the case of ammonium, the intensity in the post-edge region is well below the ionization limit. We believe this is a strong indication that the observed post-edge intensity is not due to artifacts in the description of the continuum. This contrasts the spectral feature at around 407 eV for ammonia, which is not present in the experimental spectrum, and overlaps with the ionization region. We attribute this band to artifacts due to the discretized representation of the continuum. CCSD result for ammonium is at variance with what has been previously observed at the level of TP-DFT, as illustrated in the comparative plot in Figure . As seen in Figure , the experimental XA spectrum of aqueous NH 3 is characterized by a pre-edge peak at 401.2 eV, a sharp main band at 402.8 eV and a tail due to post-edge transitions between 403 and eV. In the XA spectrum of NH + 4 , the main peak is centered at 405.7 eV, preceded by a small shoulder at 403-404 eV, corresponding to the pre-edge peak.