Within RAPPID, I will develop a relativistic real-time time-dependent density functional theory (RT-TDDFT) based approach for simulating out-of-equilibrium processes observed in modern attosecond pump−probe spectroscopies. Their theoretical description presents a formidable task owing to the intricate interplay of electronic and nuclear motions, ionization processes and relativistic effects hallmarked for high energy x-ray photons. Our objective is to address all these challenges through the formulation and implementation of mixed quantum-classical electron-nuclear dynamics based on Ehrenfest approach embedded with adaptive scheme with flexible Gaussians to solve the time-dependent electronic equation-of-motion for capturing ionization processes. The backbone of these developments will be the state-of-the-art relativistic atomic mean-field exact two-component (amfX2C) Hamiltonian within the context of RT-TDDFT. These goals will be achieved by combining my expertise on ab-initio theory of ionization processes with the expertise of Dr. Repisky in development and implementation of novel relativistic approaches. In summary, RAPPID will deliver an advanced relativistic framework which is capable of describing ultrafast electron-nuclear processes including valence and core states for elements across the periodic table, and aligns synergistically with ongoing experimental endeavours performed for instance in European-XFEL or ELI-ALPS.