Center for Molecular Modeling - F. Stevens

X- (X = O, S) Ions in Alkali Halide Lattices through Density Functional Calculations. 1. Substitutional Defect Models

wim — Fri, 30 Sep 2011 14:38:31 +0000

F. Stevens, H. Vrielinck, V. Van Speybroeck, E. Pauwels, F. Callens, M. Waroquier

Journal of Physical Chemistry B

110 (16), 8204–8212

2006

Abstract

Monoatomic X- (X = O, S) chalcogen centers in MZ (M = Na, K, Rb and Z = Cl, Br, I) alkali halide lattices are investigated within the framework of density functional theory with the principal aim to establish defect models. In electron paramagnetic resonance (EPR) experiments, X- defects with tetragonal, orthorhombic, and monoclinic g-tensor symmetry have been observed. In this paper, models in which X- replaces a single halide ion, with a next nearest neighbor and a nearest neighbor halide vacancy, are validated for the X- centers with tetragonal and orthorhombic symmetry, respectively. As such defect models are extended, the ability to reproduce experimental data is a stringent test for various computational approaches. Cluster in vacuo and embedded cluster schemes are used to calculate energy and EPR parameters for the two vacancy configurations. The final assignment of a defect structure is based on the qualitative and quantitative reproduction of experimental g and (super)hyperfine tensors.

DOI

http://dx.doi.org/10.1021/jp057008l

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06 j. phys. chem. B 110 8204 stevens.pdf

Density Functional Investigation of High-Spin XY (X = Cr, Mo, W and Y = C, N, O) Molecules

wim — Fri, 30 Sep 2011 14:35:58 +0000

F. Stevens, I. Carmichael, F. Callens, M. Waroquier

Journal of Physical Chemistry A

110 (14), 4846-4853

2006

Abstract

The performance of a density functional theory approach in calculating the equilibrium bond length, dipole moment, and harmonic vibrational frequency in a series of group 6 (Cr, Mo, W) transition metal-containing diatomic molecules is evaluated. Using flexible basis sets comprised of Slater type functions, a wide range of exchange-correlation functionals is investigated. Comparing with known experimental values and published results from high-level theoretical calculations, the most suitable functional form is selected. The importance of relativistic effects is checked, and predictions are made for several unknown dipole moments. The best agreement with experimental parameters is obtained when using a general gradient approximation, while special and hybrid functional forms give less accurate results.

DOI

http://dx.doi.org/10.1021/jp060717u

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06 j. chem. phys. A 110(14)4846 stevens.pdf

X- (X = O, S, Se) Ions in Alkali Halide Lattices through Density Functional Calculations. 2. Interstitial Defect Models

wim — Fri, 30 Sep 2011 14:06:51 +0000

V. Van Speybroeck, F. Stevens, E. Pauwels, H. Vrielinck, F. Callens, M. Waroquier

Journal of Physical Chemistry B

110 (16), 8213-8218

2006

Abstract

Density functional theory techniques are used to investigate the defect structure of X- (X = O, S, Se) ions in MZ (M = Na, K, Rb and Z = Cl, Br) alkali halides which exhibit monoclinic-I g-tensor symmetry, using cluster in vacuo, embedded cluster, and periodic embedding schemes. Although a perturbed interstitial defect model was suggested from electron paramagnetic resonance experiments (EPR), the nature of the perturbation is still unknown. An appropriate defect model is developed theoretically by comparing structural and energetical properties of various defect configurations. Further validation is achieved by cross referencing experimental and computed EPR data. On the basis of the computational results, the following defect model is proposed: the X- ion is located interstitially with a charge compensating halide vacancy in its first coordination shell.

DOI

http://dx.doi.org/10.1021/jp057010c

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06 j. phys. chem. B 110(16)8213 van speybroeck.pdf

The Rh–ligand bond: RhX (X = C, N, O, F, P and Cl) molecules

wim — Fri, 30 Sep 2011 14:04:59 +0000

F. Stevens, V. Van Speybroeck, I. Carmichael, F. Callens, M. Waroquier

Chemical Physics Letters

421 (1-3), 281-286

2006

Abstract

Bond distances, vibrational frequencies and dipole moments of the RhX (X = C, N, O, F, P and Cl) molecules were studied by density functional theory (DFT) methods. For all molecules under consideration, spectroscopic properties are computed using various functionals. This study is the first systematic ab initio investigation of the RhX molecules and in those cases where comparison with experimental data can be made, good quantitative agreement is achieved when using the BP86 functional.

DOI

http://dx.doi.org/10.1016/j.cplett.2006.01.102

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06 chem. phys. lett. 421 281 stevens.pdf

Ab initio EPR study of S and Se defects in alkali halides

wim — Fri, 30 Sep 2011 13:17:48 +0000

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, V. Van Speybroeck, M. Waroquier

International Journal of Quantum Chemistry

102 (4), 409-414

2005

Abstract

Calculations using density functional theory are performed to study the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) properties of S and Se impurities in alkali halide lattices. Cluster in vacuo models are used to describe the defect and the lattice surroundings. The trivacancy defect model proposed in the literature is able to reproduce both the experimental principal values and directions of the g tensor for S and Se defects doped in alkali halides. The alternative monovacancy model gives rise to important discrepancies with experiment and can be discarded. For the KCl lattice, the hyperfine tensors of the S and Semolecular ions also agree well with the available experimental data, giving further evidence to the trivacancy model. In addition, for NaCl:S and KCl:S computational results for the 23Na and 35Cl superhyperfine and quadrupole tensors are compared with experimental ENDOR parameters. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

DOI

http://dx.doi.org/10.1002/qua.20389

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05 int. j. quantum chem 102(4)409 stevens.pdf

Study of radical defects in crystalline lattices from first-principles molecular dynamics simulations

wim — Fri, 30 Sep 2011 13:10:57 +0000

V. Van Speybroeck, E. Pauwels, F. Stevens, F. Callens, M. Waroquier

International Journal of Quantum Chemistry

101 (6), 761-769

2005

Abstract

Theoretical calculations are presented to determine the structure of radical defects in crystalline lattices. The applications are concentrated on radical defects as they are induced by irradiation in organic crystals and paramagnetic molecular ions embedded in alkali halide lattices. Various approaches are possible to model the molecular environment: the single-molecule approach, the cluster approach, and periodic calculations. The latter are based on a Car-Parrinello formalism in which the molecular orbitals are expanded in a plane-wave basis set and in which the optimized structures at 0 K are obtained by a simulated annealing technique. The pros and cons of the various approaches are highlighted, and where possible comparison with experimental election paramagnetic resonance data are given. Due to the different natures of ionic and organic crystals, specific computational procedures are needed to get good correspondence with the experimental data. In various cases there is experimental evidence that some radical structures are submitted to noticeable changes with increasing temperature. These effects were theoretically reproduced by performing molecular dynamics calculations at elevated temperatures. (C) 2004 Wiley Periodicals, Inc. | Conference: 10th International Conference on the Applications of Density Functional Theory in Chemistry and Physics Location: Brussels, BELGIUM Date: SEP 05-12, 2003

DOI

http://dx.doi.org/10.1002/qua.20335

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05 int. j. quantum chem 101(6)761 van speybroeck.pdf

Level of theory study of magnetic resonance parameters of chalcogen XY− (X, Y = O, S and Se) defects in alkali halides

wim — Fri, 30 Sep 2011 13:05:07 +0000

F. Stevens, V. Van Speybroeck, E. Pauwels, H. Vrielinck, F. Callens, M. Waroquier

Physical Chemistry Chemical Physics (PCCP)

7 (2), 240-249

2005

Abstract

An extensive level of theory study is performed on diatomic chalcogen defects in alkali halide lattices by density functional theory methods. A variety of exchange correlation functionals and basis sets are used for the calculation of electron paramagnetic resonance (EPR) parameters of XY− (X, Y = O, S, Se) molecular ions doped in MZ (M = Na, K, Rb and Z = Cl, Br, I) lattices. Various factors contribute to the EPR values, such as geometrical effects, the choice of basis set and functional form. A sensitivity analysis is made by comparing experimental and theoretical magnetic resonance data. A flow scheme is proposed for obtaining the best agreement between experimental and calculated g-values for chalcogen defects in alkali halides.

DOI

http://dx.doi.org/10.1039/B412408A

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05 phys. chem. chem. phys. 7(2) 240 stevens.pdf

Density functional theory investigation of S2− in KCl: evidence for the existence of a di-vacancy site

wim — Fri, 30 Sep 2011 12:45:25 +0000

F. Stevens, H. Vrielinck, F. Callens, M. Waroquier

Solid State Communications

132 (11), 787-790

2004

Abstract

Electron paramagnetic resonance experiments have shown that, depending on the doping procedure, two different S2− centers may coexist in KCl. These centers have the 2B2g and the 2B3gground state, respectively. As no experimental ligand hyperfine data are available, it could not be determined whether the S2− molecular ion replaces a single halide ion (mono-vacancy site) or two nearest neighbor halide ions (di-vacancy site). Also, other defect models could a priory be considered. In this work, cluster in vacuo density functional theory calculations of the g and 33S hyperfine tensors show that the S2− ion at a mono-vacancy site has the 2B2g ground state, whereas S2− in a di-vacancy exhibits a 2B3g ground state. For the latter center, the possibility of charge compensation by a cation vacancy is also considered. The calculations indicate that a possible vacancy is not in the direct vicinity (nearest or next-nearest neighbor) of the S2− ion.

DOI

http://dx.doi.org/10.1016/j.ssc.2004.09.030

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04 solid state comm. 132(11)787 stevens.pdf

Ab initio investigation of electron paramagnetic resonance parameters of S2-, SSe-, and Se2- radicals in alkali halides

wim — Fri, 30 Sep 2011 11:51:56 +0000

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, M. Waroquier

Physical Review B

67 (10), 104429

2003

Abstract

Density functional theory (DFT) methods, as implemented in the Amsterdam Density Functional program, are used to calculate the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) parameters of S2-, SSe-, and Se2- molecular ions doped into NaZ (Z=Cl,Br,I) and KI lattices. The calculations are performed on cluster in vacuo models, involving 88 atoms for the defect and its lattice surroundings, assuming that the molecular anions replace a single halide ion. In a previous study on the S2- ion, difficulties were encountered in calculating the superhyperfine and quadrupole principal values and axes of the neighbor cation nuclei. The observed discrepancies were partially attributed to the use of the frozen core approximation. In this work, the influence of this approximation on the calculated EPR and ENDOR parameters is evaluated. The DFT results for the S2-, SSe-, and Se2- molecular ions are in good agreement with the available experimental EPR data for all considered lattices, strongly supporting the monovacancy model for these diatomic defects.

DOI

http://dx.doi.org/10.1103/PhysRevB.67.104429

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03 phys. rev. B 67(10)104429 stevens.pdf

Density-functional study of S2- defects in alkali halides

wim — Fri, 30 Sep 2011 10:47:14 +0000

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, M. Waroquier

Physical Review B

66 (13), 134103

2002

Abstract

Density-functional methods, as implemented in the Amsterdam Density Functional program, are used to calculate the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) parameters of the S2- defect in a halide monovacancy in various alkali halides (MZ:M=Na, K, Rb and Z=Cl, Br, I) lattices. The calculations were performed on cluster in vacuo models for the defect and its lattice surroundings, involving up to 88 atoms in order to limit boundary effects. For all MZ lattices, the calculated g and 33S hyperfine tensors of the S2- molecular ion are in very good agreement with the available EPR data, explicitly supporting the monovacancy model for the defect. In addition, computational results for the principal superhyperfine and quadrupole values and axes of the nearest shells of M+ and Z- ions are compared with experimental ENDOR data. The merits and shortcomings of the applied cluster in the vacuo method are critically evaluated.

DOI

http://dx.doi.org/10.1103/PhysRevB.66.134103

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02 phys. rev. B 66(13)134103 stevens.pdf