  Precise theoretical calculations of open-shell atomic systems are critical
for extracting fundamental physics parameters from precision experiments. Here
we present proof-of-principle calculations illustrating the effectiveness of
the valence-space formulation of the ab initio in-medium similarity
renormalization group, widely used in nuclear theory, as a new ab initio method
for atomic systems. We adapt this approach to study properties of closed- and
open-shell many-electron systems from helium to calcium. Ground-state energies,
excitation spectra, and ionization energies are obtained for selected atoms,
and reasonable agreement is found with benchmark coupled-cluster and many-body
perturbation theory calculations, where available.
