EOMCC cluster method

In consequence, the most expensive steps of the ground- and excited-state calculations using methods based on the MMCC(2,3) approximation are essentially identical to the n nf noniterative steps of the ground-state CCSD(T) calculations uo and are the numbers of occupied and unoccupied correlated orbitals, respectively). Similar remarks apply to the memory and disk-space requirements. Clearly, these are great simplifications in the computer effort, compared to the higher-level EOMCC approaches, such as EOMCCSDT [43,44,55,56], particularly if we realize that we only have to use the Ti and T2 clusters, obtained in the CCSD calculations, to construct matrix elements of that enter 9Jt (2), Eqs. (58) and (59). In... 

There are several single reference methods to compute electronic transition energies. Among them are configuration interaction-singles (CIS) [48], random-phase approximation (RPA) [49, 50], equation-of-motion couple cluster (EOMCC)... 

As in the ordinary EOMCC theory, in the EOMXCC method we solve the electronic Schrodinger equation (1) assuming that the excited states are represented by Eq. (7). We use the exponential representation of the ground-state wave function I S o), Eq. (8), but no longer assume that the cluster components Tn result from standard SRCC calculations (see below). The many-body expansions of the excitation operator Rk have the same form as in the ordinary EOMCC formalism. In particular, the three different forms of Rk discussed in the previous section [fi -E, R A, and REqs. (28), (30), and (26), respectively] are used to define the EE-EOMXCC, EA-EOMXCC, and IP-EOMXCC methods. As in the standard EOMCC method, by making suitable choices for the operators Qa, which define Rk, we can always extend the EOMXCC theory to other sectors of the Fock space. 

In Table II, our MMCC(2,3) results are compared with the EOMCCSD and CC3 excitation energies reported in ref 37 and with the EOMCCSDt excitation energies reported in ref 41. The EOMCCSDt approach is the EOMCC method, in which relatively small subsets of triexcited components of cluster operator T and excitation operator R are selected through active orbitals 40,41)- manifold of triexcited configurations used in the EOMCCSDt method is identical to that us in the CISDt calculations. This remark is important, since the CISDt wave functions eq (63),... 

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