My research focuses on the accurate theoretical description of spectroscopic properties of atom and molecules with few  (1-4) electrons. The recent experimental measurements go beyond the accuracy of the theoretical calculations. The uncertainty of the theoretical methods originates from the approximations, which are introduced by complex physical effects as relativity and quantum electrodynamics (QED) and the numerical uncertainty of the theoretical methods. In order to improve the accuracy I apply explicitly-correlated basis functions with close to exact description of the relativistic and QED effects.