Doctoral Defense in Chemistry - Yorick L. A. Schmerwitz

Doctoral candidate:
Yorick Leonard Adrian Schmerwitz

Title of thesis:
Saddle Point Search Methods for Calculations of Excited Electronic States

Opponents:
Dr. Neepa T. Maitra, Professor at the Department of Physics, Rutgers University at Newark, New Jersey, USA, Dr. Peter Malcolm Wallace Gill, Professor at the Faculty of Science, University of Sydney, Australia

Advisors:
Dr. Hannes Jónsson, Professor at the Faculty of Physical Sciences, University of Iceland

Also in the doctoral committee:
Dr. Gianluca Levi, Postdoc at the Science Institute of the University of Iceland Dr. Elvar Örn Jónsson, Research Scientist at the Science Institute of the University of Iceland and Adjunct Lecturer at the Faculty of Physical Sciences, University of Iceland Dr. Philipp Hansmann, Professor at the Department of Physics, Friedrich-Alexander University Erlangen-Nürnberg, Germany Dr. Asmus Ougaard Dohn, Research Engineer at the Department of Physics, Technical University of Denmark, Denmark

Chair of Ceremony:
Dr. Birgir Hrafnkelsson, Professor and Head of the Faculty of Physical Sciences, University of Iceland

Abstract:

Variational density functional calculations provide an improved description of electronic excitations with large change of the electron density compared to the frequently used adiabatic linear-response time-dependent density functional theory (TDDFT), as the orbitals are variationally optimized for the excited states. However, the fact that excited states are typically saddle points on the surface describing the variation of the energy of the system as a function of the electronic degrees of freedom requires identification of the directions along which the energy has to be maximized. A novel direct optimization generalized mode following approach (DO-GMF) is introduced that converges on an $n$\textsuperscript{th}-order saddle point by inverting the components of the gradient in the direction of the eigenvectors corresponding to the $n$ lowest eigenvalues of the electronic Hessian matrix. This approach has the distinct advantages of inherently avoiding variational collapse to the ground state without additional methods and following a chosen excited state by its saddle point order through molecular configurations where the symmetry of the single determinant wave function is broken. It thereby makes it possible to calculate challenging charge transfer excitations, avoided crossings and conical intersections where other state-specific methods tend to fail. Additional implementation challenges introduced by Perdew-Zunger self-interaction correction are overcome by extending the DO-GMF method. The method is demonstrated in calculations of potential energy curves for the ethene and dihydrogen molecules. Results of calculations are presented for challenging charge transfer excitations in organic molecules and the negatively charged nitrogen-vacancy defect in diamond.

About the candidate:
Yorick received a bachelor’s degree in FOKUS chemistry at the Julius-Maximilians University in Würzburg in Germany in 2017 and a master’s degree in 2020. During his master studies he visited Prof. Hannes Jónsson’s group at the University of Iceland for a research stay on the initial path generation for reaction path calculations which also became the topic of his master thesis. He then became a Ph. D. student in the same group.