Technologically useful indium-͑In͒ terminated c͑8 ϫ 2͒-reconstructed GaAs͑100͒ substrate surface has been studied by first-principles calculations and scanning tunneling microscopy ͑STM͒ measurements. Our totalenergy calculations demonstrate the stability of four different so-called a structures with In monomer rows and In coverage between 0.5 and 2 monolayers on the GaAs͑100͒ substrate. Thus, we introduce a surface system, which stabilizes the a reconstruction. Furthermore, an interesting trend is found. Atomic structure of the c͑8 ϫ 2͒ reconstruction depends on the surface-layer cation and substrate volumes, which, in principle, allows to tune the surface structure by cation adsorption. This phenomenon is related to the peculiar c͑8 ϫ 2͒ atomic surface structure, which shows mixed surface layer, including both anions and cations, and uncommon metallic-type cations in the a structure, which do not show covalent bonds. Our results predict a structural transition from the structure to the a structure as the surface cation size is increased at 0 K. The found transition is probably related to the disordered surface structures ͑consisting of and a building blocks͒ found experimentally by x-ray diffraction at room temperature. Comparison of the STM images, calculated for various c͑8 ϫ 2͒ models, with the former and present measured STM images of In/ GaAs͑100͒c͑8 ϫ 2͒ supports the presence of stable a reconstructions.