Multireference coupled cluster methods augmented by higher excitations from a DMRG wave function

The density matrix renormalization group (DMRG) method has proven itself as a powerful method providing qualitatively correct description of strongly correlated molecular systems, where traditional methods like CASSCF cannot be used due to high computational scaling. Nevertheless, its lack of description of the dynamical correlation is a serious drawback, which in general prevents to achieve the goal of obtaining "chemical accuracy". Various post-DMRG approaches have thus been suggested; in our group we have focused on the DMRG-tailored coupled cluster method in the recent past. The tailored CC method was able to yield satisfactory numerical results, but it still has the disadvantage that the correlation in the active space is "frozen" at the DMRG level and does not reflect interaction with the outer orbital space. In this project we plan to address this issue and explore another way to externally correct the single- and multireference CC methods using selected higher excitations obtained from the MPS wave function provided by the DMRG method.