Isospin symmetry violation in atomic nuclei is caused primarily by the
Coulomb force. Within mean-field approximation, there is another
source of the isospin symmetry violation as the product states are generally not eigenstates of isospin~[1,2,3]. In this talk, we shall report on a development of a new theoretical tool which allows for the isospin projection after variation from symmetry-unrestricted Slater
determinants and subsequent rediagonalization of the total nuclear
Hamiltonian including Coulomb interaction in order to incorporate
only physical isospin-mixing effects~[4]. Short overview of main theoretical building blocks of the formalism will be followed by specific applications pertaining to the isospin-mixing effects: (i) in ground states of N=Z and N$\ne$Z nuclei, (ii) in particle-hole excitations and odd-odd N=Z nuclei, (iv) in superdeformed rotational bands in $^{56}$Ni (v) at band termination in N=Z, A~44 nuclei. We shall focus on calculation of the isospin mixing correction $\delta_C$ to the Fermi matrix element for the set of nuclei undergoing the superallowed $0^+ \rightarrow 0^+$ beta decay~[5,6] presenting, for the first time, systematic results on $\delta_C$ stemming from the isospin and angular-momentum projected Hartree-Fock calculations.

[1] C.A. Engelbrecht and R.H. Lemmer, Phys. Rev. Lett. {\bf 24}, 607 (1970).
[2] E. Caurier, A. Poves, and A. Zucker, Phys. Lett. {\bf 96B}, 11 (1980);
Phys. Lett. {\bf 96B}, 15 (1980).
[3] W. Satula, J. Dobaczewski, W. Nazarewicz, and M. Rafalski, Phys. Rev.
Lett. {\bf 103}, 012502 (2009).
[4] J. Dobaczewski {\it et al.}, Comput. Phys. Comm. {\bf 180}, 2361 (2009).
[5] I.S. Towner and J.C. Hardy, Phys. Rev. {\bf C77}, 025501 (2008).
[6] J.C. Hardy and I.S. Towner, Phys. Rev. {\bf C79}, 055502 (2009);