Joint work with Marat Valiev, Niri Govind, Peng-Dong Fan, W.A. de Jong (William R Wiley Environmental Molecular Sciences Laboratory and Chemical Sciences Division, Pacific Northwest National Laboratory P.O. Box 999, MS K1-96, Richland, WA 99352) and Jeff R. Hammond (The University of Chicago). The coupled-cluster (CC) methodology has become a leading formalism not only in gas-phase calculations but also in modeling systems for which the inclusion of the surrounding environment is critical for a comprehensive understanding of complex photochemical reactions. At the same time it has been proven that high-level CC formalisms are capable of providing highly adequate characterization of excitation energies and excited-state potential energy surfaces. With the ever increasing power of computer platforms and highly scalable codes, very accurate QM/MM calculations for large molecules (defining the quantum region) can be routinely performed in the foreseeable future even with iterative methods accounting for the effect of triples ( CCSDT-n/EOMCCSDT-n). We will discuss several components of recently developed and implemented CC methodologies in NWChem. This includes: (1) Novel iterative/non-iterative methods accounting for the effect of triply excited configurations, (2) Massively parallel implementations of the CC theories based on the manifold of singly and doubly excited configurations. Several examples will illustrate how these approaches can be used in multiscale QM/MM framework.