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Double Excitations Within Time-Dependent Density Functional Theory Linear Response, Neepa T. Maitra, Fan Zhang, Robert J. Cave, Kieron Burke

All HMC Faculty Publications and Research

Within the adiabatic approximation, time-dependent density functional theory yields only single excitations. Near states of double excitation character, the exact exchange–correlation kernel has a strong dependence on frequency. We derive the exact frequency-dependent kernel when a double excitation mixes with a single excitation, well separated from the other excitations, in the limit that the electron–electron interaction is weak. Building on this, we construct a nonempirical approximation for the general case, and illustrate our results on a simple model.