Automatically Differentiable Model-Configuration Interaction Methodology for Exploration and Inverse Design of Functional Materials

• Applicant: Dr. Merle Röhr
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 555181242

The goal of this project is the development and implementation of inverse design strategies based on differentiable programming and its application for prediction of the structural arrangement of coupled molecules with enhanced photophysical functionality, specifically focusing on the efficiency of intermolecular Singlet Fission (iSF). The essential prerequisite of the planned studies consists in the development of an automatically differentiable electronic structure theory, that allows to treat molecular aggregates with extended size. For that purpose, we will develop a model-CI methodology in the basis of monomer molecular orbitals, accounting for all possible diabatic state configurations including locally excited (LE), Charge Transfer (CT) and singlet correlated triplet pair (TT) states in the restricted active space, that can arise in aggregates beyond dimers, incorporating also spatially separated states and three-center configurations as well as their diabatic couplings. Therefore, the methodology clearly goes beyond the commonly used dimer parametrization and allows considering the formation of the separated correlated triplet pair state in the description of SF rates, that has been identified as key intermediate in experimental studies. Employing Jordan-Wigner representation of fermionic operators and symbolic algebra, our implementation automatically delivers analytic matrix elements with arbitrary excitation level, while the automatic differentiation framework allows for reverse-engineering the diabatic Hamiltonian for an accurate parametrization, setting the stage for the inverse design strategies that will be developed in this project. Specifically, enabled by the chosen AD framework, we will develop "functionality optimization" procedures based on (i) a penalty function approach (ii) the metadynamics methodology, that allow to optimize individual matrix elements of the diabatic CI Model-Hamiltonian as well as rate expressions derived from them in diabatic or adiabatic basis with respect to the nuclear coordinates both on the monomer level and the intermolecular coordinates in extended aggregates. Such procedure will rationalize the screening of the structure-function relation, sampling the whole conformational space and will enable the inverse design of molecular aggregates with tailored photophysical properties. Furthermore, it allows for systematically exploring the parameter space of the diabatic Hamiltonian in order to study the structure sensitivity of processes and their rates. The developed methods and strategies will be applied for the targeted design of one-dimensional molecular aggregates for an efficient SF spending particular emphasis on the impact of structural long-range order as well as exploring the strategy of molecular contortion as a design principle for enhancing the SF rate.

• DFG Programme: Research Grants
• International Connection: Spain
• Cooperation Partner: Professor Dr. Coen De Graaf