Development of quantum chemical methods for strongly correlated molecular systems

An effect of crucial importance for the accuracy of quantum chemical computations is the electron correlation. In situations where a single Slater determinant dominates the wave function the problem of its description has been essentially solved by coupled cluster methods, or for larger molecules with less accuracy by DFT methods. However, in many systems there are several (or many) Slater determiants with a large contribution to the wave functions and in such situations these methods fail. Qualitatively such systems can be treated by the density matrix renormalization group method, which is nevertheless unable to capture all correlations effects (incl. co-called dynamic correlation). For this purpose we recently proposed a combination of DMRG with coupled cluster methods, and implemented it for the ground state of the Schroedinger and Dirac-Coulomb molecular Hamiltonians. We plan to extend the method to excited states and also generalize it to situations with many equally contributing Slater determinants.