Before any optimization calculation with pseudo-potential periodic DFT codes it is needed to test the convergence of k-point, energy cut-off, smearing width and ….

I study adsorption of O2 on the Ni surface. To find the minimum converged k-point mesh, I calculated single point energy of Ni primitive cell from 2*2*2 to 7*7*7 with

taking other parameters such as cutoff energy, smearing with, and … constant.

I got following energies for above k-points:

Mesh       #k-points          Energy

2*2*2             1             -1365.088491

3*3*3             2              -1354.550831

4*4*4             4              -1354.392248

5*5*5            10             -1354.301694

7*7*7            20             -1354.363604

I cannot choose converged k-point from the above data, due to the fluctuating behavior of energy for k-points. Anybody can help?