Application of multireference equation of motion coupled-cluster theory to transition metal complexes and an orbital selection scheme for the efficient calculation of excitation energies

This paper presents the first application of the multireference equation of motion coupled-cluster (MR-EOMCC) approach to the calculation and characterization of excitation energies of transition metal complexes. The calculated MR-EOM excitation energies are compared with experimental UV/Vis. band maxima, Brueckner based similarity transformed equation of motion (STEOM) calculations and Brueckner based equation of motion coupled cluster (EOM-CCSD(T)) calculations, as well as results calculated with other methods from the literature. The agreement of the excitation energies with experiment is found to be reasonable, and suitable rationalization is given for the discrepancies. An orbital selection scheme is introduced, which can be employed to extend the applicability of the MR-EOMCC methodology to considerably larger systems. (C) 2015 AIP Publishing LLC.