*hh*-TDA) density functional theory method, which is closely related to the
previously established particle-particle random phase approximation (*pp*-RPA) and its TDA
variant (*pp*-TDA). In *hh*-TDA, the *N*-electron electronic states are obtained through double
annihilations starting from a doubly anionic (*N*+2 electron) reference state. In this way, *hh*-TDA
treats ground and excited states on equal footing, thus allowing for conical intersections to be
correctly described. The treatment of dynamic correlation is introduced through the use of
commonly-employed density functional approximations to the exchange-correlation potential. *hh*-TDA appears to be a promising candidate to efficiently treat the photochemistry of organic and
biochemical systems that involve several low-lying excited states – particularly those with both
low-lying pipi* and npi* states where inclusion of dynamic correlation is essential to describe the
relative energetics. In contrast to the existing literature on *pp*-TDA, we employ a functional-
dependent choice for the response kernel in *pp*- and *hh*-TDA, which closely resembles the response
kernels occurring in linear response and collinear spin-flip TDDFT.