The co-integration of III-V semiconductors on Si (001) substrates is highly desirable for energy harvesting, electronics, and photonic applications. [1-3] Understanding surface passivation prior to any heterogeneous integration of materials through epitaxy is a cornerstone to establish the wetting characteristics in a material system. [4] Using DFT simulations, we highlight how to quantitatively determine the impact of Si surface passivation prior to III-V/Si hetero-epitaxy on the surface energy of the Si initial substrate. Then, we explore its consequences for the description of wetting properties on the specific case of GaP/Si heterogeneous growth. [5] We show that this inevitable passivation on Si prior to GaP/Si heteroepitaxy leads to a large decrease of the substrate surface energy. This, in turn, leads to a significant decrease in the spreading parameter for the studied system. In addition, we demonstrate that the stabilization of the initial Si surface through passivation is a driving force for 3D Volmer-Weber crystal growth. Finally, it is established that a complete wetting situation would become possible if the passivated Si surface could be destabilized by at least 15 meV/Ų or if the III-V (001) surface could be stabilized by the same amount. 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 A0120911434 and A0140911434 made by GENCI. [1] I. Lucci, S. Charbonnier, L. Pedesseau, M. Vallet, L. Cerutti, J.-B. Rodriguez, E. Tournié, R. Bernard, A. Létoublon, 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 (2018). [2] 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). [3] C. Cornet, S. Charbonnier, I. Lucci, L. Chen, A. Létoublon, A. Alvarez, K. Tavernier, T. Rohel, R. Bernard, J.-B. Rodriguez, L. Cerutti, E. Tournié, Y. Léger, M. Bahri, G. Patriarche, L. Largeau, A. Ponchet, P. Turban, and N. Bertru, Phys. Rev. Materials 4(5), 053401 (2020). [4] S. Pallikkara Chandrasekharan, I. Lucci, D. Gupta, C. Cornet, and L. Pedesseau, Phys. Rev. B 108(7), 075305 (2023). [5] S. Pallikkara Chandrasekharan, D. Gupta, C. Cornet, and L. Pedesseau, Phys. Rev. B 109(4), 045304 (2024).