The Exact Exchange-Correlation Potential in Time-Dependent Density Functional Theory: Choreographing Electrons with Steps and Peaks

The time-dependent exchange-correlation potential has an unusual task in directing fictitious non-interacting electrons to move with exactly the same probability density as true interacting electrons. This has intriguing implications for its structure, especially in the non-perturbative regime, leading to step and peak features that cannot be captured by bootstrapping any ground-state functional approximations. We review what has been learned about these features in the exact exchange-correlation potential in time-dependent density functional theory in the past decade or so, and implications for the performance of simulations when electrons are driven far from any ground-state.