  We propose a new approach to determine the strength of the charge symmetry
breaking (CSB) term in the framework of nuclear density functional theory. It
is shown that once ab initio calculations are available including accurate
description of isospin symmetry breaking terms in medium and heavy nuclei, the
mass difference of mirror nuclei as well as the neutron-skin thickness of
doubly-closed-shell nuclei can be used to constrain the strength of the CSB
interaction with an uncertainty less than $ 6 \, \% $, separately from other
isospin symmetry breaking forces. This method opens a new vista of ab initio
nuclear energy density functionals.
