.g., simulated annealing and nonequilibrium MD simulations.
We report an experimental and theoretical study of the strong dependence of the crystalline texture of thin films of Sr x Ba 1−x Nb 2 O 6 grown on MgO͑001͒ on the Sr-content x between 0.35ഛ x ഛ 0.75. Synchrotron-based x-ray diffraction measurements have identified three film-to-substrate crystal orientations, with contributions from domains at 0°, ±18.43°, and ±30.96°. The relative contributions from these domains change dramatically with x. Surprisingly, the ±18.43°orientation dominates, particularly for high x, although it has the largest lattice mismatch to the underlying substrate. These results can only be explained by detailed modeling of the electrostatic forces between the ions at the film-substrate interface, and not with simplistic lattice mismatch arguments. Therefore, molecular dynamics simulations of the ionic heterogeneous interface structure of thin films of such large oxide systems have been performed in an attempt to explain the diffraction data. The simulations predict that, depending on x, an initial ultrathin layer of SrNb 2 O 6 is favored, such that its Nb-O network forms the first ionic layer on the MgO͑001͒ substrate. This provides the necessary template for the change in crystallographic orientation with x. Such layers were subsequently identified by further synchrotronbased x-ray-diffraction measurements. This information has important consequences for ferroelectric and electrooptic applications, on the optimal conditions required to grow thin films with large crystalline domains of the desired orientation.
Spin density functional calculations employing the full potential linearized augmented plane wave method (FLAPW) are performed on the periodic structure of sodium electro sodalite. The density functional adopted (PW91) includes gradient corrections for exchange and correlation. A body-centered cubic lattice of bare Na43+ clusters is found to be metallic and diamagnetic. The presence of the aluminosilicate framework makes it an antiferromagnetic material with a gap of about 0.1 eV between valence and conduction band. The antiferromagnetic state is more than 110–170 kJ mol−1 more stable than a ferromagnetic state. The Heisenberg exchange integrals between nearest and next nearest neighboring Na43+ clusters, Jnn and Jnnn, are derived from an extended Hückel tight-binding approach. The parameters of this Hamiltonian were chosen such that the density functional band structure is reproduced. The Heisenberg exchange integrals between nearest and next nearest neighboring Na43+ sites, Jnn and Jnnn, are negative, i.e., both nn and nnn sites are coupled antiferromagnetically. In absolute terms Jnnn is 0.6 meV. Estimates of Jnn are between 6.4 and 9.5 meV, the most likely value being 8.1±0.5 meV. Using these values for Jnn and Jnnn the molecular field approximation yields Weiss temperatures between −160 and −230 K. The most likely result, −200±10 K, fits well to the experimental value of about −200 K.
Advancements in drilling and production technologies have made exploiting resources, which for long time were labeled unproducible such as shales, as economically feasible. In particular, lateral drilling coupled with hydraulic fracturing has created means for hydrocarbons to be transported from the shale matrix through the stimulated network of microcracks, natural fractures, and hydraulic fractures to the wellbore. Because of the degree of confinement, the ultimate recovery is just a small fraction of the total hydrocarbons in place. Our aim was to investigate how augmented pressure gradient through hydraulic fracturing when coupled with another derive mechanism such as heating can improve the overall recovery for more sustainable exploitation of unconventional resources. Knowledge on how hydrocarbons are stored and transported within the shale matrix is uncertain. Shale matrix, which consists of organic and inorganic constituents, have pore sizes of few nanometers, a degree of confinement at which our typical reservoir engineering models break down. These intricacies hinder any thorough investigations of hydrocarbon production from shale matrix under the influence of pressure and thermal gradients. Kerogen, which represents the solid part of the organic materials in shales, serves as form of nanoporous media, where hydrocarbons are stored and then expelled after shale stimulation procedure. In this work, a computational representation of a kerogen–hydrocarbon system was replicated to study the depletion process under coupled mechanisms of pressure and temperature. The extent of production enhancement because of increasing temperature was shown. Moreover, heating requirements to achieve the enhancement at reservoir scale was also presented to assess the sustainability of the proposed method.
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