  Real-time time-dependent density functional theory (RT-TDDFT) can in
principle access the whole absorption spectrum of a many-electron system
exposed to a narrow pulse. However, this requires an accurate and efficient
propagator for the numerical integration of the time-dependent Kohn-Sham
equation. While a low-order time propagator is already sufficient for the
low-lying valence absorption spectra, it is no longer the case for the X-ray
absorption spectra (XAS) of systems composed even only of light elements, for
which the use of a high-order propagator is indispensable. It is then crucial
to choose a largest possible time step and a shortest possible simulation time,
so as to minimize the computational cost. To this end, we propose here a robust
AutoPST approach to determine automatically (Auto) the propagator (P), step
(S), and time (T) for relativistic RT-TDDFT simulations of XAS.
