求助，金刚石111表面为什么会石墨化

syd2000

用vasp算金刚石111表面吸附能，弛豫后没固定的上面两层全变成石墨烯了，有没有大佬遇到过同样的情况，怎么解决呢？
还有金刚石111表面金属原子吸附能在-3eV左右正常吗，是不是太大了
INCAR
#= general control =#
ISTART = 1
ICHARG = 1
ISPIN = 1
PREC = accurate
LREAL = .False.
LWAVE = .True.
LCHARG = .True.
ADDGRID = .True.
ALGO = Fast
ENCUT = 520

#=electronic relaxation=#
ISMEAR = 0
SIGMA = 0.05
LORBIT = 11
NELM = 60
NELMIN = 5
EDIFF = 1e-5

#=ionic relaxation=#
NSW = 150
IBRION = 1
ISIF = 2
POTIM = 0.2
EDIFFG = -0.02

#=vdW interaction=#
IVDW     = 1
VDW_RADIUS = 5