[CP2K-user] [CP2K:19327] question on using CDFT with different constraints

[John Zhang]

Dear cp2k users and developers

I tried to use cp2k (2023.2) to perform CDFT calculations. I tried the 
example (He+ + H, HeH-cdft-1) supplied with the code distribution. In this 
example, the constraint is defined as
TARGET 0.0
STRENGTH 0.0 
&ATOM_GROUP
       ATOMS 1..2
       COEFF   1 -1
 &END ATOM_GROUP

I can run this example successfully.  In a second try, I modified the 
definition of the constraint following the online tutorial on Zn2+, the 
constraint is defined as
TARGET 1.0
STRENGTH 0.0 
&ATOM_GROUP
       ATOMS 1
       COEFF   1 
       CONSTRAINT_TYPE CHARGE
 &END ATOM_GROUP
&DUMMY_ATOMS
       ATOMS 2
&END DUMMY_ATOMS

The calculation finished smoothly, but produced different finial energy ( 
-2.626510202989254 vs -2.623351221443904 ). The two cdftlog showed that 
both calculations converged to the desired charge state (see blow). Any 
suggestion on why this is the case and which calculation is correct?

Thanks for your help!

Best 

John


The original constraint definition. 

  --------------------- Becke constraint information ---------------------

  Atomic group                :                                          1

  Type of constraint          :                  Charge density constraint

  Target value of constraint  :                             0.000000000000

  Current value of constraint :                            -0.000025394646

  Deviation from target       :                                 -2.539E-05

  Strength of constraint      :                             0.591096253419

  ------------------------------------------------------------------------


  ------------------------------------------ CDFT atomic charges 
-------------------------------------------

  #Atom  Element   Is_constraint   Core charge    Population (alpha, beta)  
  Net charge      Spin population

      1       He               T         2.000         0.499         0.499 
        1.003                0.000

      2        H               T         1.000         0.499         0.499 
        0.003                0.000


  Total Charge and Spin Moment:                                            
        1.005                0.000


The modified constraint definition

  --------------------- Becke constraint information ---------------------

  Atomic group                :                                          1

  Type of constraint          :                  Charge density constraint

  Target value of constraint  :                             1.000000000000

  Current value of constraint :                             1.000004946096

  Deviation from target       :                                  4.946E-06

  Strength of constraint      :                             1.173800395659

  ------------------------------------------------------------------------


  ------------------------------------------ CDFT atomic charges 
-------------------------------------------

  #Atom  Element   Is_constraint   Core charge    Population (alpha, beta)  
  Net charge      Spin population

      1       He               T         2.000         0.500         0.500   
      1.000                0.000

      2        H               F         1.000         0.497         0.497   
      0.005                0.000


  Total Charge and Spin Moment:                                            
        1.005                0.000




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