Solution LAB1 ecutwfc convergence 3

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Convergence of the total energy differences and forces wrt the kinetic energy cutoff

We can use the same script as before, adding a displacement as suggested (0.05 or 0.10 alat to the component of one of the atoms in the cell).

We also want to modify the scheme for file naming (i.e. te label variable).

In practice, we can modify the script according to the following snippets:

 #
 # set vars
 #
 nk=8
 alat=6.741
 displ=0.01
 #
 ecutwfc_list="20 30 40 50 60 80 100 120 140 160 200 300 400"
 [...]
 
 label="nk${nk}_displ${displ}_ecut${ecutwfc}"
 [...]
 
 # input file
 &CONTROL  
    [...]
    tprnfor=.true.
 /
 [...]   
 ATOMIC_POSITIONS (alat)
 C 0.00 0.00 0.00
 C 0.25 0.25 $((0.25+$displ))

As discussed, by displacing one atom we are breaking the symmetry of the system and atomic forces different from zero can arise. This is indeed the case.

Here we can first compare the total energy differences (wrt the pristine undistorted case) as a function of kinetic energy cutoff. Plot is given in the figure:

Next, we can also plot the behaviour of atomic forces wrt ecutwfc.