  Electronic resonances are metastable states that can decay by electron loss.
They are ubiquitous across various fields of science, such as chemistry,
physics, and biology. However, current theoretical and computational models for
resonances cannot yet rival the level of accuracy achieved by bound-state
methodologies. Here, we generalize selected configuration interaction (SCI) to
treat resonances using the complex absorbing potential (CAP) technique. By
modifying the selection procedure and the extrapolation protocol of standard
SCI, the resulting CAP-SCI method yields resonance positions and widths of full
configuration interaction quality. Initial results for the shape resonances of
\ce{N2-} and \ce{CO-} reveal the important effect of high-order correlation,
which shifts the values obtained with CAP-augmented equation-of-motion
coupled-cluster with singles and doubles by more than \SI{0.1}{\eV}. The
present CAP-SCI approach represents a cornerstone in the development of
highly-accurate methodologies for resonances.
