Partially internally contracted Hilbert space multireference coupled cluster method

Abstract: The topic of this project is development of a new multireference coupled cluster method based on aspects of both internally contracted and Hilbert space approaches. The aim is to obtain a method which is applicable to systems with active space consistingof hundreds references. The scheme of the new approach consists of two steps. First, internally contructed cluster equations are solved, yielding the transformed Hamiltonian. In this step, the common cluster operator is truncated to connected singles anddoubles. Second, the Hilbert space calculation is performed using cluster operators truncated to connected singles. Here, the standard Hamiltonian is replaced by the transformed Hamiltonian from the previous step. Similarly to IC MRCC, connected doubles are described in a internally contracted manner, which enables application to systems with large model spaces. Similarly to Hilbert space methods, differential orbital relaxation is included and the internal excitations in the formalism are excluded.