CC/EOMCC

We are now equipped with all of the basic concepts of the CC/EOMCC theory which are necessary to explain the noniterative MMCC approaches to ground and excited electronic states. In this section, we focus on the exact MMCC theory. The approximate MMCC schemes for excited electronic states, including the externally corrected MMCC approaches and the CR-EOMCCSD(T) theory, and their most recent analog based on the left eigenstates of the similarity-transformed Hamiltonian, are discussed in Section 3. 

It can be demonstrated that once the cluster and excitation operators, and R l, respectively, and the ground- and excited-state energies are determined by solving the relevant CC/EOMCC equations, Eqs. (9), (23), and (26), we can obtain the exact, full Cl, energies by adding the... 

CC/EOMCC equations corresponding to the approximate method A, as defined by Eq. (48). In particular, if we want to recover the full Cl energies E from the CCSD/EOMCCSD energies (the niA = 2 case), we... 

Equation (50) (or its CCSD/EOMCCSD-based analog, Eq. (53)) defines the exact MMCC formalism for ground and excited states. This equation allows us to improve the CC/EOMCC (e.g. CCSD/EOMCCSD) results, in a state-selective manner, by adding the noniterative corrections (in practice, one of the approximate forms of or obtained using the... 

TABLE 3. Total energies (E), adiabatic excitation energies (Te), reduced excitations level (REL) values, and dipole moments (/r) of the ground and low-lying excited states of the CH radical, as obtained with the aug-cc-pVTZ (E, Te, and REL) and aug-cc-pVDZ (/r) basis sets [110,111]. Experimental data and nuclear geometries used in the CC/EOMCC calculations are taken from Refs. [113-119]. 

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