To shed light on the probable mechanism and active sites for reaction, density functional theory (DFT) calculations were performed. In the first step, FCC bulk structure of Ni was taken from Materials Project to generate the FCC-HEA surface. Optimized lattice parameters are (a=b=c=3.52 Å) which is in good agreement with previous report . The (111) facet was selected for DFT calculations, as it shows the most prominent pick in the XRD pattern. It was generally believed that for CO oxidation (CO + OCO 2 ), O 2 adsorption takes place on the surface. Its dissociation follows this on transition metals as O 2 can easily dissociate on all transition metals at room temperature . Therefore, the reaction follows Langmuir-Hinshelwood mechanism, and the elementary steps involved in the reactions are (i) CO adsorption on the surface, (ii) O 2 dissociation into O adatoms on surfaces, and (iii) CO + O  CO 2 reaction on the surface. Accordingly, the reaction rate depends on the favourable activation of both adsorbed CO and O 2 . This occurs by the transfer of electrons from the Fermi level of CoFeZnNiGa alloy surface to the lowest unoccupied molecular orbital (LUMO) of CO or O 2 , which are antibonding in nature for the both . To understand the factors contributing to the initial adsorption of reactant molecules, energies of Fermi level (E F ) of surface of the CoFeZnNiGa alloy and LUMO (E LUMO ) of CO and O 2 were compared (Figure ). E F of metallic CoFeZnNiGa alloy surface is -4.29 eV, which is energetically lower from E LUMO (-0.24 eV) of O 2 than E LUMO (-2.71 eV) of CO. Therefore, electron transfer from the catalytic surface to O 2 is more difficult than to CO. Adsorption of CO on the surface depletes the electron availability for the activation of O 2 . Hence, the adsorption and the dissociation of O 2 were considered to be the first step followed by adsorption of CO. To understand the most favourable adsorption sites of O 2 and CO, their orbital interaction with the surface was considered. For both O 2 and CO, the LUMO is Π* orbital and hence more prone to interact with the p x or p y orbitals of Ga and d xz or d yz orbitals of Co, Ni, Fe, Zn atoms on the surface Figure ). Therefore, a particular atom having more filled states contributed from previously considered orbitals near the Fermi level has more potentiality to transfer the electron for adsorption and activation of O 2 and CO. Being more stabilized on the hollow sites, for the adsorption of O 2 , the average value of the filled states within 1 eV near the Fermi level of different hollow sites on the surface was calculated. A particular hollow site containing only Ni atoms came out to be the most suitable adsorption site according to the criteria; Figure The free energy diagram was calculated to understand the rate-determining step in the considered mechanistic pathway (Figure ). Adsorption of O on the surface is less endothermic (0.56 eV) as compared to that of (1.13 eV) of adsorption of CO. This is obvious as initially adsorbed O pulls surface electrons, making the stabilization CO difficult. Desorption of CO 2 from the surface is highly exothermic, putting CoFeZnNiGa alloy surface forward as an active catalyst for CO oxidation reaction.