  The X-ray absorption (XA) spectra of aqueous ammonia and ammonium are
computed using a combination of coupled cluster singles and doubles (CCSD) with
different quantum mechanical and molecular mechanical embedding schemes.
Specifically, we compare frozen Hartree--Fock (HF) density embedding,
polarizable embedding (PE), and polarizable density embedding (PDE).
Integrating CCSD with frozen HF density embedding is possible within the
CC-in-HF framework, which circumvents the conventional system-size limitations
of standard coupled cluster methods. We reveal similarities between PDE and
frozen HF density descriptions, while PE spectra differ significantly. By
including approximate triple excitations, we also investigate the effect of
improving the electronic structure theory. The spectra computed using this
approach show an improved intensity ratio compared to CCSD-in-HF. Charge
transfer analysis of the excitations shows the local character of the pre-edge
and main-edge, while the post-edge is formed by excitations delocalized over
the first solvation shell and beyond.
