The adsorption and diffusion behavior of a Ga adatom on the GaAs (001)鈥?em>c(4 脳 4)-heterodimer surface were studied by employing ab initio density functional theory (DFT) computations in the local density approximation. Structural and bonding features of the c(4 脳 4)-heterodimer reconstruction surface were examined. A comparison with the c(4 脳 4)-ss reconstruction was performed. Minimum energy sites (MES) on the c(4 脳 4)-heterodimer surface were located by mapping the potential energy surface for a Ga adatom. Barriers for diffusion of a Ga adatom between the neighboring MES were calculated by using top hopping- and exchange-diffusion mechanisms. We proposed two unique diffusion pathways for a Ga adatom diffusing between the global minimums of two neighboring unit cells. Signature differences between electronic structures of top hopping- and exchange鈥恉iffusion mechanisms were studied for relevant atoms. We observed a higher diffusion barrier for exchange mechanism compared to top hopping.