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events:2018_summer_school:converging_cutoff [2018/05/31 10:08]
mwatkins [Analysis]
events:2018_summer_school:converging_cutoff [2018/06/11 09:54] (current)
ibethune ↷ Page moved from exercises:2018_uol_school:converging_cutoff to events:2018_summer_school:converging_cutoff
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 This exercise is similar to the previous one, but uses a setup and system more typical of CP2K usage. We will use a system of 32 H<​sub>​2</​sub>​O water molecules within a periodic box. Here is the input template: This exercise is similar to the previous one, but uses a setup and system more typical of CP2K usage. We will use a system of 32 H<​sub>​2</​sub>​O water molecules within a periodic box. Here is the input template:
  
-<​code>​+<​code ​cp2k input_template.inp>
 &GLOBAL &GLOBAL
   PRINT_LEVEL MEDIUM   PRINT_LEVEL MEDIUM
Line 91: Line 91:
 Compared to the Si example, this is a larger system, we are using the OT optimizer in a good setup for a small to medium insulating system: Compared to the Si example, this is a larger system, we are using the OT optimizer in a good setup for a small to medium insulating system:
  
-<​code>​+<​code ​cp2k>
     &SCF     &SCF
       SCF_GUESS RESTART       SCF_GUESS RESTART
Line 109: Line 109:
 and we are also saving the forces on the atoms and we are also saving the forces on the atoms
  
-<​code>​+<​code ​cp2k>
   &PRINT   &PRINT
     &FORCES     &FORCES
Line 192: Line 192:
  
 we see that the largest exponent is only 2.7 Bohr<​sup>​-2</​sup>,​ so can be represented on a much coarser grid. we see that the largest exponent is only 2.7 Bohr<​sup>​-2</​sup>,​ so can be represented on a much coarser grid.
 +
 +<​note>​**Task**
 +If you like, have a look at the BASIS_MOLOPT file (in the data directory, or online [[https://​sourceforge.net/​p/​cp2k/​code/​HEAD/​tree/​trunk/​cp2k/​data/​BASIS_MOLOPT|here]]) and see how the exponents change across the periodic table
 +</​note>​
 +
 +The convergence is largely dominated by the calculation of the gradient terms in a GGA functional (compare a simulation with LDA to the PBE used here). The evaluation of these terms on the grids are demanding, and very dependent on the functional.
 +
 +<​code>​
 +    &XC
 +      &​XC_FUNCTIONAL PBE
 +      &END XC_FUNCTIONAL
 +      &​XC_GRID
 +        ! defaults
 +        XC_SMOOTH_RHO NONE
 +        XC_DERIV PW
 +      &END XC_GRID
 +    &END XC
 +</​code>​
 +
 +For BLYP functional some smoothing needs to be applied. The smoothing may also converge forces more rapidly than the default settings, but at the expense of modifying the functional slightly.
 +
 +<​note>​**TASKS**
 +
 +compare to the previous calculation,​ but using a smoothing section in the XC section.
 +
 +<​code>​
 +    &XC
 +      &​XC_FUNCTIONAL PBE
 +      &END XC_FUNCTIONAL
 +      &​XC_GRID
 +        XC_SMOOTH_RHO NN50
 +        XC_DERIV NN50_SMOOTH
 +      &END
 +    &END XC
 +</​code>​
 +  ​
 +compare the convergence of LDA and BLYP to PBE.
 +
 +<​code>​
 +&​XC_FUNCTIONAL PADE # or BLYP
 +&END XC_FUNCTIONAL
 +</​code>​
 +</​note>​
 +
 +<note tip>
 +Also change the psuedo potential to the appropriate functional.
 +<​code>​
 +    &KIND O
 +      BASIS_SET DZVP-MOLOPT-SR-GTH-q6
 +      POTENTIAL GTH-PADE-q6
 +    &END KIND
 +</​code>​
 +PADE is a synonym for LDA.
 +</​note>​
 +
events/2018_summer_school/converging_cutoff.1527761282.txt.gz · Last modified: 2018/05/31 10:08 by mwatkins