Changes in polarization dictate necessary approximations for modeling electronic deexcitation intensity: Application to x-ray emission

We systematically investigate the underlying relations among different levels of approximation for simulating electronic de-excitations, with a focus on modeling X-ray emission spectroscopy (XES). Using Fermi's golden rule and explicit initial, core-excited state final, valence-hole state, we show that XES can be accurately modeled by using orbital optimization various final states within Slater-determinant framework. However, in this paper, introduce much cheaper approach reliant only single self-consistent field all states, it is typically sufficient. Further approximations reveal these fundamentally many-body transitions reasonably approximated projections ground orbitals, but alone insufficient. Furthermore, except cases where core-ionization induces negligible changes polarization, linear-response approaches adiabatic will have difficulty de-excitation to core level. Therefore, change net dipole moment valence electrons serve as metric validity approximation.