Not getting correct optimised geometry for Au(111) surface with ligands (BDT or DMBT)

[Vikash Khokhar]

I'm trying to optimize a small unit cell of  Au(111) surface with ligands (BDT or DMBT) but I'm not getting the correct optimized geometry.

I'm using Cp2k 8.1 version.

I'm attaching the Input file here, please have a look and suggest to me what's wrong with my calculation. 

&GLOBAL
  PROJECT Au_64_2BDT
  RUN_TYPE GEO_OPT
  PRINT_LEVEL MEDIUM
&END GLOBAL

&MOTION
  &GEO_OPT
    TYPE MINIMIZATION
    OPTIMIZER BFGS
    MAX_DR    3.00E-03
    MAX_FORCE 4.50E-04
    RMS_DR    1.50E-03
    RMS_FORCE 3.00E-04
    MAX_ITER 3000
  &END GEO_OPT

  &CONSTRAINT
    &FIXED_ATOMS
      COMPONENTS_TO_FIX XYZ
      LIST 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
    &END FIXED_ATOMS
  &END CONSTRAINT

&END MOTION

&FORCE_EVAL
  METHOD QS

  &DFT
    BASIS_SET_FILE_NAME BASIS_MOLOPT
    POTENTIAL_FILE_NAME GTH_POTENTIALS

    &QS
      METHOD GPW
      EXTRAPOLATION USE_GUESS
      EPS_DEFAULT 1.0E-10
    &END QS

    &MGRID
      CUTOFF 350
      NGRIDS 4
      REL_CUTOFF 60
    &END MGRID

    &SCF
      SCF_GUESS ATOMIC
      EPS_SCF 1.0E-05
      MAX_SCF 200
      ADDED_MOS 10

      &OUTER_SCF
       EPS_SCF 1.0E-04
       MAX_SCF 200
      &END OUTER_SCF

      &DIAGONALIZATION T
        ALGORITHM STANDARD
      &END DIAGONALIZATION

      &SMEAR ON
        METHOD FERMI_DIRAC
        ELECTRONIC_TEMPERATURE [K] 300
      &END SMEAR

      &MIXING T
        METHOD BROYDEN_MIXING
        ALPHA 0.4
        NBROYDEN 8
      &END MIXING
    &END SCF

    &XC
      &XC_FUNCTIONAL PBE
      &END XC_FUNCTIONAL

      &vdW_POTENTIAL
        DISPERSION_FUNCTIONAL PAIR_POTENTIAL
        &PAIR_POTENTIAL
            PARAMETER_FILE_NAME dftd3.dat
            TYPE DFTD3
            REFERENCE_FUNCTIONAL PBE
            R_CUTOFF 25.0
        &END PAIR_POTENTIAL
     &END vdW_POTENTIAL
    &END XC

    &KPOINTS
      SCHEME GAMMA 1 1 1
      FULL_GRID .TRUE.
      PARALLEL_GROUP_SIZE  0
    &END KPOINTS

  &END DFT

  &SUBSYS
    &CELL
      PERIODIC XYZ
      A    17.3099747000000015    0.0000000000000000    0.0000000000000000
      B     0.0000000000000000    9.9939184000000001    0.0000000000000000
      C     0.0000000000000000    0.0000000000000000   30.0000000000000000
    &END CELL

    &COORD
      @INCLUDE Au_64_2BDT.xyz
    &END COORD

    &KIND Au
      BASIS_SET DZVP-MOLOPT-SR-GTH
      POTENTIAL GTH-PBE
    &END KIND
    &KIND H
      BASIS_SET DZVP-MOLOPT-GTH
      POTENTIAL GTH-PBE
    &END KIND
    &KIND C
      BASIS_SET DZVP-MOLOPT-GTH
      POTENTIAL GTH-PBE
    &END KIND
    &KIND S
      BASIS_SET DZVP-MOLOPT-GTH
      POTENTIAL GTH-PBE
    &END KIND
  &END SUBSYS
&END FORCE_EVAL

I'm also attaching the input and output geometry pics. In the input 3rd layer of the gold surface is fixed and the above 2 layers optimize above each other(which breaks 111 symmetry) which is not expected.

I hope you understand my problem and I expect a solution, earliest possible. 

Regards 

Vikash Khokhar

Indian Institute of Technology, Madras 

[Marcella Iannuzzi]

dear Vikash

The system is probably too small.

To stabilise the Au slab one should use more layers, we use 6.

With such small lateral dimension you should use a mesh of k-points. 

Moreover, the tolerance for the convergence of the wavefunction is not very tight  and the PW cutoff seems quite low. 

Regards

Marcella

[Vikash Khokhar]

Thank you, Marcella, 
I would like to know the exact PW cutoff I can use.
And yes, I  have used mesh k-points 2 3 1, are these correct, or do I need to take a bigger grid? If yes, what should it be?
I thought the default tolerance would be okay. If not, then how much tolerance would be enough?

Regards 

Vikash

[Vikash Khokhar]

And one more thing. I only need the correct geometry. Energetics or forces are not required at the moment.

[Marcella Iannuzzi]

Dear Vikash

In the posted input, the k-points are not used. 

If you already used them, try with a denser mesh and with the MONKHORST-PACK scheme.

I suggest that you test on one configuration (possibly already a bit distorted) the effects on energies and forces by changing k-point mesh and cutoff.

If energies and forces are not accurate, the geometry will be wrong. EPS_SCF 1.0E-07 should be good enough.

OUTER_SCF is used only to reinitialise the preconditioner for the OT approach, hence it is useless with the standard diagonalization method.

 3 layers for a Au slab are insufficient and are keen to lead to weird  distortions in the lattice. 

Regards

Marcella

[Vikash Khokhar]

Thank you, Marcella
I'll look into this. 

Regards 

Vikash