adsorption energy is too high when using optb88 correction

[weiwen_pu]

Hello all,

when I use opt88 vdwcorrection to calculate 3 layers 12*12 Ag(111) slab with a 18 A vacuum layer , the total energy is positive
and this lead to a large adsorption energy.
however when I use PBE without any correction, the adsorpiton energy is small and resonable.
could anyone tell me why?

Thanks a lot

[support_vasp]

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