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 dyz + π → dz(2) character of the D1 state increases with respect to the axial base distance, corresponding to a lowering in energy of anti-bonding dz(2) orbitals, leading to near a degeneracy between the ground, and D1 states in the base-off form.