RESUMEN
Using quantum mechanical calculations within density functional theory, we provide a comprehensive analysis of infrared-active excitation of water molecules confined in nanocages of a beryl crystal lattice. We calculate infrared-active modes including the translational, librational, and mixed-type resonances of regular and heavy water molecules. The results are compared to the experimental spectra measured for the two principal polarizations of the electric field: parallel and perpendicular to the crystallographic c-axis. Good agreement is achieved between calculated and measured isotopic shifts of the normal modes. We analyze the vibrational modes in connection with the structural characteristics and arrangements of water molecules within the beryl crystal. Specific atomic displacements are assigned to each experimentally detected vibrational mode resolving the properties of nano-confined water on scales not accessible by experiments. Our results elucidate the applicability and efficiency of a combined experimental and computational approach for describing and an in-depth understanding of nano-confined water, and pave the way for future studies of more complex systems.
RESUMEN
Results on the structural details of Kirkwood-Buff integrals obtained from the optimization of ionic force fields are presented. We have proposed and make use of an optimization scheme for ionic force fields, which is based on the modification of the cation-anion mixing rules, the calculation of the thermodynamics properties of various monovalent salt solutions according to the Kirkwood-Buff theory of solutions and the comparison to relevant experimental findings. Here, we complete and extend our calculations and analysis as we focus on the technical details of this optimization procedure and the case of fluorides, which have been proven difficult to handle. Important insight is given on the dependence of the radial distribution functions, the short-ranged potentials of mean force, and the Kirkwood-Buff integrals of the salt solutions on the different scaling factors in the mixing rules. Specifically, the way the structural details and inherent characteristics of the above properties are affected by the quantitative and qualitative differences in the mixing rules for a variety of common biologically relevant monovalent salts is mainly addressed. We conclude on the efficiency of this scheme, again with a focus on the fluorides. In the end, we provide a variation of the ion-pair mixing rules scaling factors with salt concentration to identify regimes for which different mixing rules prefactors lead to well-optimized force fields. All results are obtained through Molecular Dynamics simulations using previously optimized force fields for the monovalent ions.
RESUMEN
Tight-binding molecular dynamics simulations shed light into the fracture mechanisms and the ideal strength of tetrahedral amorphous carbon and of nanocomposite carbon containing diamond crystallites, two of the hardest materials. It is found that fracture in the nanocomposites, under tensile or shear load, occurs intergrain and so their ideal strength is similar to the pure amorphous phase. The onset of fracture takes place at weakly bonded sites in the amorphous matrix. On the other hand, the nanodiamond inclusions significantly enhance the elastic moduli, which approach those of diamond.