Models of Aged Magnesium–Silicate–Hydrate Cements Based on the Lizardite and Talc Crystals: A Periodic DFT-GIPAW Investigation

Classical MD simulations and periodic DFT-GIPAW calculations have been employed to investigate possible structural models of aged magnesium–silicate–hydrate (M–S–H) cements. Four models were built, two based, respectively, on the lizardite Mg₃Si₂O₅(OH)₄ and talc Mg₃Si₄O₁₀(OH)₂ crystals in which we have modified the Mg/Si ratio and distribution of Q^<i>n</i> silicon species (<i>n</i> is the number of bridging oxygens for each quaternary silicon atom) according to previous experimental evidence. The other two models were still based on the lizardite and talc crystals, but the original Mg/Si ratios were retained. The latter two models were used to simulate MSH binders containing different proportions of talc- and lizardite-based amorphous phases as proposed in several experimental works. From these initial models, additional structures were generated by adding increasing amounts of water molecules to investigate its effect on the structure and on the ²⁹Si and ²⁵Mg MAS NMR spectra. Our results demonstrate that MSH phases with an Mg/Si ratio close to 1 can be described as a defective talc-based model or as the combination of talc and lizardite amorphous models with the former being the dominant phase. In both cases, the hydrated models provide the best agreement with NMR experiments

TD-DFT Benchmark on Inorganic Pt(II) and Ir(III) Complexes

We report in the present paper a comprehensive investigation of representative Pt­(II) and Ir­(III) complexes with special reference to their one-photon absorption spectra employing methods rooted in density functional theory and its time dependent extension. We have compared nine different functionals ranging from generalized gradient approximation (GGA) to global or range-separated hybrids, and two different basis sets, including pseudopotentials for 4 iridium and 7 platinum complexes. It turns out that hybrid functionals with the same exchange part give comparable results irrespective of the specific correlation functional (i.e., B3LYP is very close to B3PW91 and PBE0 is very close to MPW1PW91). More recent functionals, such as CAM-B3LYP and M06-2X, overestimate excitation energies, whereas local functionals (BP86 -GGA-, M06-L -Meta GGA-) strongly underestimate transition energies with respect to experimental results. As expected, basis set effects are weak, and the use of a triple-ζ polarized (def2-TZVP) basis set does not significantly improve the computed excitation energies with respect to a classical double-ζ basis set (LANL2DZ) augmented by polarization functions, but it significantly raises the computational effort