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

[Wei Lai]

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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