First-principles transition-metal catalysis : efficient and accurate approaches for studying enzymatic systems

[摘要] Transition metals are ever-present as reactive centers in biological and inorganic catalytic cycles. However, the open shell character which gives 3d transition metals unique reactive properties also makes transition metal complexes challenging to study using traditional first principles approaches. Density functional theory is a widely popular computational approach because it recasts a many-body problem of interacting electrons into an equivalent problem of non-interacting electrons, greatly reducing computational cost. Each electron lives in the electric field of the total electron density, giving rise to a problem known as self-interaction; that is, each electron sees the total field including itself, and is therefore repelled by itself. Such an error is maximal in systems with highly localized electrons, in particular transition metals. We introduce an approach in which we augment standard density functionals with a Hubbard U term that helps to counteract the unphysical delocalization of electrons due to errors in exchange-correlation functionals. A Hubbard U approach has already been successfully applied to highly correlated systems in the solid state, but we introduce it for the first time to study the transition metal centers of molecules. This approach, we will show, is even more fitting for single-site molecules where the Hubbard U term need only counteract local effects (e.g. excessive hybridization with ligands) as opposed to multi-site systems where both short-range and long-range self-interaction problems are simultaneously present. The simplified, linear-response formulation we use in conjunction with density functional theory permits direct calculation of the Hubbard U, which is an intrinsic property of the system. We also extend this DFT+U approach by obtaining the linear-response U self-consistently as a property of the DFT+U density, further increasing accuracy.

[发布机构] Massachusetts Institute of Technology