Heteroepitaxy, such as the co-integration of III-V semiconductors on Si (001) substrates, is poised to meet tomorrow’s energy harvesting and opto-electronic applications needs [1-3]. A fundamental understanding of in-situ hetero-epitaxial growth requires a deep knowledge of nucleation at the atomistic level. Using density functional theory, we established the methodology to obtain the absolute surface and interface energies, allowing us to quantitatively determine the wetting behavior of the system [4,5]. In this work, we show how a freshly-prepared Si substrate, is naturally passivated when inserted in an epitaxial chamber, by either a group-III or a group-V monoatomic coverage, resulting in large stabilization of its surface energy [6]. Especially, we show its drastic impact on the wetting properties of GaP on Si, promoting 3D Volmer-Weber growth mode. With these findings, a synthetic description of surface and interface energies ranges expected during III-V/Si epitaxy can be given. Finally, we demonstrate how the knowledge of surface and interface energies in the system allows to give a full ab initio prediction of the Wulff-Kaishew equilibrium morphology of III-V/Si islands at the equilibrium. This research was supported by the French National Research NUAGES Project (Grant no. ANR-21-CE24-0006). DFT calculations were performed at FOTON Institute, and the work was granted access to the HPC resources of TGCC/CINES under the allocation A0160911434 and A0140911434 made by GENCI. [1] I. Lucci, S. Charbonnier, M. Vallet, P. Turban, Y. Léger, T. Rohel, N. Bertru, A. Létoublon, J. Rodriguez, L. Cerutti, E. Tournié, A. Ponchet, G. Patriarche, L. Pedesseau, and C. Cornet, Adv Funct Materials 28(30), 1801585 (2018). [2] M. Feifel, J. Ohlmann, J. Benick, M. Hermle, J. Belz, A. Beyer, K. Volz, T. Hannappel, A. W. Bett, D. Lackner et al., IEEE J. Photovolt. 8, 1590 (2018). [3] M. Piriyev, G. Loget, Y. Léger, L. Chen, A. Létoublon, T. Rohel, C. Levallois, J. Le Pouliquen, B. Fabre, N. Bertru et al., Sol. Energy Mater. Sol. Cells 251, 112138 (2023). [4] I. Lucci, S. Charbonnier, L. Pedesseau, M. Vallet, L. Cerutti, J.-B. Rodriguez, E. Tournié, R. Bernard, A. Lé-toublon, N. Bertru, A. Le Corre, S. Rennesson, F. Semond, G. Patriarche, L. Largeau, P. Turban, A. Ponchet, and C. Cornet, Phys. Rev. Materials 2(6), 060401(R) (2018). [5] S. Pallikkara Chandrasekharan, I. Lucci, D. Gupta, C. Cornet, and L. Pedesseau, Phys. Rev. B 108(7), 075305 (2023). [6] S. Pallikkara Chandrasekharan, D. Gupta, C. Cornet, and L. Pedesseau, Phys. Rev. B 109(4), 045304 (2024).