Hybrid Local Molecular Orbital: Molecular Orbital Calculations for Open Shell Systems

In this work we discuss first applications of the local molecular orbital:molecular orbital (LMOMO) scheme on open-shell systems, focusing on the advantages of isolating the orbital space of (or near) metal centers. We have used as a benchmark ligand exchange reactions, discussing the multireference character observed in local and canonical calculations, the impact of the local domain approximations, and the convergence of the hybrid scheme. After drawing some conclusions on how to build a selection for high level regions, we applied the method to the rate-determining steps in a nitrite reductase catalyzed reaction step. We have been able to obtain an overall description of the proton, electron transfers energetics occurring in the system which provide a picture for the first time in consistence with the experimental findings. The use of the hybrid scheme is particularly useful in the calculation of the electronic affinities which had previously only been calculated at lower levels of theory. These results show the potential of such local orbital approaches to deal with specific correlation effects confined to a metal center or their direct vicinity.