Tag Archives: Jolla Kullgren

Curvature Constrained Splines for DFTB Repulsive Potential Parametrization

Authors: Akshay Krishna Ammothum Kandy, Eddie Wadbro, Balint Aradi, Peter Broqvist, and Jolla Kullgren 

The Curvature Constrained Splines (CCS) methodology has been used for fitting repulsive potentials to be used in SCC-DFTB calculations. The benefit of using CCS is that the actual fitting of the repulsive potential is performed through quadratic programming on a convex objective function. This guarantees a unique (for strictly convex) and optimum two-body repulsive potential in a single shot, thereby making the parametrization process robust, and with minimal human effort. Furthermore, the constraints in CCS give the user control to tune the shape of the repulsive potential based on prior knowledge about the system in question. Herein, we developed the method further with new constraints and the capability to handle sparse data. We used the method to generate accurate repulsive potentials for bulk Si polymorphs and demonstrate that for a given Slater-Koster table, which reproduces the experimental band structure for bulk Si in its ground state, we are unable to find one single two-body repulsive potential that can accurately describe the various bulk polymorphs of silicon in our training set. We further demonstrate that to increase transferability, the repulsive potential needs to be adjusted to account for changes in the chemical environment, here expressed in the form of a coordination number. By training a near-sighted Atomistic Neural Network potential, which includes many-body effects but still essentially within the first-neighbor shell, we can obtain full transferability for SCC-DFTB in terms of describing the energetics of different Si polymorphs.

J. Chem. Theory Comput. 2021, 17, 3, 1771–1781
https://doi.org/10.1021/acs.jctc.0c01156

Phonon-Assisted Hot Carrier Generation in Plasmonic Semiconductor Systems

Authors: Yocefu Hattori, Jie Meng, Kaibo Zheng, Ageo Meier de Andrade, Jolla Kullgren, Peter Broqvist, Peter Nordlander, and Jacinto Sá

Plasmonic materials have optical cross sections that exceed by 10-fold their geometric sizes, making them uniquely suitable to convert light into electrical charges. Harvesting plasmon-generated hot carriers is of interest for the broad fields of photovoltaics and photocatalysis; however, their direct utilization is limited by their ultrafast thermalization in metals. To prolong the lifetime of hot carriers, one can place acceptor materials, such as semiconductors, in direct contact with the plasmonic system. Herein, we report the effect of operating temperature on hot electron generation and transfer to a suitable semiconductor. We found that an increase in the operation temperature improves hot electron harvesting in a plasmonic semiconductor hybrid system, contrasting what is observed on photodriven processes in nonplasmonic systems. The effect appears to be related to an enhancement in hot carrier generation due to phonon coupling. This discovery provides a new strategy for optimization of photodriven energy production and chemical synthesis.

Nano Lett. 2021, 21, 2, 1083–1089
https://doi.org/10.1021/acs.nanolett.0c04419

CCS: A software framework to generate two-body potentials using Curvature Constrained Splines

Authors: Akshay Krishna A. K., Eddie Wadbro, Christof Köhler, Pavlin Mitev, Peter Broqvist, and Jolla Kullgren

We have developed an automated and efficient scheme for the fitting of data using Curvature Constrained Splines (CCS), to construct accurate two-body potentials. The approach enabled the construction of an oscillation-free, yet flexible, potential. We show that the optimization problem is convex and that it can be reduced to a standard Quadratic Programming (QP) problem. The improvements are demonstrated by the development of a two-body potential for Ne from ab initio data. We also outline possible extensions to the method.

Program summary
Program Title: CCS

CPC Library link to program files: http://dx.doi.org/10.17632/7dt5nzxgbs.1

Developer’s repository link: http://github.com/aksam432/CCS

Licensing provisions: GPLv3

Programming language: Python

External routines/libraries: NumPy, matplotlib, ASE, CVXOPT

Nature of problem: Ab initio quantum chemistry methods are often computationally very expensive. To alleviate this problem, the development of efficient empirical and semi-empirical methods is necessary. Two-body potentials are ubiquitous in empirical and semi-empirical methods.

Solution method: The CCS package provides a new strategy to obtain accurate two body potentials. The potentials are described as cubic splines with curvature constraints.

Computer Physics Communications, 258, 107602, (2021);

https://doi.org/10.1016/j.cpc.2020.107602

The water/ceria(111) interface: Computational overview and new structures

Authors: Andreas Röckert, Jolla Kullgren, Peter Broqvist, Seif Alwan, and Kersti Hermansson
 
Thin film structures of water on the CeO2(111) surface for coverages between 0.5 and 2.0 water monolayers have been optimized and analyzed using density functional theory (optPBE-vdW functional). We present a new 1.0 ML structure that is both the lowest in energy published and features a hydrogen-bond network extending the surface in one-dimension, contrary to what has been found in the literature, and contrary to what has been expected due to the large bulk ceria cell dimension. The adsorption energies for the monolayer and multilayered water structures agree well with experimental temperature programmed desorption results from the literature, and we discuss the stability window of CeO2(111) surfaces covered with 0.5–2.0 ML of water.
 

Quantitative and qualitative performance of density functional theory rationalized by reduced density gradient distributions

Authors: Ageo Meier de Andrade, Jolla Kullgren and Peter Broqvist 

We evaluate the qualitative and quantitative accuracy of various flavors of density functionals with and without accounting for dispersion corrections. Our test system is nickel in the form of bulk, surfaces, and nanoparticles for which we compute structural properties, bulk cohesive energies, surface energies, and work functions and compare to experimental data. We find that the inclusion of any dispersion, either by an a posteriori correction or by a self-consistent treatment by explicitly computing the nonlocal correlation contribution to the total energy, has a significant effect on the calculated properties and improves the quantitative comparison to experiments. Besides the quantitative agreement, we also investigate qualitative features by comparing Wulff shapes of metal nanoparticles as obtained using the different density functionals. We find that all tested functionals predict similar Wulff shapes for nickel nanoparticles but still have some small differences. These results show that the relative energies calculated using the semilocal GGA and meta-GGA functionals, with and without dispersion, are quite similar. Our findings can also be generalized to other systems when rationalized in terms of the computed reduced density gradients. We find that the distribution of reduced density gradients in a material is correlated to the steepness of the exchange enhancement factor and propose that this information can be used as a quantitative guide when it comes to picking the most appropriate density functional for specific target systems as well as when it comes to extrapolating DFT data to predict experiments.

Phys. Rev. B 102, 2020, 075115

https://doi.org/10.1103/PhysRevB.102.075115

Anion-mediated electronic effects in reducible oxides: Tuning the valence band of ceria via fluorine doping

Authors:  Miroslav Kettner,  Tomáš Duchoň,  Matthew J. Wolf,  Jolla Kullgren,  Sanjaya D. Senanayake,  Kersti Hermansson,  Kateřina Veltruská, and  Václav Nehasil

Combining experimental spectroscopy and hybrid density functional theory calculations, we show that the incorporation of fluoride ions into a prototypical reducible oxide surface, namely, ceria(111), can induce a variety of nontrivial changes to the local electronic structure, beyond the expected increase in the number of Ce3+ ions. Our resonant photoemission spectroscopy results reveal new states above, within, and below the valence band, which are unique to the presence of fluoride ions at the surface. With the help of hybrid density functional calculations, we show that the different states arise from fluoride ions in different atomic layers in the near surface region. In particular, we identify a structure in which a fluoride ion substitutes for an oxygen ion at the surface, with a second fluoride ion on top of a surface Ce4+ ion giving rise to F 2p states which overlap the top of the O 2p band. The nature of this adsorbate F–Ce4+ resonant enhancement feature suggests that this bond is at least partially covalent. Our results demonstrate the versatility of anion doping as a potential means of tuning the valence band electronic structure of ceria.

J. Chem. Phys. 151, 044701 (2019)

https://doi.org/10.1063/1.5109955

From Ceria Clusters to Nanoparticles: Superoxides and Supercharging

Authors: Dou Du, J. Kullgren, K. Hermansson and P. Broqvist

Several studies have reported a dramatically increased oxygen storage capacity (OSC) for small ceria nanoparticles (∼5 nm). Both experiments and theory have correlated this effect with superoxide ion formation. In previous studies, density functional theory (DFT) calculations with the PBE+U density functional have been used, and the obtained results were only in qualitative agreement with the experimental observations. One severe problem is the underbinding of the O2 molecule upon superoxide ion formation, which suggests that such species should not exist above room temperature. In this work, we use hybrid DFT functional to resolve this problem. We find that the discrepancy between theory and experiment originates from an incorrect estimate of the energy associated with the localized f-electrons with respect to the oxygen p-levels. By using average O2 adsorption energies from hybrid DFT calculations, extrapolated to large nanoparticles (3−10 nm), in conjunction with first-order desorption kinetics, we find that superoxide ions are indeed stable on nanosized ceria well above room temperature, in accordance with experiments.

Multiscale Modeling of Agglomerated Ceria Nanoparticles: Interface Stability and Oxygen Vacancy Formation

Authors: Byung-Hyun Kim, Jolla Kullgren, Matthew J. Wolf, Kersti Hermansson and Peter Broqvist

The interface formation and its effect on redox processes in agglomerated ceria nanoparticles (NPs) have been investigated using a multiscale simulation approach with standard density functional theory (DFT), the self-consistent-charge density functional tight binding (SCC-DFTB) method, and a DFT-parameterized reactive force-field (ReaxFF). In particular, we have modeled Ce40O80 NP pairs, using SCC-DFTB and DFT, and longer chains and networks formed by Ce40O80 or Ce132O264 NPs, using ReaxFF molecular dynamics simulations. We find that the most stable {111}/{111} interface structure is coherent whereas the stable {100}/{100} structures can be either coherent or incoherent. The formation of {111}/{111} interfaces is found to have only a very small effect on the oxygen vacancy formation energy, Evac. The opposite holds true for {100}/{100} interfaces, which exhibit significantly lower Evac values than the bare surfaces, despite the fact that the interface formation eliminates reactive {100} facets. Our results pave the way for an increased understanding of ceria NP agglomeration.

Front. Chem., Vol. 7, article id 203,  22 May 2019

https://doi.org/10.3389/fchem.2019.00203

Initial Steps in PEO Decomposition on a Li Metal Electrode

Authors: Amina Mirsakiyeva, Mahsa Ebadi, C. Moyses Araujo, Daniel Brandell, Peter Broqvist, Jolla Kullgren

Poly(ethylene oxide) (PEO) is the most widely used compound as a solid-state (solvent-free) polymer electrolyte for Li batteries, mainly due to its low glass transition temperature (Tg) and ability to dissolve Li salts. It is also frequently suggested that its cathodic stability renders it possible to operate with Li metal anodes in the design of high energy density storage devices. However, little is still known about the true interfacial chemistry between Li metal and PEO and how these two materials interact with each other. We are here exploring this relationship by the means of density functional theory (DFT)-based modeling. Using bulk structures and isolated PEO chains, we have found that there is a strong thermodynamic driving force to oxidize Li metal into lithium oxide (Li2O) when PEO is decomposed into C2H4 and H2, irrespectively of the PEO oligomer length. Explicit modeling of PEO on a Li(100) surface reveals that all steps in the decomposition are exothermic and that the PEO/Li metal system should have a layer of Li2O between the polymer electrolyte and the metal surface. These insights and the computational strategy adopted here could be highly useful to better tailor polymer electrolytes with favorable interfacial properties.


J. Phys. Chem. C, 2019, 123, 37, p. 22851-22857

https://doi.org/10.1021/acs.jpcc.9b07712

Dynamical and Structural Characterization of the Adsorption of Fluorinated Alkane Chains onto CeO2

Authors: Giovanni Barcaro , Luca Sementa, Susanna Monti , Vincenzo Carravetta, Peter Broqvist, Jolla Kullgren, and Kersti Hermansson

The widespread use of ceria-based materials and the need to design suitable strategies to prepare eco-friendly CeO2 supports for effective catalytic screening induced us to extend our computational multiscale protocol to the modeling of the hybrid organic/oxide interface between prototypical fluorinated linear alkane chains (polyethylene-like oligomers) and low-index ceria surfaces. The combination of quantum chemistry calculations and classical reactive molecular dynamics simulations provides a comprehensive picture of the interface and discloses, at the atomic level, the main causes of typical adsorption modes. The data show that at room temperature a moderate percentage of fluorine atoms (around 25%) can enhance the interaction of the organic chains by anchoring strongly pivotal fluorines to the channels of the underneath ceria (100) surface, whereas an excessive content can remarkably reduce this interaction because of the repulsion between fluorine and the negatively charged oxygen of the surface.

J. Phys. Chem. C, Volume 41, 2018, Page 23405
https://doi.org/10.1021/acs.jpcc.8b05554