The excited, ionized and electron attached states within the EOM-CC approach with full inclusion of connected triple excitations

A full inclusion of the connected single (S), double (D) and triple (T) excitations into the equation-of-motion (EOM) coupled-cluster (CC) approach is discussed in the context of the calculations of excitation energies (EE), ionization potentials (IP) and electron affinity (EA). The EOM-CC formalism relies on the diagonalization of the CI-like matrix representing similarity transformed Hamiltonian H, defined as [image omitted] where T is a cluster operator. The H operator is non-Hermitian and - in the case of the EOM-CCSDT model - includes also three- and four-body elements which have to be treated in a special factorized manner to avoid very high scaling of the method and - at the same time - to keep the method fully rigorous. An analysis of the results indicates that the inclusion of triples for the EE reduces the average error twice and for the IP - three times. The performance of the EOM-CCSDT scheme for the electron affinity is more difficult to assess due to scarcity of the experimental data.