  Isospin-symmetry breaking is responsible for the energy difference of excited
states in mirror nuclei. It also influences the coefficient of the isobaric
multiplet mass equation. In the present work, we extensively investigate
isospin-symmetry breaking in medium mass nuclei within ab initio frameworks.
For this, we employ the ab initio valence-space in-medium similarity
renormalization group approach, in which charge-symmetry and
charge-independence breakings are included in the adopted nuclear force. The
mirror energies of sd- and pf- shell nuclei are computed for that matter. The
effects of single-particle states on weakly bound and unbound nuclear states,
especially those of the s-wave, are discussed. Predictions are also made
concerning proton drip-line nuclei bearing large mirror energy difference.
Finally, the coefficient of the isobaric multiplet mass equation in T = 1/2 and
T = 1 isospin multiplets for A = 18 to A = 76 is calculated.
