[wanzt]

I want to study the interaction of hydrogen atoms on the surface of graphene. Create a supercell 2*2*1 with a vacuum layer thickness of 40 Å.

At a distance of 20 Å perpendicular to the graphene layer, I added hydrogen atoms (the internuclear distance is 2.2 Å) parallel to the y-axis and calculated the single point energy of this system. However, the energy oscillated and did not converge (as shown in the figure below).


The relevant parameters are set as follows:



Why doesn't the energy converge? What can I do to correctly calculate the energy of this system?
Thanks for the guidance.