Tag Archives: Christof Köhler

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

Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) Parameters for Ceria in 0D to 3D

Authors: Jolla Per Kullgren, Matthew Jason Wolf, Kersti Hermansson, Christof Köhler, Bálint Aradi, Thomas Frauenheim, and Peter Broqvist

Reducible oxides such as CeO2 are challanging to describe
with standard density functional theory (DFT) due to the mixed valence states of the cations, and often require the use of additional correction schemes, an
d/or more computationally expen- sive methods. This adds a new layer of complexity when it comes to the generation of Slater-Koster tables and the corresponding repulsive potentials for self-consistent density functional based tight binding (SCC-DFTB) calculations of such materials. In this work, we provide guidelines for how to set up a parameterisation scheme for mixed valence oxides within the SCC-DFTB framework, with a focus on reproducing structural and electronic properties as well as redox reaction energies calculated using a reference DFT method. This parameterisation procedure has been used to generate parameters for Ce–O interactions, with Ce in its +III or +IV formal oxidation states. The generated parameter set is validated through comparison to DFT calculations for various ceria (CeO2) and reduced ceria (CeO2−x ) systems of different dimensionalities ranging from 0D (nano-particles) to 3D (bulk). As oxygen vacancy defects in ceria are of crucial importance to many technological applications, special focus is directed towards the capability of describing such defects accurately.

J. Phys. Chem. C2017, 121 (8), pp 4593–4607
DOI: 10.1021/acs.jpcc.6b10557