In-Medium Similarity Renormalization Group for Open-Shell Nuclei

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