Direct interactions over big distances become negligible, making indirect interactions (please see Fig. ) through a medium with higher carrier density more significant. It's well-established in surface chemistry and materials science that defects tend to attract each other, with the attractive forces arising from changes in the elastic tensor and/or electrostatic interactions. Since STM allows to observation of relative charge density that is related to LDOS, it has become a tool to visualize the interference pattern from FOs, generated by different defects (please see Fig. ) as well as it's dependence on the defect separation distance (please see Fig. )) Repp et al., have experimentally determined the interaction energy between two Cu atoms on Cu(111) system to be about approximately -0.4 ∼ 5 meV (see Fig. ). It can be seen that in the case of the first and second energy minima of pair potential, a decrease of LDOS is observed (Fig. )), and the reverse has also been detected for the potential energy maxima (Fig. ). They have used the theory developed by Hyldgaard and Persson 49 (see section 3.7 of this review) to fit their STM data. The decay rate and periodicity of 1/r 2 and 12.5 Å, respectively, that they have obtained are both in good agreement with theoretical values. The calculated value of periodicity is only slightly smaller (∼11 Å). They have shown that Cu is able to form adatom islands at a distance up to the first potential energy minima at 12.5 Å (see Fig. )).