[CP2K-user] EXTERNAL_PRESSURE and PRESSURE_TOLERANCE settings in cell optimisations for crystals

[Huaiyang Sun]

Deal all,

I'm doing a cell optimisation run of a YAG (Y3Al5O12) crystal. I wonder 
about the meaning of the tags EXTERNAL_PRESSURE and PRESSURE_TOLERANCE in 
the CELL_OPT section of the input. The default values are both 100 bar. 
Here are the few points I don't understand,

1. If the cell optimisation run is targeted at the input EXTERNAL_PRESSURE, 
shouldn't the default PRESSURE_TOLERANCE be given a much smaller value, 
like 5.0 or 1.0 bar?

2. Why 100 bar? 1 atm = 1.013 bar sounds more reasonable.

3. My simple understanding of cell optimisation is to find the minimum 
point of the energy-volume (E-V) curve. Then what's the role of external 
pressure?

Here is my input file.

&GLOBAL
  project  pristine-opt
  run_type CELL_OPT
  print_level MEDIUM
&END GLOBAL

&FORCE_EVAL
  METHOD QS

  &SUBSYS
    &CELL 
      ABC  12.0003 12.0003 12.0003  
      PERIODIC XYZ 
      MULTIPLE_UNIT_CELL  1   1   1
      SYMMETRY CUBIC 
    &END CELL

    &TOPOLOGY
      MULTIPLE_UNIT_CELL  1   1   1
      COORD_FILE_FORMAT XYZ
      COORD_FILE_NAME yag_from_cif_conven_unit.xyz 
    &END TOPOLOGY

    &KIND Y
      ELEMENT Y
      BASIS_SET DZV-GTH-PADE
      POTENTIAL GTH-PBE-q11
    &END KIND 


    &KIND Al
      ELEMENT Al
      BASIS_SET DZVP-MOLOPT-SR-GTH 
      POTENTIAL GTH-PBE-q3
    &END KIND 

    &KIND O
      ELEMENT O
      BASIS_SET DZVP-GTH-PBE
      POTENTIAL GTH-PBE-q6
    &END KIND

    &KIND Ce
      ELEMENT Ce
      BASIS_SET DZVP-MOLOPT-SR-GTH-q12
      POTENTIAL GTH-PBE-q12
    &END KIND

    &KIND Yb
      ELEMENT Yb
      BASIS_SET DZVP-MOLOPT-SR-GTH-q24
      POTENTIAL GTH-PBE-q24
    &END KIND

  &END SUBSYS

  &DFT
    BASIS_SET_FILE_NAME BASIS_SET_YAG
    POTENTIAL_FILE_NAME POTENTIAL_YAG

    &MGRID
      NGRIDS 4
      CUTOFF 600
      REL_CUTOFF 60
    &END MGRID

    &QS
      EPS_DEFAULT 1.0E-12
    &END QS

    &POISSON
      PERIODIC XYZ
    &END POISSON

    &SCF
      SCF_GUESS ATOMIC
      EPS_SCF 5.0E-7
      MAX_SCF 200 

      CHOLESKY INVERSE 

      &OT ON
        MINIMIZER DIIS
      &END OT
      #&DIAGONALIZATION 
      #  ALGORITHM STANDARD
      #&END DIAGONALIZATION 

      # ADDED_MOS 30
      #
      # &SMEAR ON
      # METHOD FERMI_DIRAC
      # ELECTRONIC_TEMPERATURE [K] 300
      #&END SMEAR 

      # &MIXING
      #  METHOD BROYDEN_MIXING
      #  ALPHA 0.4
      #  BETA 0.5
      #  NBROYDEN 8
      # &END MIXING 
    &END SCF

    &XC
      &XC_FUNCTIONAL PBE
      &END XC_FUNCTIONAL 
    &END XC

    &KPOINTS
      EPS_GEO 1.0E-5
      SCHEME MONKHORST 3 3 3
    &END KPOINTS
  &END DFT

  &PRINT
    &TOTAL_NUMBERS ON
    &END TOTAL_NUMBERS
  &END PRINT
  
  STRESS_TENSOR ANALYTICAL 
&END FORCE_EVAL

&MOTION
  &GEO_OPT
    OPTIMIZER LBFGS
    TYPE MINIMIZATION
    MAX_DR   5.0E-4
    MAX_FORCE 5.0E-4
    RMS_DR  5.0E-4
    RMS_FORCE 5.0E-4
    MAX_ITER 500
  &END GEO_OPT

  &CELL_OPT
    TYPE DIRECT_CELL_OPT
    OPTIMIZER LBFGS
    KEEP_SYMMETRY
    MAX_DR 5.0E-4
    MAX_FORCE 1.0E-4
    RMS_DR 5.0E-4
    RMS_FORCE 5.0E-4
    # defaults for p and delta p [bar]
    EXTERNAL_PRESSURE   100.0 0.0 0.0    0.0 100.0 0.0   0.0 0.0 100.0
    PRESSURE_TOLERANCE  100.0
  &END CELL_OPT
&END MOTION 




Thank you,
Huaiyang Sun

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