In the $m$-scheme Hartree-Fock (HF) basis, we have developed an ab initio deformed single-reference in-medium similarity renormalization group (IMSRG) approach for open-shell nuclei. A deformed wave function may be more efficient in describing the deformed nucleus. The broken rotational symmetry can be restored using the angular momentum projection. However, a full angular momentum projection at the IMSRG level is still a challenge in both theory itself and computation. The angular momentum restoration mainly recaptures the static correlations, and in the present work we estimate the angular momentum projection effect by projecting the HF state as a leading-order approximation. As a test ground, we have calculated the deformed ${}^{8,10}\mathrm{Be}$ isotopes with the optimized chiral interaction ${\mathrm{NNLO}}_{\mathrm{opt}}$. The results are benchmarked with the no-core shell model and valence-space IMSRG calculations. Then we systematically investigate the ground-state energies and charge radii of even-even nuclei from light beryllium to medium-mass magnesium isotopes. The calculated energies are extrapolated to infinite basis space by an exponential form, and compared with extrapolated valence-space IMSRG results and experimental data.

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• Binding energy & masses
• Nuclear many-body theory
• Nuclear structure & decays
• 20 ≤ A ≤ 38
• 6 ≤ A ≤ 19
• Ab initio calculations