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We present an implementation of equation-of-motion oscillator strengths forthe multilevel CCSD (MLCCSD) model where CCS is used as the lower level method(CCS/CCSD). In this model, the double excitations of the cluster operator arerestricted to an active orbital space, whereas the single excitations areunrestricted. Calculated nitrogen K-edge spectra of adenosine, adenosinetriphosphate (ATP), and an ATP-water system are used to demonstrate theperformance of the model. Projected atomic orbitals (PAOs) are used topartition the virtual space into active and inactive orbital sets. Choleskydecomposition of the Hartree-Fock density is used to partition the occupiedorbitals. This Cholesky-PAO partitioning is cheap, scaling as$\mathcal{O}(N^3)$, and is suitable for the calculation of core excitationswhich are localized in character. By restricting the single excitations of thecluster operator to the active space, as well as the double excitations, theCCSD-in-HF model is obtained. A comparison of the two models---MLCCSD andCCSD-in-HF---is presented for the core excitation spectra of the adenosine andATP systems.

Equation-of-motion MLCCSD and CCSD-in-HF oscillator strengths and their application to core excitations