DMRG-tailored coupled cluster method in the 4c-relativistic domain
Degenerate systems are a long-standing source of problems for quantum chemists. Many compounds, particularly reaction intermediates, feature nearly degenerate energy levels. This causes the single-reference methods, such as density functional theory (DFT), to break down. Moreover, when a molecule contains heavy atoms, electrons move so fast, that the relativistic treatment is necessary. Further problems may occur when the movement of electrons is correlated. The combination of all the three effects emerges in case of heavier transition metal compounds, lanthanides or actinides with open d or f shells. Part of these systems can be solved by CASSCF, MRCI and CASPT2 methods, however, certain molecules are yet to be covered, largely limiting the possibilities of chemists. We propose a method capable to treat heavy metal compounds with combined effects of degeneracy and dynamical correlation. As the most appropriate approach, we find the 4-component coupled clusters, which should be externally corrected by the output from a 4-component density matrix renormalization group calculation.