  In the realm of photochemistry, the significance of double excitations (also
known as doubly-excited states), where two electrons are concurrently elevated
to higher energy levels, lies in their involvement in key electronic
transitions essential in light-induced chemical reactions as well as their
challenging nature from the computational theoretical chemistry point of view.
Based on state-of-the-art electronic structure methods (such as high-order
coupled-cluster, selected configuration interaction, and multiconfigurational
methods), we improve and expand our prior set of accurate reference excitation
energies for electronic states exhibiting a substantial amount of double
excitations [http://dx.doi.org/10.1021/acs.jctc.8b01205; Loos et al. J. Chem.
Theory Comput. 2019, 15, 1939]. This extended collection encompasses 47
electronic transitions across 26 molecular systems that we separate into two
distinct subsets: (i) 28 "genuine" doubly-excited states where the transitions
almost exclusively involve doubly-excited configurations and (ii) 19 "partial"
doubly-excited states which exhibit a more balanced character between singly-
and doubly-excited configurations. For each subset, we assess the performance
of high-order coupled-cluster (CC3, CCSDT, CC4, and CCSDTQ) and
multiconfigurational methods (CASPT2, CASPT3, PC-NEVPT2, and SC-NEVPT2). Using
as a probe the percentage of single excitations involved in a given transition
($\%T_1$) computed at the CC3 level, we also propose a simple correction that
reduces the errors of CC3 by a factor of 3, for both sets of excitations. We
hope that this more complete and diverse compilation of double excitations will
help future developments of electronic excited-state methodologies.
