Time-dependent restricted-active-space configuration-interaction method for the photoionization of many-electron atoms

We introduce the time-dependent restricted-active-space configuration-interaction method to solve the time-dependent Schr\"odinger equation for many-electron atoms, and particularly apply it to the treatment of photoionization processes in atoms. The method is presented in a very general formulation and incorporates a wide range of commonly used approximation schemes, such as the single-active electron approximation, time-dependent configuration interaction with single-excitations, or the time-dependent R-matrix method. We prove the applicability of the method by calculating the photoionization cross sections of helium and beryllium, as well as the x-ray--IR pump-probe ionization of beryllium.