It is a long-standing controversy whether the whole metal clusters are saturated with dissolved carbon atoms during the growth of single-walled carbon nanotubes(SWCNTs).In current work,we are trying to elucidate the carbon nucleation on nickel carbides during the initial stages based on density functional theory,by calculating the formation energy,and chemical potential for a series of Ni55Cn carbides(n is the number of carbon atoms dissolved in Ni55 cluster).The formation energies of the Ni55Cn carbides decrease gradually with the increase of carbon concentration,which suggests the Ni55Cn carbides are thermodynamically stable.The chemical potential energy surface indicates not only would be nickel carbides preferentially formed during the initial stage of the SWCNT nucleation,but also saturated nickel carbides are essential for forming the small carbon rings and caps on carbide cluster surface.The adsorption energies show that the saturated nickel carbides have a better selectivity for the carbon structures with the five-membered carbon ring.The charge transfer and DOS analysis indicate the nickel carbides have comparable reactivity with the pure nickel clusters for growing carbon structures.These properties of the Ni55Cn carbides strongly correlate with the nucleation and growth of the SWCNTs.