Recent advances in attosecond spectroscopy open the door to the understanding correlated motion of valence and core electrons in molecular systems. In this thesis, the density matrix-based TD-RASCI method is used to study the electron and spin-flip dynamics. The dephasing effect of nuclear vibrations is incorporated implicitly making use of an electronic system/vibrational bath partitioning. We theoretically addressed the ultrafast spin-flip dynamics in a transition metal complex which is triggered by isolated sub-fs soft-X-ray pulses as well as X-ray pulse trains
