Real-time Complete Active Space Configuration Interaction

Ultrafast spectroscopies provide valuable insights into electron dynamics at atto- and femtosecond scales. Numerical simulations play a pivotal role in understanding the physics that give rise to experimental results. However, accurate, correlated computational methods for simulating electron dynamics in systems with relativistic effects are still in active development. Specifically, simulating electron dynamics in multireference systems with a variational treatment of spin-orbit coupling presents a significant challenge. In light of this need, this study introduces the relativistic real-time complete active space configuration interaction (RT-X2C-CASCI) method. This method is designed to offer a comprehensive understanding of ultrafast spectroscopy in multireference, relativistic systems. We present the RT-X2C-CASCI method and provides an initial assessment of its performance. This contribution not only expands the computational toolkit for exploring electron dynamics in complex systems but also paves the way for a deeper comprehension of ultrafast processes in the realm of relativistic quantum mechanics.