Authors: Jolla Kullgren, Matthew J. Wolf, Pavlin D. Mitev, Kersti Hermansson and Wim J. Briels
The interplay between energetics and entropy in determining defect distributions at ceria(111) is studied using a combination of DFT+U and lattice Monte Carlo simulations. Our main example is fluorine impurities, although we also present preliminary results for surface hydroxyl groups. A simple classical force-field model was constructed from a training set of DFT+U data for all symmetrically inequivalent (F−)n(Ce3+)n nearest-neighbor clusters with n = 2 or 3. Our fitted model reproduces the DFT energies well. We find that for an impurity concentration of 15% at 600 K, straight and hooked linear fluorine clusters are surprisingly abundant, with similarities to experimental STM images from the literature. We also find that with increasing temperature the fluorine cluster sizes show a transition from being governed by an attractive potential to being governed by a repulsive potential as a consequence of the increasing importance of the entropy of the Ce3+ ions. The distributions of surface hydroxyl groups are noticeably different.
J. Phys. Chem. C, 2017, 121 (28), pp 15127–15134
Authors: Imre Bakó, Anikó Lábas, Kersti Hermansson, Ákos Bencsura and Julianna Oláh
The significant cooperative effect between water molecules substantially affects the properties of liquid water. The cooperativity of hydrogen bonds means that the hydrogen bond strength is influenced by the neighboring water molecules. Another descriptor related to cooperativity is degree correlation (or static correlation) describing the probability of hydrogen-bonded molecule pairs participating in additional hydrogen-bonds. Herein we analyze the latter one in liquid water at various temperatures and densities in a series of molecular dynamics simulations with the help of knowledge from network science. We investigated how the applied hydrogen bond criteria (energetic or geometric) influence the obtained results, and showed that the energetic criterion is much more rigorous and reliable, therefore should be used for similar studies. We found that the structure of the subsystems of water molecules with 3 and 4 hydrogen-bonds is distinctly different at low temperature, 3‑hydrogen-bonded water molecules form branched chain structures at all temperature. Deconvolution of the descriptors of the mixing pattern of water molecules according to their donor and acceptor numbers showed that species with complementary hydrogen bonding properties are likely to correlate and form H-bonds with each other, while species with similar H-bond pattern tend to avoid each other. Pearson’s coefficient (global descriptor of the local cooperativity) of the studied networks suggests that at normal density the H-bonded network in liquid water can be described by an uncorrelated network.
Journal of Molecular Liquids, 245, 2017, pp 140-146
Author: Kersti Hermansson
This report discusses some of the most pressing challenges that need to be overcome for computational condensed matter chemistry to become fully accepted, at par with experiments. The prospects are rather bright. By means of a few examples, all connected to the bound water molecule and the hydroxide ion, and their mysteries, the unique capabilities of theoretical calculations to provide new insights and sometimes even surpass experiments in accuracy, will be demonstrated.
Contributions, Sec. Nat. Math. Biotech. Sci., MASA, 38 (1), 2017, pp 17–26
Authors: Samual R. Zukowski, Pavlin D. Mitev, Kersti Hermansson, and Dor Ben-Amotz
The hydration-shell of CO2 is characterized using Raman multivariate curve resolution (Raman-MCR) spectroscopy combined with ab initio molecular dynamics (AIMD) vibrational density of states simulations, to validate our assignment of the experimentally observed high-frequency OH band to a weak hydrogen bond between water and CO2. Our results reveal that while the hydration-shell of CO2 is highly tetrahedral, it is also occasionally disrupted by the presence of entropically stabilized defects associated with the CO2-water hydrogen bond. Moreover, we find that the hydration-shell of CO2 undergoes a temperature-dependent structural transformation to a highly disordered (less tetrahedral) structure, reminiscent of the transformation that takes place at higher temperatures around much larger oily molecules. The biological significance of the CO2 hydration shell structural transformation is suggested by the fact that it takes place near physiological temperatures.
J. Phys. Chem. Lett., 8 (13), 2017, pp 2971–2975
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.
The Journal of Chemical Physics 146, 064703 (2017);
Authors: Jolla Per Kullgren, Matthew Jason Wolf, Kersti Hermansson, Christof Köhler, Bálint Aradi, Thomas Frauenheim, and Peter Broqvist
Reducible oxides such as CeO2 are challanging to describe
with standard density functional theory (DFT) due to the mixed valence states of the cations, and often require the use of additional correction schemes, an
d/or more computationally expen- sive methods. This adds a new layer of complexity when it comes to the generation of Slater-Koster tables and the corresponding repulsive potentials for self-consistent density functional based tight binding (SCC-DFTB) calculations of such materials. In this work, we provide guidelines for how to set up a parameterisation scheme for mixed valence oxides within the SCC-DFTB framework, with a focus on reproducing structural and electronic properties as well as redox reaction energies calculated using a reference DFT method. This parameterisation procedure has been used to generate parameters for Ce–O interactions, with Ce in its +III or +IV formal oxidation states. The generated parameter set is validated through comparison to DFT calculations for various ceria (CeO2) and reduced ceria (CeO2−x ) systems of different dimensionalities ranging from 0D (nano-particles) to 3D (bulk). As oxygen vacancy defects in ceria are of crucial importance to many technological applications, special focus is directed towards the capability of describing such defects accurately.
J. Phys. Chem. C, 2017, 121 (8), pp 4593–4607
Authors: Matthew J. Wolf, Jolla Kullgren, Peter Broqvist, and Kersti Hermansson
We investigate the effects of anion doping with fluorine impurities on the chemistry of the CeO2 (111) facet, using the results of DFT + U
calculations. We consider three prototypical processes: the formation of oxygen vacancies, the adsorption of O2 and H2O molecules, and the re-oxidation of the surface with fragments of the two molecules. We find that the first two of these processes are not strongly affected, but that the presence of F lowers the energy gained in the re-oxidation of the surface in comparison to the healing of an oxygen vacancy, by 1.47 eV in the case of O2 (provided that the F is part of a cluster) and by 0.92 eV in the case of H2O. Based on these results, we suggest that F could enhance the redox chemistry of ceria by toggling between being in the surfaceand on the surface, effectively facilitating the release of lattice O by acting as a “place holder” for it. Finally, we find that the desorption of F as either 1212
F2 or HF is energetically unfavourable, suggesting that F doped ceria should be stable in the presence of O2 and H2O.
J. Chem. Phys. 2017, 146, 044703
Authors: Pemikar Srifa, Maxim V. Galkin, Joseph S. M. Samec, Kersti Hermansson, and Peter Broqvist
Density functional theory (DFT) calculations, combined with a constrained minima hopping algorithm (global minimum search while preserving the molecular identity), have been performed to investigate important reaction intermediates for the heterogeneously catalyzed β-O-4′ bond cleavage in lignin derivatives. More specifically, we have studied the adsorption properties of a keto tautomer (1-methoxypropan-2-one) and its enol form on a catalytically active Pd(111) surface. In agreement with experiments, we find that for the gas-phase molecules the keto tautomer is the most stable. Interestingly, the enol tautomer has a higher affinity to the Pd catalyst than the keto form, and becomes the most stable molecular form when adsorbed on the catalyst surface. The global minimum complex found on the metal surface corresponds to an enolate structure formed when the enol tautomer chemisorbs onto the surface and donates its π-electrons from the C═C region to two adjacent palladium atoms. The actual formation of a chemical bond to the surface in the case of the enol molecule could be the key to understanding why the enol derivative is needed for an efficient β-O-4′ bond cleavage.
J. Phys. Chem. C, 2016, 120 (41), pp 23469–23479
Authors: C. M. Yim, M. B. Watkins, M. J. Wolf, C. L. Pang, K. Hermansson, and G. Thornton
Polarons in metal oxides are important in processes such as catalysis, high temperature superconductivity, and dielectric breakdown in nanoscale electronics. Here, we study the behavior of electron small polarons associated with oxygen vacancies at rutile TiO2(110), using a combination of low temperature scanning tunneling microscopy (STM), density functional theory, and classical molecular dynamics calculations. We find that the electrons are symmetrically distributed around isolated vacancies at 78 K, but as the temperature is reduced, their distributions become increasingly asymmetric, confirming their polaronic nature. By manipulating isolated vacancies with the STM tip, we show that particular configurations of polarons are preferred for given locations of the vacancies, which we ascribe to small residual electric fields in the surface. We also form a series of vacancy complexes and manipulate the Ti ions surrounding them, both of which change the associated electronic distributions. Thus, we demonstrate that the configurations of polarons can be engineered, paving the way for the construction of conductive pathways relevant to resistive switching devices.
Phys. Rev. Lett. 117, 116402, (2016)
Authors: Anik Sen, Pavlin D. Mitev, Anders Eriksson, and Kersti Hermansson
We present periodic plane-wave density functional theory (DFT) Perdew–
Burke–Ernzerhof (PBE-D2) calculations for four highly hydrated crystals, Na2CO3·10H2O, MgSO4·7H2O, MgSO4·11H2O, and Al(NO3)3·9H2O, containing 37 structurally unique water molecules and 74 unique hydrogen bonds. The calculated R(H···O) distances lie in the range 1.60–2.05 Å, the anharmonic OH frequencies in the range 2570–3425 cm−1, and the water dipole moments lie in the range 2.9–4.3 Debye, as calculated from the Wannier function centers and the nuclei. We present the following findings. (i) Our optimized intramolecular r(OH) distances are always larger than the gas-phase value and thus more accurate than those derived from neutron diffraction experiments; (ii) The local in situ electric field over the molecule, calculated from the positions of the nuclei and the Wannier centers in the surrounding crystal, appears to be a good descriptor of the pertturbation from the water molecule’s surroundings as the internal molecular properties (re, ν, μ) are found to correlate well with the crystal-generated electric field; (iii) We have added DFT-calculated data points to the well-known experimental ‘OH frequency versus R(H···O)’ correlation curve in a region where the experimental data points are scarce; (iv) For all 37 water molecules, the Wannier centers located in the lone-pair region, and those located in the OH bonds, displace about equally much due to the polarizing environment. Finally, we propose that our resulting ‘OH frequency versus Wannier-type electric field’ correlation curve may constitute a useful tool for predicting OH vibrational frequencies from snapshots from PBE-D2-based ab initio molecular dynamics simulations of water-containing systems.
International Journal of Quantum Chemistry, 116, ( 2016), 67-80