Advancement and Application of TDDFT-based Non-Adiabatic Molecular Dynamics Methods for Triplet States

• Applicant: Dr. Robin Grotjahn
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 501114520

Non-adiabatic molecular dynamics (NAMD) simulations are an important theoretical tool to study ultrafast light-induced processes occurring on a femtosecond time scale. Previous research has been predominantly focused on processes in which the electronic transitions involve no change in electron spin (singlet excitations and internal conversion). Processes involving spin reversal (triplet excitations and intersystem crossing, ISC) typically proceed on a much longer time scale. However, important exceptions to this rule are increasingly found, such as ultrafast ISC processes in transition-metal complexes, which are relevant for the development of dye-sensitized solar cells or OLEDs. However, the consideration of triplet states in NAMD simulations also introduces various problems, some of which will be addressed in this project.For larger systems, time-dependent density functional theory (TDDFT) is a popular method for calculating the quantum chemical quantities needed for NAMD simulations, as it offers an attractive cost-performance ratio. However, conventional density functionals exhibit large errors for triplet excitation energies, which can lead to incorrect results in NAMD simulations. The relatively recent class of local hybrid functionals, on the other hand, offers significant advantages in describing triplet states, so a first goal is to develop non-adiabatic coupling matrix elements for these functionals to make them available for NAMD simulations.To describe ISC in NAMD simulations, the underlying spin-orbit coupling (SOC) effect must be described. For efficiency and simplicity, this is usually done using perturbative approaches. However, for strong couplings, such as those expected in complexes with heavy elements, these approaches may fail. The second goal of this project is to instead use variational, two-component TDDFT methods that are more reliable for strong couplings to determine SOC matrix elements in NAMD simulations.For the calculation of the forces governing the motion of atomic nuclei in NAMD simulations, the nuclear gradients of the excited states are needed. However, contemporary NAMD methods neglect the gradients of the SOC terms because they are not yet analytically computable by any quantum chemical program. The third goal of this project is to derive and implement these gradients in the framework of two-component TDDFT.The new methods will be used to explore ISC processes in molecules that are highly relevant in the context of solar energy harvesting but have so far only been studied using simpler approaches or not at all. The accuracy of the different methods will also be compared in this context.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Filipp U. Furche