Electronically excited states of cob(II)alamin: Insights from CASSCF/XMCQDPT2 and TD-DFT calculations

The low-lying excited states of cob(ii)alamin were investigated using time-dependent density functional theory (TD-DFT). The performance of TD-DFT calculations was further evaluated using CASSCF/XMCQDPT2, where both four-coordinate and five-coordinate models of cob(ii)alamin were considered. Dependence of electronic structure on the axial base was then investigated using TD-DFT. Consistent with previous benchmarks, the BP86 functional provides a reliable description of the electronically excited states. It was found that the d + π → dz character of the D state increases with respect to the axial base distance, corresponding to a lowering in energy of anti-bonding dz orbitals, leading to near a degeneracy between the ground, and D states in the base-off form.