[CP2K-user] [CP2K:11061] comparing CP2K-DFTB (5.1) and DFTB+ (18.2)

[hut... at chem.uzh.ch]

Hi

the parameter in the EWALD section are not very user friendly
(for historical reasons). In your case you should not keep
RCUT at a small value and change ALPHA. 
It is better to use the default (RCUT calculated from EWALD_ACCURACY
and ALPHA internally).
Also you have to make sure that GCUT is big enough for your system
to give converged values. 
With all these settings you should see the same behaviour as in DFTB+.

regards

Juerg Hutter
--------------------------------------------------------------
Juerg Hutter                         Phone : ++41 44 635 4491
Institut für Chemie C                FAX   : ++41 44 635 6838
Universität Zürich                   E-mail: hut... at chem.uzh.ch
Winterthurerstrasse 190
CH-8057 Zürich, Switzerland
---------------------------------------------------------------

-----cp... at googlegroups.com wrote: -----
To: "cp2k" <cp... at googlegroups.com>
From: "Wei Lai" 
Sent by: cp... at googlegroups.com
Date: 12/19/2018 04:50AM
Subject: [CP2K:11061] comparing CP2K-DFTB (5.1) and DFTB+ (18.2)

Dear CP2K users and developers,
   
  I was testing the DFTB functionality of CP2K and comparing it to DFTB+.  I was using Li2O (324 atoms) as an example and the CP2K input file is shown in the following (without the full coordinates).  
   
  I found CP2K-DFTB results are very sensitive to the choice of Ewald (SPME which is the only Ewald method available for DFTB) parameter (ALPHA in A^-1).  For a RCUT of 6 A, an estimate of ALPHA is usually 3.5/6=0.583.  These results actually suggest indeed this is a good estimate.
  ALPHA Li-Mulliken-charge    H0-energy      Repulsive energy            SCC-energy    Tot energy
  0.64     0.571831        -414.5978261 1.790186269            8.302889648  -404.5047502
  0.583   0.57183          -414.5978042 1.790186269            8.302909311  -404.5047086
  0.5       0.57183          -414.5978156 1.790186269            8.302899133  -404.5047302
  0.35     0.572372        -414.6135268 1.790186269            8.288678932  -404.5346616
  0.22     0.610178        -415.6039561 1.790186269            7.126741326  -406.6870285
  0.219   0.611188        -415.6274692 1.790186269            7.090867312  -406.7464156
   
  On the other hand, DFTB+ (using Ewald instead of SPME) results are NOT very sensitive to ALPHA.  I am not sure what is the unit of ALPHA in DFTB+ but I tried several values.  A value of 0.219 (hand tuned) in CP2K gave similar results to those in DFTB+ with an automatic ALPHA value.
  ALPHA Li-Mulliken-charge    H0-energy      Repulsive energy            SCC-energy    Tot energy
  auto    0.61175023    -415.6390952 1.790189279            7.070521175  -406.7783847
  0.338   0.61175319    -415.6391637 1.790189279            7.07041506    -406.7785594
  0.64     0.6117653      -415.6394434 1.790189279            7.069981919  -406.7792722
  1          0.61178357    -415.6398657 1.790189279            7.069327807  -406.7803486
   
  I am puzzled by this comparison and wondering if you have any insight on this.


Thanks, Wei
   
  &GLOBAL
    PROJECT Li2O
    RUN_TYPE ENERGY_FORCE
    PRINT_LEVEL LOW
  &END GLOBAL
   
  &FORCE_EVAL
    STRESS_TENSOR ANALYTICAL
    METHOD QS
    &DFT  
      &QS
        METHOD DFTB
        &DFTB
          SELF_CONSISTENT T
          DO_EWALD        T
          DISPERSION      F
          &PARAMETER
              SK_FILE Li Li     ./Li-Li.skf
              SK_FILE Li O      ./Li-O.skf
              SK_FILE O Li      ./O-Li.skf
              SK_FILE O O       ./O-O.skf
          &END PARAMETER
        &END DFTB      
        EPS_DEFAULT 1.0E-12
        EXTRAPOLATION ASPC
        EXTRAPOLATION_ORDER 2
      &END QS
      
      &SCF                              
        SCF_GUESS RESTART ! can be used to RESTART an interrupted calculation
        MAX_SCF 100
        EPS_SCF 1.0E-7
        ADDED_MOS 216
        CHOLESKY INVERSE
        &SMEAR ON
          METHOD FERMI_DIRAC
          ELECTRONIC_TEMPERATURE [K] 300
        &END SMEAR      
        &MIXING
            METHOD DIRECT_P_MIXING
            ALPHA   0.1
        &END
      &END SCF
         
      &POISSON
         PERIODIC XYZ 
        &EWALD
          EWALD_TYPE SPME
          ALPHA  .64
          GMAX   21
          RCUT 5
        &END EWALD        
      &END
    &END DFT
    
    &SUBSYS
     &CELL
      PERIODIC XYZ
        A     13.9765253067         0.0000000000         0.0000000000
        B     0.0000000000        13.9765253067         0.0000000000
        C     0.0000000000         0.0000000000        13.9765253067
      &END CELL
      
      &COORD
  Li            1.1647105      1.1647105      1.1647105
  Li            1.1647105      1.1647105      5.8235526
  Li            1.1647105      1.1647105     10.4823942  
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