The use of configuration interaction method for describing fine-   splitting effects in the bound two- quark system
  The screened quasi-relativistic potential is used for describing spin-orbit splitting in $^{3}P_{J}$ waves of quark-antiquark system. Fermi-Breit equation is solved numerically in configuration interaction approximation. This approximation takes into account the mixing of wave functions up to fifth order and corrects substantially perturbation calculations. The nature of potentials Lorenz transformation property is elucidated. Very good quantitative results for $b\bar{b}$ and $c\bar{c}$ quarkonia and quite acceptable qualitative characteristics as well as for systems with unequal masses are obtained for $u\bar{u}$. 
V. Lengyel, V. Rubish, A. Shpenik, S.Chalupka, M.Salak