In the present work, using VASP and AIRSS, we have modelled possible structures of Si(111)(2/3)√3×(2/3)√3-Mg. Ab initio density functional theory (DFT) calculations using pseudopotentials were employed to investigate structural models of the Si(111)(2/3)√3×(2/3)√3-Mg surface. A number of models were analyzed in a 2×2 surface cell. The study identifies the most energetically favorable structure in the 2×2 configuration as the “7-3_random” model, which contains 1.75 monolayers (ML) of Mg and 0.75 ML of Si. The results of ab initio calculations are well confirmed by experimental methods (STM and LEED observations).