What do
you think if I just choose the same energy shift for carbon atom
as the one used in diamond solid system calculation?
I think that if you'll take a single atom surrounded by ghost atoms, all
having the same basis as in bulk, that would give you the total energy
lower (i.e. more complete basis) than you would get with any very small
energy shift but only centered on a single atom.
I wouldn't be surprised if you would see the convergence vs energy shift
in the system with a single atom surrounded by ghost atoms.
