One can reduce the computational costs of, for example, core excitation energies, by using some selection criteria on the used occupied and virtual orbitals. This can also be used in case of spin-orbit coupling. One possibility is to allow only selected occupied orbitals and or selected virtual orbitals in the TDDFT calculations. In this scheme the complete one-electron excited state configuration space is reduced to the subspace where, for example, only the core electrons are excited. Another possibility is to restrict the space of excitation energies, by allowing only pairs of occupied and virtual orbitals, for which the difference in orbital energy is between a certain range.
