RESUMO
First-principles molecular dynamics (FPMD) calculations were performed on liquid GeSe4 with the aim of inferring the impact of dispersion (van der Waals, vdW) forces on the structural properties. Different expressions for the dispersion forces were employed, allowing us to draw conclusions on their performances in a comparative fashion. These results supersede previous FPMD calculations obtained in smaller systems and shorter time trajectories by providing data of unprecedented accuracy. We obtained a substantial agreement with experiments for the structure factor regardless of the vdW scheme employed. This objective was achieved by using (in addition to FPMD with no dispersion forces) a selection of vdW schemes available within density functional theory. The first two are due to Grimme, D2 and D3, and the third one is devised within the so-called maximally localized Wannier functions approach (MLWF). D3 results feature a sizeable disagreement in real space with D2 and MLWF in terms of the partial and total pair correlation functions as well as the coordination numbers. More strikingly, total and partial structure factors calculated with D3 exhibit an unexpected sharp increase at low k. This peculiarity goes along with large void regions within the network, standing for a phase separation of indecipherable physical meaning. In view of these findings, further evidence of unconventional structural properties found by employing D3 is presented by relying on results obtained for a complex ionic liquid supported on a solid surface. The novelty of our study is multifold: new, reliable FPMD data for a prototypical disordered network system, convincing agreement with experimental data and assessment of the impact of dispersion forces, with emphasis on the intriguing behavior of one specific recipe and the discovery of common structural features shared by drastically dissimilar physical systems when the D3 vdW scheme is employed.
RESUMO
We provide a microscopic insight, both structural and electronic, into the multifold interactions occurring in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI] currently targeted for applications in next-generation low-power electronics and optoelectronic devices. To date, practical applications have remained hampered by the lack of fundamental understanding of the interactions occurring both inside the IL and at the interface with the substrate. Our first principles dynamical simulations provide accurate insights into the nature of bonding and non-bonding interactions, dynamical conformational changes and induced dipole moments, along with their statistical distributions, of this ionic liquid, that have so far not been completely unraveled. The mobilities of the two ionic species are obtained by long-lasting dynamical simulations at finite temperature, allowing simultaneous monitoring and quantification of the isomerization occurring in the IL. Moreover, a thorough analysis of the electronic structure and partial charge distributions characterizing the two components, the cation and anion, allow rationalization of the nature of the electrostatic interactions, hydrogen bonding properties of the two ionic counterparts, and the infra-red and dielectric response of the system, especially in the low frequency range, for the full characterization of the IL.
