Embedding strategies currently provide the best compromise between accuracy and computational cost in modeling chemical properties and processes of large and complex systems. In this framework, different methods have been proposed all over the years, from the very popular QM/MM approaches to the more recent and very TOC GRAPHICS KEYWORDS. Embedding methods, QM/QM' techniques, extremely localized molecular orbitals, Local Self-Consistent Field approach.

Equation-of-Motion Coupled Cluster with single and double excitations (EOM-CCSD) is currently one of the most accurate quantum chemical methods for the investigation of excited-states, but its non-negligible computational cost unfortunately limits its application to small molecules. To extend its range of applicability, one possibility consists in its coupling with the so-called multi-scale embedding techniques. Along this line, in this work we propose the interface of the EOM-CCSD method with the recently developed quantum mechanics / extremely localized molecular orbital (QM/ELMO) strategy, an approach where the chemically relevant region of the investigated system is treated at fully quantum chemical level (QM region), while the remaining part (namely, the chemical environment) is described through transferred and frozen extremely localized molecular orbitals (ELMO subsystem). In order to determine capabilities and limitations of the novel EOM-CCSD/ELMO approach, some validation tests were properly designed and carried out. They indicated that the new approach is particularly useful and efficient in describing local electronic transitions in relatively large systems, for both covalently and non-covalently bonded QM and ELMO regions. In particular, it has been shown that, including only a limited number of atoms in the chemically active subunit, the ELMO-embedded computations enable the reproduction of excitation energies and oscillator strengths resulting from full EOM-CCSD calculations within the limit of chemical accuracy, but with a significantly reduced computational cost. Furthermore, despite the approximation of an embedding potential given by frozen extremely localized molecular orbitals, it was observed that the new strategy is able to satisfactorily account for the effects of the environment.