First principle computation of spin-orbit coupling effects on iron centers of proteins and bioinorganic complexes

These studies are aimed at elucidating magnetic properties of metal complexes via ab initio electronic structure calculations. The major part of this thesis is focused on the effects of spin-orbit coupling, such as zero-field splittings (ZFS), on metal complexes. The prediction of ZFS parameters has been done using a combined Kohn-Sham spin density functional theory (KS-SDFT) and sum-over-states perturbation theory (PT) methodology: SDFT-PT. This work consists of three main parts: i) exhaustive derivation of the theory, ii) implementation and validation of the source code, and iii) application of the implemented source code to study a selected set of bioinorganic iron-containing complexes. In addition, I have further developed the theory to elucidate, within the context of SDFT, the physical origin of axial, D, and rhombic, E, ZFS parameters. This feature of the SDFT-PT determines the particular occupied-empty KS orbital interactions with largest contributions to the ZFS. In addition, by contrast to most experiments on bioinorganic complexes, our SDFT-PT methodology allows a direct mapping of the magnitudes and orientations of the ZFS tensor to the molecular structures. This in turn provides a detailed microscopic interpretation of magnetic parameters measured by M ossbauer spectroscopy, EPR, magnetic susceptibility, etc. To accurately predict ZFS parameters via sum-over-states perturbation theory it is necessary to calculate the energy difference between ground state and excited state configurations with accuracy. Accordingly, I have developed and implemented an approximate expression to compute such energy difference within the generalized gradient approximation (GGA) of SDFT. Finally, I have applied SDFT-PT to a selected set of Fe complexes of biochemical relevance