S. Cottenier

Is the error on first-principles volume predictions absolute or relative?

K. Lejaeghere, L. Vanduyfhuys, T. Verstraelen, V. Van Speybroeck, S. Cottenier
Computational Materials Science
117, 390-396
2016
A1

Abstract 

Many benchmarks of density-functional theory with respect to experiment suggest the error on predicted equilibrium volumes to scale with the volume. Relative volume errors are therefore often used as a decisive argument to select one exchange-correlation functional over another. We show that the error on the volume (after correcting for systematic deviations) is only approximately relative. A simple analytic model, validated by rigorous Monte Carlo simulations, reveals that a more accurate error estimate can be derived from the inverse of the bulk modulus. This insight is not only instrumental for the selection or design of suitable functionals. It also calls for a new attitude towards computational errors: to report computational errors on electronic-structure calculations, identify systematic deviations and distinguish between relative and absolute effects. (C) 2016 Elsevier B.V. All rights reserved.

Open Access version available at UGent repository

DOI 

http://dx.doi.org/10.1016/j.commatsci.2016.01.039

Ab initio study of the trapping of polonium on noble metals

K. Rijpstra, A. Van Yperen-De Deyne, E. A. Maugeri, J. Neuhausen, M. Waroquier, V. Van Speybroeck, S. Cottenier
Journal of Nuclear Materials
472, 35-42
2016
A1

Abstract 

In the future MYRRHA reactor, lead bismuth eutectic (LBE) will be used both as coolant and as spallation target. Due to the high neutron flux a small fraction of the bismuth will transmute to radiotoxic 210Po. Part of this radiotoxic element will evaporate into the gas above the coolant. Extracting it from the gas phase is necessary to ensure a safe handling of the reactor. An issue in the development of suitable filters is the lack of accurate knowledge on the chemical interaction between a candidate filter material and either elemental polonium or polonium containing molecules. Experimental work on this topic is complicated by the high radiotoxicity of polonium. Therefore, we present in this paper a first-principles study on the adsorption of polonium on noble metals as filter materials. The adsorption of monoatomic Po is considered on the candidate filter materials palladium, platinum, silver and gold. The case of the gold filter is looked upon in more detail by examining how bismuth pollution affects its capability to capture polonium and by studying the adsorption of the heavy diatomic molecules Po2, PoBi and PoPb on this gold filter.

Open Access version available at UGent repository

DOI 

http://dx.doi.org/10.1016/j.jnucmat.2016.01.026

Reproducibility in density functional theory calculations of solids

K. Lejaeghere, G. Bihlmayer, T. Björkman, P. Blaha, S. Blügel, V. Blum, D. Caliste, I.E. Castelli, S.J. Clark, A. Dal Corso, S. de Gironcoli, T. Deutsch, J.K. Dewhurst, I. Di Marco, C. Draxl, M. Dułak, O. Eriksson, J.A. Flores-Livas, K.F. Garrity, L. Genovese, P. Giannozzi, M. Giantomassi, S. Goedecker, X. Gonze, O. Grånäs, E.K.U. Gross, A. Gulans, F. Gygi, D.R. Hamann, P.J. Hasnip, N.A.W. Holzwarth, D. Iușan, D.B. Jochym, F. Jollet, D. Jones, G. Kresse, K. Koepernik, E. Küçükbenli, Y.O. Kvashnin, I.L.M. Locht, S. Lubeck, M. Marsman, N. Marzari, U. Nitzsche, L. Nordström, T. Ozaki, L. Paulatto, C.J. Pickard, W. Poelmans, M.I.J. Probert, K. Refson, M. Richter, G.-M. Rignanese, S. Saha, M. Scheffler, M. Schlipf, K. Schwarz, S. Sharma, F. Tavazza, P. Thunström, A. Tkatchenko, M. Torrent, D. Vanderbilt, M.J. van Setten, V. Van Speybroeck, J.M. Wills, J.R. Yates, G.-X. Zhang, S. Cottenier
Science
351 (6280), 1415-aad3000-7
2016
A1

Abstract 

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.

DOI 

http://dx.doi.org/10.1126/science.aad3000

Controlling the Size of Hot Injection Made Nanocrystals by Manipulating the Diffusion Coefficient of the Solute

K. De Nolf, R.K. Capek, S. Abé, M. Sluydts, Y. Jang, J.C. Martins, S. Cottenier, E. Lifshitz, Z. Hens
JACS (Journal of the American Chemical Society)
137 (7), 2495–2505
2015
A1

Abstract 

We investigate the relation between the chain length of ligands used and the size of the nanocrystals formed in the hot injection synthesis. With two different CdSe nanocrystal syntheses, we consistently find that longer chain carboxylic acids result in smaller nanocrystals with improved size dispersions. By combining a more in-depth experimental investigation with kinetic reaction simulations, we come to the conclusion that this size tuning is due to a change in the diffusion coefficient and the solubility of the solute. The relation between size tuning by the ligand chain length and the coordination of the solute by the ligands is further explored by expanding the study to amines and phosphine oxides. In line with the weak coordination of CdSe nanocrystals by amines, no influence of the chain length on the nanocrystals is found, whereas the size tuning brought about by phosphine oxides can be attributed to a solubility change. We conclude that the ligand chain length provides a practical handle to optimize the outcome of a hot injection synthesis in terms of size and size dispersion and can be used to probe the interaction between ligands and the actual solute.

DOI 

http://dx.doi.org/10.1021/ja509941g

Binary and Ternary Po-containing Molecules Relevant for LBE Cooled Reactors at Operating Temperature

A. Van Yperen-De Deyne, K. Rijpstra, M. Waroquier, V. Van Speybroeck, S. Cottenier
Journal of Nuclear Materials
458, 288-295
2015
A1

Abstract 

Quantum-chemical calculations at several levels of theory were used to assess the stability at different temperatures of a set of 13 binary and ternary Po-containing molecules that could possibly be formed in an environment with lead, bismuth, oxygen and water. The conclusions are that especially PoPb, PbPoO and PoOH and to a lesser extent Po2 and PoO are stable. These small molecules are therefore likely to be found near the Lead-Bismuth eutectic (LBE) coolant at operational temperatures in a heavy liquid metal cooled fission reactor. In contrast, Po3 and PoBi are unlikely to be present under the assumed conditions. Several stability criteria, such as the dissociation into free atoms or into molecular fragments at realistic Po-concentrations or in the thermodynamic limit are discussed at different temperatures. The results obtained with a medium level of theory (Density Functional Theory, PBE0 with relativistic effective core potentials) show good qualitative correspondence with calculations performed at a much higher level of theory (Multi Reference Configuration Interaction, with spin–orbit coupling and scalar relativistic Hamiltonian).

Open Access version available at UGent repository

DOI 

http://dx.doi.org/10.1016/j.jnucmat.2014.11.064

Aliovalent doping of CeO2: DFT study of oxidation state and vacancy effects

D.E.P. Vanpoucke, P. Bultinck, S. Cottenier, V. Van Speybroeck, I. Van Driessche
Journal of Materials Chemistry A
2 (3), 13723-13737
2014
A1

Abstract 

The modification of CeO2 properties by means of aliovalent doping is investigated within the ab initio density functional theory framework. Lattice parameters, dopant atomic radii, bulk moduli and thermal expansion coefficients of fluorite type Ce1-xMxO2-y (with M = Mg, V, Co, Cu, Zn, Nb, Ba, La, Sm, Gd, Yb, and Bi) are presented for 0.00 < x < 0.25. The relative stability of the doped systems is discussed, and the influence of oxygen vacancies is investigated. It is shown that oxygen vacancies tend to increase the lattice parameter, and strongly decrease the bulk modulus. Defect formation energies are correlated with calculated crystal radii and covalent radii of the dopants, and are shown to present no simple trend. The previously observed inverse relationship between the thermal expansion coefficient and the bulk modulus in group IV doped CeO2 [J. Am. Ceram. Soc., 2014, 97(1), 258] is shown to persist independent of the inclusion of charge compensating vacancies.

Open Access version available at UGent repository

DOI 

http://dx.doi.org/10.1039/c4ta02449d

Tetravalent doping of CeO2: The impact of valence electron character on group IV dopant influence

D.E.P. Vanpoucke, S. Cottenier, V. Van Speybroeck, I. Van Driessche, P. Bultinck
Journal of the American Ceramic Society
97 (1), 258-266
2014
A1

Abstract 

Fluorite CeO2 doped with group IV elements is studied within the DFT and DFT+U framework. Concentration dependent formation energies are calculated for Ce1−xZxO2 (Z= C, Si, Ge, Sn, Pb, Ti, Zr, Hf) with 0≤x≤0.25 and a roughly decreasing trend with ionic radius is observed. The influence of the valence and near valence electronic configuration is discussed, indicating the importance of filled d and f shells near the Fermi level for all properties investigated. A clearly different behavior of group IVa and IVb dopants is observed: the former are more suitable for surface modifications, the latter are more suitable for bulk modifications.\\ \indent For the entire set of group IV dopants, there exists an inverse relation between the change, due to doping, of the bulk modulus and the thermal expansion coefficients. Hirshfeld-I atomic charges show that charge transfer effects due to doping are limited to the nearest neighbor oxygen atoms.

DOI 

http://dx.doi.org/10.1111/jace.12650

Ab initio based thermal property predictions at a low cost: An error analysis

K. Lejaeghere, J. Jaeken, V. Van Speybroeck, S. Cottenier
Physical Review B
89, 014304
2014
A1

Abstract 

Ab initio calculations often do not straightforwardly yield the thermal properties of a material yet. It requires considerable computational efforts, for example, to predict the volumetric thermal expansion coefficient αV or the melting temperature Tm from first principles. An alternative is to use semi-empirical approaches. They relate the experimental values to first-principles predictors via fits or approximative models. Before applying such methods, however, it is of paramount importance to be aware of the expected errors. We therefore quantify these errors at the DFT-PBE level for several semi-empirical approximations of αV and Tm , and compare them to the errors from fully ab initio methods, which are computationally more intensive. We base our conclusions on a benchmark set of 71 ground-state elemental crystals. For the thermal expansion coefficient, it appears that simple quasiharmonic theory, in combination with different approximations to the Gruneisen parameter, provides a similar overall accuracy as exhaustive first-principles phonon calculations. For the melting temperature, expensive ab initio molecular-dynamics simulations still outperform semi-empirical methods.

Open Access version available at UGent repository

DOI 

http://dx.doi.org/10.1103/PhysRevB.89.014304

Reactivity of CO on carbon covered cobalt surfaces in Fischer-Tropsch Synthesis

L. Joos, I. Filot, S. Cottenier, E. Hensen, M. Waroquier, V. Van Speybroeck, R.A. van Santen
Journal of Physical Chemistry C
118 (10), 5317–5327
2014
A1

Abstract 

Fischer–Tropsch synthesis is an attractive process to convert alternative carbon sources, such as biomass, natural gas, or coal, to fuels and chemicals. Deactivation of the catalyst is obviously undesirable, and for a commercial plant it is of high importance to keep the catalyst active as long as possible during operating conditions. In this study, the reactivity of CO on carbon-covered cobalt surfaces has been investigated by means of density functional theory (DFT). An attempt is made to provide insight into the role of carbon deposition on the deactivation of two cobalt surfaces: the closed-packed Co(0001) surface and the corrugated Co(112̅1) surface. We also analyzed the adsorption and diffusion of carbon atoms on both surfaces and compared the mobility. Finally, the results for Co(0001) and Co(112̅1) are compared, and the influence of the surface topology is assessed.

DOI 

http://dx.doi.org/10.1021/jp4109706

Crystal structure prediction for supersaturated AZO : the case of Zn3Al2O6

K. Rijpstra, S. Cottenier, M. Waroquier, V. Van Speybroeck
CrystEngComm
2013 (15), 10440-10444
2013
A1

Abstract 

Increasing the Al concentration in Al-doped ZnO (AZO) is one way of improving the conductivity of this transparent conductive oxide (TCO). Beyond a certain concentration, an unwanted secondary phase develops with a low conductivity. Its stoichiometry is Zn3Al2O6, and its crystal structure has not yet been convincingly determined. By applying unbiased ab initio structure prediction tools, we predict the crystal structure of Zn3Al2O6 to be monoclinic with space group Pm. It can be described as a nanofabric, with one-dimensional Al2O3 wires penetrating a ZnO matrix. This crystal has a formation energy that is lower than any structure proposed before, and is consistent with all available experimental information. Knowledge of the nature of this phase can help to avoid its formation and therefore to engineer AZO crystals with an increased level of Al-doping and associated increased conductivity.

DOI 

http://dx.doi.org/10.1039/C3CE41009A

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