Cell dimensions

[samala nagaprasad reddy]

Hi all,
  I am trying to optimize (cell and geometry) water box (taken from cp2k
benchmarks) with 128 water molecules using DFTB method. I am taking
initially 20 20 20 as cell dimensions and asking to minimize cell and
geometry simultaneously. I think it is well known that the dimensions
should be around 15 ang (In the benchmark study given as 15.6404 15.6404
15.6404). I am getting the dimensions 12.539 12.539 12.539 from my
calculations. The input and end of output given here. Why DFTB is far away
from the benchmark study? Am I missing something in my input? I really
thankful to them, if someone help in this regard.

I look forward to hear the solutions
Thanks in advance
Nagaprasad

Input:
&FORCE_EVAL
  METHOD Quickstep
  STRESS_TENSOR ANALYTICAL
  &DFT
    &POISSON
      PERIODIC XYZ
      &EWALD
       EWALD_TYPE SPME
       GMAX 25
       O_SPLINE 5
      &END EWALD
    &END POISSON

    &LOCALIZE ON
      &PRINT
        &TOTAL_DIPOLE
          COMMON_ITERATION_LEVELS 40000
          FILENAME ./TOTAL_DIPOLE.dat
          ADD_LAST NUMERIC
          PERIODIC TRUE
          REFERENCE COAC
          &EACH
            MD 1
          &END EACH
        &END TOTAL_DIPOLE
      &END PRINT
    &END LOCALIZE
    &QS
      METHOD DFTB
      &DFTB
         DIAGONAL_DFTB3    T
        SELF_CONSISTENT    T
        HB_SR_GAMMA        T
        DO_EWALD           T
        DISPERSION         T
        &PARAMETER
          PARAM_FILE_PATH  /opt/cp2k_libint/cp2k/data/DFTB/scc
          PARAM_FILE_NAME  scc_parameter
          DISPERSION_TYPE  D3
          DISPERSION_RADIUS           15.
          COORDINATION_CUTOFF         1.e-4
          D3_SCALING                  1.0 1.5 1.5
          DISPERSION_PARAMETER_FILE   /opt/cp2k_libint/cp2k/data/dftd3.dat
        &END PARAMETER
      &END DFTB
    &END QS

    &SCF
      SCF_GUESS ATOMIC  #RESTART  #ATOMIC
      EPS_SCF 1.0E-7
      MAX_SCF 2000
      &OT
        PRECONDITIONER FULL_ALL
        MINIMIZER  DIIS
      &END
      &OUTER_SCF
        OPTIMIZER DIIS
        MAX_SCF 100
        EPS_SCF 1.0E-7
      &END OUTER_SCF
    &END SCF
    &PRINT
      &MOMENTS
       PERIODIC TRUE
       &EACH
         MD 1
       &END
       MAX_MOMENT 1
       FILENAME =dipolemet.traj
      &END MOMENTS

      &MULLIKEN
      &END MULLIKEN
    &END PRINT
 &END DFT
  &SUBSYS
    &CELL
      ABC [angstrom] 20.0000 20.0000 20.0000
      SYMMETRY CUBIC
      PERIODIC XYZ
    &END
    &TOPOLOGY
     COORD_FILE_NAME ./h2o.xyz
     COORDINATE xyz
    &END
  &END SUBSYS
&END FORCE_EVAL
&GLOBAL
  PRINT_LEVEL MEDIUM
  PROJECT wat
  RUN_TYPE CELL_OPT
  EXTENDED_FFT_LENGTHS
&END GLOBAL
&MOTION
   &CELL_OPT
     EXTERNAL_PRESSURE  1.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0
     TYPE DIRECT_CELL_OPT
    MINIMIZER LBFGS
    KEEP_SYMMETRY
    MAX_ITER 5000
    &PRINT
      &CELL ON
         &EACH
            CELL_OPT 1
         &END EACH
      &END CELL
    &END PRINT
  &END CELL_OPT

  &PRINT
    &RESTART
      LOG_PRINT_KEY T
      &EACH
        MD 20
      &END EACH
      ADD_LAST NUMERIC
    &END RESTART

    &TRAJECTORY
      LOG_PRINT_KEY T
      FORMAT XYZ
      UNIT angstrom
      &EACH
        MD 1
      &END EACH
      ADD_LAST NUMERIC
    &END TRAJECTORY

    &VELOCITIES
      LOG_PRINT_KEY T
      FORMAT XYZ
      UNIT angstrom
      &EACH
        MD 1
      &END EACH
      ADD_LAST NUMERIC
    &END VELOCITIES
  &END PRINT
&END MOTION





few steps in Output:

 --------  Informations at step =  1092 ------------
  Optimization Method        =                LBFGS
  Total Energy               =      -526.7648934300
  Internal Pressure [bar]    =        20.7598453579
  Real energy change         =        -0.0000068927
  Decrease in energy         =                  YES
  Used time                  =                2.757

  Convergence check :
  Max. step size             =         0.0024311351
  Conv. limit for step size  =         0.0030000000
  Convergence in step size   =                  YES
  RMS step size              =         0.0005225723
  Conv. limit for RMS step   =         0.0015000000
  Convergence in RMS step    =                  YES
  Max. gradient              =         0.0003651668
  Conv. limit for gradients  =         0.0004500000
  Conv. in gradients         =                  YES
  RMS gradient               =         0.0000715192
  Conv. limit for RMS grad.  =         0.0003000000
  Conv. in RMS gradients     =                  YES
  Pressure Deviation [bar]   =        19.7598453579
  Pressure Tolerance [bar]   =       100.0000000000
  Conv. for  PRESSURE        =                  YES
 ---------------------------------------------------

 *******************************************************************************
 ***                    GEOMETRY OPTIMIZATION
COMPLETED                      ***
 *******************************************************************************

 17:54:28 NOTE in cp_output_handling:cp_print_key_unit_nr :: Moving file
 wat-1.restart.bak-2 into file wat-1.restart.bak-3.


 17:54:28 NOTE in cp_output_handling:cp_print_key_unit_nr :: Moving file
 wat-1.restart.bak-1 into file wat-1.restart.bak-2.

 17:54:28 NOTE in cp_output_handling:cp_print_key_unit_nr :: Moving file
 wat-1.restart into file wat-1.restart.bak-1.


 17:54:28 NOTE in cp_output_handling:cp_print_key_unit_nr :: Writing RESTART
 1_1092 to wat-1.restart


                    Reevaluating energy at the minimum

 CELL| Volume [angstrom^3]:
1971.378
 CELL| Vector a [angstrom]:      12.539     0.000     0.000    |a| =
12.539
 CELL| Vector b [angstrom]:       0.000    12.539     0.000    |b| =
12.539
 CELL| Vector c [angstrom]:       0.000     0.000    12.539    |c| =
12.539
 CELL| Angle (b,c), alpha [degree]:
90.000
 CELL| Angle (a,c), beta  [degree]:
90.000
 CELL| Angle (a,b), gamma [degree]:
90.000
 CELL| Requested initial symmetry:
CUBIC
 CELL| Numerically
orthorhombic:                                             YES

 DISTRIBUTION OF THE NEIGHBOR LISTS
              Total number of particle pairs:
31467
              Total number of matrix elements:
31467
              Average number of particle pairs:
1259
              Maximum number of particle pairs:
1354
              Average number of matrix element:
1259
              Maximum number of matrix elements:
1354

 Number of electrons:
1024
 Number of occupied
orbitals:                                                512
 Number of molecular
orbitals:                                               512

 Number of orbital
functions:                                                768
 Number of independent orbital
functions:                                    768

 Parameters for the always stable predictor-corrector (ASPC) method:

  ASPC order: 3
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