Acetylene adsorption on silicon (100)-(4 × 2) revisited

► Revisit of acetylene adsorption on silicon by theoretical methods including van der Waals interactions. ► Adsorption configuration found to be di-sigma bonded in all cases. ► Two-dimer footprint in scanning tunneling microscopy (STM) images unambiguously found to be due to two adjacent molecules. ► Full agreement between STM images and theoretical simulations concerning shape and apparent height of molecules on the surface.