  We present the first ab initio construction of valence-space Hamiltonians for
medium-mass nuclei based on chiral two- and three-nucleon interactions using
the in-medium similarity renormalization group. When applied to the oxygen
isotopes, we find experimental ground-state energies are well reproduced,
including the flat trend beyond the drip line at 24O. Similarly, natural-parity
spectra in 21,22,23,24O are in agreement with experiment, and we present
predictions for excited states in 25,26O. The results exhibit a weak dependence
on the harmonic-oscillator basis parameter and reproduce spectroscopy within
the standard sd valence space.
