Yangchuan Li's notes: Converge test in Quantum Espresso

Several parameters (like ecutwfc, kpoints and also a reasonable pseudopotential) need to be determined before running ultimate model. To determine these, we need to make convergence test.

1. Energy cutoff

ecutwfc (Kinetic energy cutoff for wavefunctions) and ecutrho (Kinetic energy cutoff for charge density and potential) are set according to the pseudopotential. When using ultrasoft PP, ecutrho should be 8~12*ecutwfc. Under norm-conserving PP, ecutrho don't need to be assigned. Just use the default value = 4*ecutwfc.

To get ecutwfc, do a convergence test by input different value of ecutwfc. Or by a loop, like the following. Don't forget the last "done".

 for cutoff in 16 20 24 28 32 ; do  
  cat > scf.$cutoff.in <<EOF  
  ! Your other code  
   ecutwfc=$cutoff  
  ! ...  
 EOF  
 pw.x < scf.$cutoff.in > scf.$cutoff.out  
 Done

Here shows a result use Si model

2. kpoints
You can write you own kpoints or generate a list of it by kpoints.x or use "automatic" to make a kpoints mesh. Again, to decide how dense the mesh should be, use the loop above to do convergence test.
Here shows a test result use Si model.