The effect of the distribution of dislocation density in the core region is taken into account in the discrete elastic approach. The self-energies, effective inner cut radii, as functions of the position of dislocation center are calculated for screw dislocations with different core structures in isotropic elastic fcc and bcc lattices. The P-N forces for different slip planes are estimated.The effect of the thermal motion of atoms on P-N force is discussed. This discrete elastic approach is compared with other methods. We believe that the discrete elastic approach is an effective method for comprehensive investigation