Double-Pole Approximation in Time-Dependent Density Functional Theory
  A simple approximate solution to the linear response equations of time-dependent density functional theory (TDDFT) is given. This extends the single-pole approximation (SPA) to two strongly-coupled poles. The analysis provides both an illustration of how TDDFT works when strong exchange-correlation effects are present and insight into such corrections. For example, interaction can cause a transition to vanish entirely from the optical spectrum. 
H. Appel, E.K.U. Gross and K. Burke