  Time-dependent density functional theory (TD-DFT) and correlated ab initio
methods have been applied to the electronically excited states of vitamin B12
(cyanocobalamin or CNCbl). Different experimental techniques have been used to
probe the excited states of CNCbl, revealing many issues that remain poorly
understood from an electronic structure point of view. Due to its efficient
scaling with size, TD-DFT emerges as one of the most practical tools that can
be used to predict the electronic properties of these fairly complex molecules.
However, the description of excited states is strongly dependent on the type of
functional used in the calculations. In the present contribution, the choice of
a proper functional for vitamin B12 was evaluated in terms of its agreement
with both experimental results and correlated ab initio calculations. Three
different functionals, i.e. B3LYP, BP86, and LC-BLYP, were tested. In addition,
the effect of relative contributions of DFT and HF to the exchange-correlation
functional was investigated as a function of the range-separation parameter,
{\mu}. The issues related to the underestimation of charge transfer (CT)
excitation energies by TD-DFT was validated by Tozer's L diagnostic, which
measures the spatial overlap between occupied and virtual orbitals involved in
the particular excitation. The nature of low-lying excited states was also
analyzed based on a comparison of TD-DFT and ab initio results. Based on an
extensive comparision against experimental results and ab initio benchmark
calculations, the BP86 functional was found to be the most appropriate in
describing the electronic properties of CNCbl. Finally, an analysis of
electronic transitions and a new re-assignment of some excitations are
discussed.

Chemical Physics;Biological Physics;Biomolecules