Augmented coupled cluster theories

In two recent publications we have tried to characterize the excited state properties of 1 and 3 in order to facilitate their detection by LIF-spectroscopy. Our main tool in this effort has been equation of motion coupled cluster theory (EOM-CC). The EOM-CCSD method, which is equivalent to linear response CCSD, has been shown to provide an accurate description of both valence and excited states even in systems where electron correlation effects play an important role [39]. Computed transition energies for excitations that are of mainly single substitution character are generally accurate to within 0.1 eV. We have found the EOM-CCSD method to perform particularly well in combination with the doubly-augmented cc-pVDZ (d-aug-cc-pVDZ) basis set. This basis seems to provide equally balanced descriptions of ground and excited states,... 

EOM-CC), to projected coupled cluster theory with all single and double excitations (CCSD) ° As an additional requirements there are (e) the use of enough large basis sets with augmented basis functions to obtain reliable evalution of paramagnetic spin-orbit (PSO) and spin-dipole (SD) terms (f) the inclusion of vibrational corrections for the SSCC, and (g) the consideration of solvent effects on SSCCs. " The latter two requirements concern the comparison of calculated and measured SSCCs. 

The CISD method produces typical errors of 0.4-0.7 eV for the ground states of elements from manganese to copper even after the inclusion of relativistic effects. The Coupled Cluster method called CPF produces an error of 0.4 eV for the d s to d s splitting in nickel. The basis set cited in the main text comes from a study in which an elaborate quadratic Cl method was used in which the already large basis set was augmented with numerous diffuse orbitals (Raghavachari and Trucks, 1989). The use of M-P perturbation theory produced what the authors of this article describe as wild oscillations for the same excitation energy. 

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