Description
Over the past decade, the ultrafast paradigm in physics and chemistry has shifted from the femtosecond to the sub-femtosecond domain. To keep pace with experiment, accurate and efficient theoretical methods are required. In this talk, I will present multi-reference computational protocols for describing highly excited electronic states in molecules, which serve as the foundation for simulating both static spectroscopies and ultrafast electron dynamics. In addition to X-ray photon-in/photon-out techniques, I will focus on photon-in/electron-out spectroscopic observables, employing various flavors of the central-potential method and extensions beyond it. I will also introduce the density-matrix-based time-dependent restricted active space configuration interaction method (ρ-TD-RASCI) for modeling ultrafast electron dynamics. This framework enables a general formulation of processes with different degrees of coherence and offers flexibility in basis selection. For example, using a basis of correlated multiconfigurational states allows one to keep the representation relatively compact. Furthermore, I will discuss the influence of decoherence processes—such as nuclear dynamics, ionization, and autoionization—on electron dynamics. Applications of these theoretical approaches will be illustrated by simulations of linear X-ray spectra, high harmonic generation, ultrafast charge migration, and spin-flip dynamics in molecules and transition-metal complexes excited by extreme ultraviolet and soft X-ray light.
