Topics in Current Chemistry

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes 127
Scheme 2
equal size of the other electronic interactions [27]. When the treatment of
SOC is required on the same footing with other interactions (heavy elements)
it is necessary to go beyond the perturbation theory using a variational
approach based on a spin-orbit Hamiltonian [28]. A very promising
approach has been recently proposed accounting for spin-orbit polarization
effects on multiconfigurational wave functions [29]. A sophisticated correlation
treatment in a scalar relativistic approximation is carried out in a first
step. A model space which includes a set of reference configurations able to
represent all the wanted states along with singly excited configurations selected
with the Spin-Orbit operator is defined in a second step. An extension
of the combined DFT/MRCI method to spin-orbit effects has been proposed
recently which is able to evaluate spin-dependent properties for excited electronic
states in large molecular organic systems [30]. To the state of our
knowledge this promising method has never been applied to the electronic
spectroscopy of transition metal complexes.
2.4
Computational Methods
The goal is to obtain accurate transition energies (within 0.10 eV–0.15 eV)
and reliable dipole transition moments in order to assign bands located in
the UV-visible spectral domain of energy in various transition metal complexes.
The size of the molecular system, its symmetry and the density of
states as well as the saturation of the metal centre d shells or the metal-ligand
interactions will generate particular difficulties. The choice of the
method will be a compromise taking into account the following factors: i)
the feasibility and computational cost; ii) the validity of some approximations;
iii) the wanted level of accuracy; iv) the control that can be performed
on the analysis of the results. Four types of methods based on different