Adsorption energy with vdW-DF2 method (vasp.5.2.12)

[akuritu]

Dear admin

I am facing a problem in calculation of adsorption energy of silver ad atom over 3-layer graphite with vdW-DF2 method. I have AB-type of stacking within in graphite layers . Currently I am using Vasp.5.2.12.
Within vdW-DF2 method, cohesive energy of a clean 3-layer graphite slab is -925.979773 eV, and cohesive energy of silver atom deposited over graphite is -922.16616 eV. I have also calculated cohesive energy for reference silver atom with in this method which is +1.2613 eV.
When I do the subtraction E(clean_gr) + E(Ag_ref) - E(ag+gr), it give me a -ve adsorption energy, which means no binding.
To cross check this calculation I also did a calculation for adsorption energy of silver adatom deposited over graphene(single layer). In which I found a positive adsorption, which is matched with old published result.

Hence I am getting a -ve adsorption when I add more layers to graphene from bottom.
Can you help me in this regard. Why this is happening. Am I doing some mistake.

Here is my INCAR

SYSTEM = 3_layer_graphite

ISTART = 0 # 0 -start job: 1&2-continued job
INIWAV = 1 # 1- random wavefunction

ELECTRONIC RELAXATION
IALGO = 38
LDIAG = T
ENCUT = 400 # KE cut-off
NELM = 60 # no of electronic steps
EDIF = 1E-04
ISPIN = 2 # 1- non-pplarised 2- spin-polarised
MAGMOM = 96*0.2 1*1.2
INIMIX = 0 # Linear mixing
AMIX = 0.2
BMIX = 0.0001
MAXMIX = -12
LORBIT = 11 # PAW METHOD
LREAL = AUTO

IONIC RELAXATION
NSW = 200
NBLOCK = 1 # Pair-correlation and DOS are calculated after NBLOCK ionic step
IBRION = 2 # Conjugate gradient
POTIM = 0.5 # Scaling constant
ISYF = 2 # Stress and Relaxation
ISYM = 2 # PAW DATA SET
IWAVPR = 1 #SIMPLE EXTRAPOLATION OF C_D
EDIFFG =-0.01

PARALLEL TAGS
NPAR = 32
NSIM = 1
LPLANE = .TRUE

DOS RELATED VALUES
ISMEAR = 0
SIGMA = 0.05
LUSE_VDW = .TRUE.
AGGAC = 0.0000
GGA = ML
Zab_vdW = -1.8867


Thanks in advance.

[support_vasp]

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