Calculation of zero-field splitting parameters by density functional theory and ab-initio methods

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The spectroscopic analysis of the excited triplet states plays an important role in diverse research fields. In particular, studying such states is essential for understanding the structure and reactivity of many important biological systems. Theoretical treatments can contribute to this research in various directions: (1) Calculation of the structure and energy surfaces of electronically excited states. (2) Qualitative analysis of the electronic structure of excited states. (3) Calculation of spectroscopic properties of excited triplet states, for (a) testing the quality of the calculated wave function or rather the electron and spin densities, and (b) the interpretation of the complex experimental spectra.This proposal is aimed at the calculation of the magnetic properties of the excited triplet states on the basis of Hartree-Fock (HF) and density functional theory (DFT) as well as correlated ab initio approaches. One of the important accessible parameters from EPR experiments is the zerofield splitting tensor, which describes the splitting of the MS= 0, ±1 components of the S = 1 state in zero applied magnetic field. Reliable quantum chemical methods and programs for the calculation of ZFS parameters for large systems are still scarce. A generally applicable program for the calculation of such properties will be developed and tested. Tasks to be completed include:1. Development and implementation of a resolution of identity (or density fitting) technique to speed up calculations of the spin-spin interaction for large systems;2. Implementation of a fully variational treatment that simultaneously accounts for the spin-spin and spin-orbit interactions on the complete active space self-consistent field ab initio level;3. Validation of the newly implemented methods;4. Application to biologically relevant systems.