  We have developed an {\it ab initio} deformed in-medium similarity
renormalization group (IMSRG) for open-shell nuclei. This is a single-reference
IMSRG in deformed Hartree-Fock (HF) basis. Deformed wave functions are more
efficient in describing deformed nuclei. The broken spherical symmetry needs to
be restored by angular momentum projection, which is computational expensive.
The angular momentum mainly capture the static correlations and can be
estimated by the projection of the HF state. In this work, we do deformed IMSRG
calculation and add the correlation energy from projected HF as a leading order
approximation. As the test ground, we have calculated the deformed $^{8,10}\rm
Be$ isotopes with the optimized chiral interaction NNLO$_{\rm opt}$. The
results are benchmarked with the no-core shell model and valence space IMSRG
calculations. Then we systematically investigated the ground-state energies and
charge radii of even-even isotopes from light beryllium to medium-mass
magnesium. The calculated energies are extrapolated to infinite basis space by
an exponential form, and compared with the extrapolated valence-space IMSRG
results and experimental data available.
