  We present an implementation of equation-of-motion oscillator strengths for
the 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 are
restricted to an active orbital space, whereas the single excitations are
unrestricted. Calculated nitrogen K-edge spectra of adenosine, adenosine
triphosphate (ATP), and an ATP-water system are used to demonstrate the
performance of the model. Projected atomic orbitals (PAOs) are used to
partition the virtual space into active and inactive orbital sets. Cholesky
decomposition of the Hartree-Fock density is used to partition the occupied
orbitals. This Cholesky-PAO partitioning is cheap, scaling as
$\mathcal{O}(N^3)$, and is suitable for the calculation of core excitations
which are localized in character. By restricting the single excitations of the
cluster operator to the active space, as well as the double excitations, the
CCSD-in-HF model is obtained. A comparison of the two models---MLCCSD and
CCSD-in-HF---is presented for the core excitation spectra of the adenosine and
ATP systems.
