Phase Cycling RT-TDDFT Simulation Protocol for Nonlinear XUV and X-ray Molecular Spectroscopy

Real-time time-dependent density functional theory (RT-TDDFT) provides a practical algorithm for propagating a many-electron system driven by external laser fields. The fields are included nonperturbatively in the propagation, and the molecular reduced single-electron density operator and various spectroscopic and diffraction signals can be computed directly, avoiding the expensive calculation of many-body states. Nonlinear optical signals contain contributions of multiple pathways. A phase cycling protocol is implemented in order to separate these pathways. Simulations of XUV four-wave mixing signals in the CO molecule are compared with ab initio sum-over-states calculations.