Symmetry-projected quantum-chemical methods for magnetic molecules

• Applicant: Dr. Shadan Ghassemi Tabrizi
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 535298924

Complexes of open-shell metal ions, including system with multiple magnetically coupled metal centers, are not only found in all known forms of life – the water-oxidizing cluster (photosystem II) and the FeMo cofactor (nitrogenase enzymes) are exemplary for harnessing solar energy and for nitrogen fixation – but are also of outstanding interest for quantum technologies, where such systems are explored as molecular data stores (single-molecule magnets, SMMs), as qubits in quantum computers, and for use in magnetic refrigeration or spintronic devices. For gaining a detailed theoretical understanding of existing molecules and materials and to rationally design new ones, simple and efficient quantum-chemical methods are needed that enable accurate electronic-structure calculations. In this respect, dealing with the so-called strong-correlation problem poses a particular challenge for the indicated system classes. Improved treatments for strong correlation could thus help to achieve breakthroughs in bioinorganic chemistry, catalysis, or quantum materials. Quantum chemistry already makes invaluable contributions to all these research fields, although strong correlation indeed remains a demanding problem if an overwhelming number of electronic configurations must be superposed to describe a system qualitatively correctly. Distributions of about 20 electrons in 20 orbitals can be superimposed with the Complete Active-Space Self-Consistent Field method, which imposes a strict limitation on the types of problems and molecules that can be addressed. As an alternative to routinely applied computational techniques I plan to implement and further develop methods based on symmetry projection to allow a comprehensive theoretical modeling of distinctive properties of monoand polynuclear transition-metal complexes, with a focus on molecular magnetism. Calculations of magnetic-resonance parameters will be deployed to quantify spin-vibronic coupling in SMMs as a basis for subsequent simulations of the spin dynamics. I hope to thereby advance rational design principles for improved SMMs. In addition, I will address exchange interactions in SMMs whose intriguing properties are not fully understood. Specifically, cooperative effects in mixed transition-metal/lanthanide clusters, including new systems that will be shortly created, shall be investigated. Lastly, symmetry projection methods also appear promising and shall be further explored for the Heisenberg spin model, which is very challenging to solve accurately for all but the smallest systems. The various subprojects complement each other and will facilitate comprehensive predictions of the properties of magnetic molecules.

• DFG Programme: Research Grants