P. Bultinck

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
2013
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.

Can The Electronegativity Equalization Method Predict Spectroscopic Properties?

T. Verstraelen, P. Bultinck
Spectrochimica Acta Part A (Mol. & biomol.)
2013
A1

Abstract 

The electronegativity equalization method is classically used as a method allowing the fast generation of atomic charges using a set of calibrated parameters and provided knowledge of the molecular structure. Recently, it has started being used for the calculation of other reactivity descriptors and for the development of polarizable and reactive force fields. For such applications, it is of interest to know whether the method, through the inclusion of the molecular geometry in the Taylor expansion of the energy, would also allow sufficiently accurate predictions of spectroscopic data. In this work, relevant quantities for IR spectroscopy are considered, namely the dipole derivatives and the Cartesian Hessian. Despite careful calibration of parameters for this specific task, it is shown that the current models yield insufficiently accurate results.

A New Mean-Field Method Suitable for Strongly Correlated Electrons: Computationally Facile Antisymmetric Products of Nonorthogonal Geminals

P.A. Limacher, P.W. Ayers, S. De Baerdemacker, D. Van Neck, P. Bultinck
Journal of Chemical Theory and Computation (JCTC)
9 (3), 1394-1401
2013
A1

Abstract 

We propose an approach to the electronic structure problem based on noninteracting electron pairs that has similar computational cost to conventional methods based on noninteracting electrons. In stark contrast to other approaches, the wave function is an antisymmetric product of nonorthogonal geminals, but the geminals are structured so the projected Schrödinger equation can be solved very efficiently. We focus on an approach where, in each geminal, only one of the orbitals in a reference Slater determinant is occupied. The resulting method gives good results for atoms and small molecules. It also performs well for a prototypical example of strongly correlated electronic systems, the hydrogen atom chain.

A size-consistent approach to strongly correlated systems using a generalized antisymmetrized product of nonorthogonal geminals

P.A. Johnson, P.W. Ayers, P.A. Limacher, S. De Baerdemacker, D. Van Neck, P. Bultinck
Computational and Theoretical Chemistry
1003 (2013), 101-113
2013
A1

Abstract 

Inspired by the wavefunction forms of exactly solvable algebraic Hamiltonians, we present several wavefunction ansatze. These wavefunction forms are exact for two-electron systems; they are size consistent; they include the (generalized) antisymmetrized geminal power, the antisymmetrized product of strongly orthogonal geminals, and a Slater determinant wavefunctions as special cases. The number of parameters in these wavefunctions grows only linearly with the size of the system. The parameters in the wavefunctions can be determined by projecting the Schrödinger equation against a test-set of Slater determinants; the resulting set of nonlinear equations is reminiscent of coupled-cluster theory, and can be solved with no greater than O (N5) scaling if all electrons are assumed to be paired, and with O (N6) scaling otherwise. Based on the analogy to coupled-cluster theory, methods for computing spectroscopic properties, molecular forces, and response properties are proposed.

Extended random phase approximation method for atomic excitation energies from correlated and variationally optimized second-order density matrices

H. van Aggelen, B. Verstichel, G. Acke, M. Degroote, P. Bultinck, P.W. Ayers, D. Van Neck
Computational and Theoretical Chemistry
1003 (2013), 50-54
2013
A1

Reply to ‘comment on “extending hirshfeld-I to bulk and periodic materials”’

D.E.P. Vanpoucke, I. Van Driessche, P. Bultinck
Journal of Computational Chemistry
Volume 34, Issue 5, pages 422-427
2013
A1
Published while none of the authors were employed at the CMM

Abstract 

The issues raised in the comment by Manz are addressed through the presentation of calculated atomic charges for NaF, NaCl, MgO, SrTiO3 , and La2Ce2O7 , using our previously presented method for calculating Hirshfeld-I charges in solids (Vanpoucke et al., J. Comput. Chem. doi: 10.1002/jcc.23088). It is shown that the use of pseudovalence charges is sufficient to retrieve the full all-electron Hirshfeld-I charges to good accuracy. Furthermore, we present timing results of different systems, containing up to over 200 atoms, underlining the relatively low cost for large systems. A number of theoretical issues are formulated, pointing out mainly that care must be taken when deriving new atoms in molecules methods based on “expectations” for atomic charges.

Open Access version available at UGent repository

Extending Hirshfeld-I to bulk and periodic materials

D.E.P. Vanpoucke, P. Bultinck, I. Van Driessche
Journal of Computational Chemistry
Volume 34, Issue 5, pages 405-417
2013
A1
Published while none of the authors were employed at the CMM

Abstract 

In this work, a method is described to extend the iterative Hirshfeld-I method, generally used for molecules, to periodic systems. The implementation makes use of precalculated pseudopotential-based electron density distributions, and it is shown that high-quality results are obtained for both molecules and solids, such as ceria, diamond, and graphite. The use of grids containing (precalculated) electron densities makes the implementation independent of the solid state or quantum chemical code used for studying the system. The extension described here allows for easy calculation of atomic charges and charge transfer in periodic and bulk systems. The conceptual issue of obtaining reference densities for anions is discussed, and the delocalization problem for anionic reference densities originating from the use of a plane wave basis set is identified and handled.

Open Access version available at UGent repository

Tuning of CeO2 buffer layers for coated superconductors through doping

D.E.P. Vanpoucke, S. Cottenier, V. Van Speybroeck, P. Bultinck, I. Van Driessche
Applied Surface Science
260, 32-35
2012
A1

Abstract 

The appearance of microcracks in cerium oxide (CeO 2) buffer layers, as used in buffer layer architectures for coated superconductors, indicates the presence of stress between this buffer layer and the substrate. This stress can originate from the differences in thermal expansion or differences in lattice parameters between the CeO 2 buffer layer and the substrate. In this article, we study, by means of ab initio density functional theory calculations, the influence of group IV doping elements on the lattice parameter and bulk modulus of CeO 2. Vegard's law behavior is found for the lattice parameter in systems without oxygen vacancies, and the Shannon crystal radii for the doping elements are retrieved from the lattice expansions. We show that the lattice parameter of the doped CeO 2 can be matched to that of the La 2Zr 2O 7 coated NiW substrate substrate for dopant concentrations of about 5%, and that bulk modulus matching is either not possible or would require extreme doping concentrations. [All rights reserved Elsevier].

Open Access version available at UGent repository

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