Posted on04/01/2019byPavlin Mitev|Comments Off on Fifty Shades of Water: Benchmarking DFT Functionals against Experimental Data for Ionic Crystalline Hydrates
Authors: Getachew Kebede, Peter Broqvist, Anders Eriksson, and Kersti Hermansson
We propose that crystalline ionic hydrates constitute a valuable resource for benchmarking theoretical methods for aqueous ionic systems. Many such structures are known from the experimental literature, and they contain a large variety of water–water and ion–water structural motifs. Here we have collected a data set (CRYSTALWATER50) of 50 structurally unique “in-crystal” water molecules, involved in close to 100 nonequivalent O–H···O hydrogen bonds. A dozen well-known DFT functionals were benchmarked with respect to their ability to describe these experimental structures and their OH vibrational frequencies. We find that the PBE, RPBE-D3, and optPBE-vdW methods give the best H-bond distances and that anharmonic OH frequencies generated from B3LYP//optPBE-vdW energy scans outperform the other methods, i.e., here we performed B3LYP energy scans along the OH stretching coordinate while the rest of the structure was kept fixed at the optPBE-vdW-optimized positions
Authors: Getachew G. Kebede , Pavlin D. Mitev, Peter Broqvist, Jolla Kullgren , and Kersti Hermansson
This paper concerns thin water films and their hydrogen-bond patterns on ionic surfaces. As far as we are aware, this is the first time H-bond correlations for surface water and hydroxide species are presented in the literature while hydrogen-bond relations in the solid state have been scrutinized for at least five decades. Our data set, which was derived using density functional theory, consists of 116 unique surface OH groups–intact water molecules as well as hydroxides–on MgO(001), CaO(001) and NaCl(001), covering the whole range from strong to weak to no H-bonds. The intact surface water molecules are found to always be redshifted with respect to the gas-phase water OH vibrational frequency, whereas the surface hydroxide groups are either redshifted (OsH) or blueshifted (OHf) compared to the gas-phase OH– frequency. The surface H-bond relations are compared with the traditional relations for bulk crystals. We find that the “ν(OH) vs R(H···O)” correlation curve for surface water does not coincide with the solid state curve: it is redshifted by about 200 cm–1 or more. The intact water molecules and hydroxide groups on the ionic surfaces essentially follow the same H-bond correlation curve.
Posted on15/02/2017byPavlin Mitev|Comments Off on Comparing van der Waals DFT methods for water on NaCl(001) and MgO(001)
Authors: Getachew G. Kebede, Daniel Spångberg, Pavlin D. Mitev, Peter Broqvist, and Kersti Hermansson
In this work, a range of van der Waals type density functionals are applied to the H2O/NaCl(001) and H2O/MgO(001) interface systems to explore the effect of an explicit dispersion treatment. The functionals we use are the self-consistent vdW functionals vdW-DF, vdW-DF2, optPBE-vdW, optB88-vdW, optB86b-vdW, and vdW-DF-cx, as well as the dispersion-corrected PBE-TS and PBE-D2 methods; they are all compared with the standard PBE functional. For both NaCl(001) and MgO(001), we find that the dispersion-flavoured functionals stabilize the water-surface interface by approximately 20%-40% compared to the PBE results. For NaCl(001), where the water molecules remain intact for all overlayers, the dominant contribution to the adsorption energy from “density functional theory dispersion” stems from the water-surface interactions rather than the water-water interactions. The optPBE-vdW and vdW-DF-cx functionals yield adsorption energies in good agreement with available experimental values for both NaCl and MgO. To probe the strengths of the perturbations of the adsorbed water molecules, we also calculated water dipole moments and found an increase up to 85% for water at the MgO(001) surface and 70% at the NaCl(001) surface, compared to the gas-phase dipole moment.