In-medium similarity renormalization group for open-shell nuclei

We present a nonperturbative derivation of effective valence-shell Hamiltonians in the framework of the re-cently developed in-medium similarity renormalization group (IM-SRG). As a first application, we calculate the spectra of p- and $sd$-shell nuclei, ⁶Li and ¹⁸O, based on evolved chiral nucleon-nucleon interactions. For ⁶Li, the spectrum is in very good agreement with ab initio results. For ¹⁸O, the IM-SRG provides a new method for the shell model to systematically go beyond effective interaction techniques based on diagrammatic expansions.