  We present an approach to derive effective shell-model interactions from
microscopic nuclear forces. The similarity-transformed coupled-cluster
Hamiltonian decouples the single-reference state of a closed-shell nucleus and
provides us with a core for the shell model. We use a second similarity
transformation to decouple a shell-model space from the excluded space. We show
that the three-body terms induced by both similarity transformations are
crucial for an accurate computation of ground and excited states. As a proof of
principle we use a nucleon-nucleon interaction from chiral effective field
theory, employ a $^4$He core, and compute low-lying states of $^{6-8}$He and
$^{6-8}$Li in $p$-shell model spaces. Our results agree with benchmarks from
full configuration interaction.
