*hh*-TDA) density functional
theory for this purpose. The *hh*-TDA method is the hole-hole counterpart
to the more established particle-particle TDA (*pp*-TDA) method, both of
which are derived from the particle-particle random phase approximation
(*pp*-RPA). 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. We show that hh-TDA is 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 and *pp*-RPA,
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.