  The Dirac equation for an electron in a finite dipole potential has been
studied within the method of linear combination of atomic orbitals (LCAO). The
Coulomb potential of the nuclei that compose a dipole is regularized, by
considering the finite nuclear size. It is shown that if the dipole momentum
reaches a certain critical value, the novel type of supercriticality occurs;
namely, the wave function of the highest occupied electron bound state changes
its localization from the negatively charged nucleus to the positively charged
one. This phenomenon can be interpreted as a spontaneous creation of an
electron-positron pair in vacuum, with each of the created particles being in
the bound state with the corresponding nucleus and partially screening it.
