Hybrid functional study of the structural and electronic properties of Co and Ni

Density functional theory calculations for the ground-state properties of Co and Ni have been performed using conventional generalized gradient approximation (GGA) and screened hybrid functionals for an exchange-correlation functional. Here, we have calculated structural (equilibrium lattice constants, bulk moduli, and cohesive energies) and electronic (magnetic moments and densities of states) properties. The GGA functional calculations gave the correct ferromagnetic (FM) ground states [FM hcp (Co) and FM fcc (Ni) ground states], while the hybrid functional calculations gave incorrect ground states. In the experimental ground-state configurations [FM hcp (Co) and FM fcc (Ni)], except for the equilibrium lattice constant c ₀ and bulk modulus of Co, the structural parameters obtained using the hybrid functional calculations were in good agreement with experimental values. Further inspection showed that the description of cohesive energies and magnetic moments in the ground state exhibited different behaviors. An unusually small cohesive energy and a large magnetic moment were observed in the hybrid functional calculations, which are in contrast to the GGA calculation results. The reason for this difference is discussed in connection with the calculated electronic structures.