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Hello,
I am attempting to study adsorption of O atoms on a metal surface using the vdW-DF2 functional. While my jobs for the the surface+O and surface have converged, the job involving the O atom placed in a box needed to calculate the binding energy of O on the surface gives me unreasonable positive energy ( free energy TOTEN = 1.14957393 eV) . Please let me know what is wrong. Below is my INCAR file:

SYSTEM =0
NWRITE = 1
ISTART = 0

PREC = HIGH
ENCUT = 400.0
EDIFF = 1E-05
EDIFFG = -1.0E-03

IBRION = 2
ISPIN = 2
ISIF = 2
NSW = 20
LCORR = T
ISMEAR = 0
SIGMA = 0.01
LORBIT = 0

IALGO=38
LDIAG = T
LREAL = F
LPLANE = T
NSIM = 4
NPAR = 1

NBANDS = 12
MAGMOM = 2.0

LWAVE = F

GGA = ML
LUSE_VDW = .TRUE.
ZAB_VDW = -1.8867
AGGAC = 0.0
LASPH = .TRUE.

Hello,
I am attaching the input and output files of my unsuccessful run.
I appreciate your guidance on how to run this job.
Thanks!

Dear serenity_3,

I have some suggestions regarding your calculation. To begin with, since you are running a single atom in a box, it is probably easier that you:

• Make all side equal

• Remove the van der Waals interactions

• Deactivate the ionic-minimisation options

The first suggestion just makes the calculation easier, since the system is now symmetric, while the others remove unnecessary complexity. The box should be big enough so that there are no fictitious forces between replicas, thus there is no need to optimise lattice parameters or atomic positions. Also adding a long range term like vdW will only force you to use bigger boxes until this interaction is negligible.

I attach an example of the modified INCAR and POSCAR files that I used. With it I obtained a total energy of -0.01247611 eV.

Let me know if this helps. Kind regards,
Pedro