E. Pauwels

Assessment of Periodic and Cluster-in-Vacuo Models for First Principles Calculation of EPR Parameters of Paramagnetic Defects in Crystals: Rh2+ Defects in NaCl as Case Study

N. Sakhabutdinova, A. Van Yperen-De Deyne, E. Pauwels, V. Van Speybroeck, H. Vrielinck, F. Callens, M. Waroquier
Journal of Physical Chemistry A
115(9), 1721-1733
2011
A1

Abstract 

In order to find a reliable and efficient calculation scheme for electron paramagnetic resonance (EPR) spectroscopic parameters for transition metal complexes in ionic solids from first principles, periodic and finite cluster-in-vacuo density functional theory (DFT) simulations are performed for g tensors, ligand hyperfine tensors (A), and quadrupole tensors (Q) for Rh2+-related centers in NaCl. EPR experiments on NaCl:Rh single crystals identified three Rh2+ monomer centers, only differing in the number of charge compensating vacancies in their local environment, and one dimer center. Periodic and cluster calculations, both based on periodically optimized structures, are able to reproduce experimentally observed trends in the ligand A and Q tensors and render very satisfactory numerical agreement with experiment. Taking also computation time into account as a criterion, a full periodic approach emerges as most appropriate for these parameters.The g tensor calculations, on the other hand, prove to be insufficiently accurate for model assessment. The calculations also reveal parameters of the complexes which are not directly accessible through experiments, in particular related to their geometry.

Assessment of atomic charge models for gas-phase computations on polypeptides

T. Verstraelen, E. Pauwels, F. De Proft, V. Van Speybroeck, P. Geerlings, M. Waroquier
Journal of Chemical Theory and Computation (JCTC)
8 (2), 661-676
2012
A1

Abstract 

The concept of the atomic charge is extensively used to model the electrostatic properties of proteins. Atomic charges are not only the basis for the electrostatic energy term in biomolecular force fields but are also derived from quantum mechanical computations on protein fragments to get more insight into their electronic structure. Unfortunately there are many atomic charge schemes which lead to significantly different results, and it is not trivial to determine which scheme is most suitable for biomolecular studies. Therefore, we present an extensive methodological benchmark using a selection of atomic charge schemes [Mulliken, natural, restrained electrostatic potential, Hirshfeld-I, electronegativity equalization method (EEM), and split-charge equilibration (SQE)] applied to two sets of penta-alanine conformers. Our analysis clearly shows that Hirshfeld-I charges offer the best compromise between transferability (robustness with respect to conformational changes) and the ability to reproduce electrostatic properties of the penta-alanine. The benchmark also considers two charge equilibration models (EEM and SQE), which both clearly fail to describe the locally charged moieties in the zwitterionic form of penta-alanine. This issue is analyzed in detail because charge equilibration models are computationally much more attractive than the Hirshfeld-I scheme. Based on the latter analysis, a straightforward extension of the SQE model is proposed, SQE+Q0, that is suitable to describe biological systems bearing many locally charged functional groups.

Open Access version available at UGent repository

Accurate spin-orbit and spin-other-orbit contributions to the g-tensor for transition metal containing systems

A. Van Yperen-De Deyne, E. Pauwels, V. Van Speybroeck, M. Waroquier
Physical Chemistry Chemical Physics (PCCP)
14 (30), 10690 - 10704
2012
A1

Abstract 

In this paper an overview is presented of several approximations within Density Functional Theory (DFT) to calculate g-tensors in transition metal containing systems and a new accurate description of the spin–other-orbit contribution for high spin systems is suggested. Various implementations in a broad variety of software packages (ORCA, ADF, Gaussian, CP2K, GIPAW and BAND) are critically assessed on various aspects including (i) non-relativistic versus relativistic Hamiltonians, (ii) spin–orbit coupling contributions and (iii) the gauge. Particular attention is given to the level of accuracy that can be achieved for codes that allow g-tensor calculations under periodic boundary conditions, as these are ideally suited to efficiently describe extended condensed-phase systems containing transition metals. In periodic codes like CP2K and GIPAW, the g-tensor calculation schemes currently suffer from an incorrect treatment of the exchange spin–orbit interaction and a deficient description of the spin–other-orbit term. In this paper a protocol is proposed, making the predictions of the exchange part to the g-tensor shift more plausible. Focus is also put on the influence of the spin–other-orbit interaction which becomes of higher importance for high-spin systems. In a revisited derivation of the various terms arising from the two-electron spin–orbit and spin–other-orbit interaction (SOO), new insight has been obtained revealing amongst other issues new terms for the SOO contribution. The periodic CP2K code has been adapted in view of this new development. One of the objectives of this study is indeed a serious enhancement of the performance of periodic codes in predicting g-tensors in transition metal containing systems at the same level of accuracy as the most advanced but time consuming spin–orbit mean-field approach. The methods are first applied on rhodium carbide but afterwards extended to a broad test set of molecules containing transition metals from the fourth, fifth and sixth row of the periodic table. The set contains doublets as well as high-spin molecules.

The vibrational fingerprint of the electronic excitation energy of molecular systems via molecular dynamics

ISBN/ISSN:
Poster

Conference / event / venue 

Next generation quantum based molecular dy-namics: challenges and perspectives
Bremen, Germany
Monday, 13 July, 2015 to Friday, 17 July, 2015

"Uncovering radiation chemistry in the solid state through periodic density-functional calculations: confrontation with experimental results and beyond" in Applications of EPR in Radiation Research (Eds. A. Lund, M. Shiotani)

Three questions are crucial to unravel the radiation chemistry of any solid-state molecular system: what is the structure of the radicals formed, how are they formed and why? Molecular modeling methods based on Density Functional Theory – in confrontation with experimental Electron Paramagnetic Resonance (EPR) results – can help in finding an answer to all three questions.

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