  We describe our efforts of the past few years to create a large set of more
than 500 highly-accurate vertical excitation energies of various natures ($\pi
\to \pi^*$, $n \to \pi^*$, double excitation, Rydberg, singlet, doublet,
triplet, etc) in small- and medium-sized molecules. These values have been
obtained using an incremental strategy which consists in combining high-order
coupled cluster and selected configuration interaction calculations using
increasingly large diffuse basis sets in order to reach high accuracy. One of
the key aspect of the so-called QUEST database of vertical excitations is that
it does not rely on any experimental values, avoiding potential biases
inherently linked to experiments and facilitating theoretical cross
comparisons. Following this composite protocol, we have been able to produce
theoretical best estimate (TBEs) with the aug-cc-pVTZ basis set for each of
these transitions, as well as basis set corrected TBEs (i.e., near the complete
basis set limit) for some of them. The TBEs/aug-cc-pVTZ have been employed to
benchmark a large number of (lower-order) wave function methods such as CIS(D),
ADC(2), CC2, STEOM-CCSD, CCSD, CCSDR(3), CCSDT-3, ADC(3), CC3, NEVPT2, and
others (including spin-scaled variants). In order to gather the huge amount of
data produced during the QUEST project, we have created a website
[https://lcpq.github.io/QUESTDB_website] where one can easily test and compare
the accuracy of a given method with respect to various variables such as the
molecule size or its family, the nature of the excited states, the type of
basis set, etc. We hope that the present review will provide a useful summary
of our effort so far and foster new developments around excited-state methods.
