Solution LAB1 ecutwfc convergence 2

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Convergence of total energy differences wrt the kinetic energy cutoff

Use the same machinery (script to run the calculations, scripts to extract data from the output file) developed for the previous exercise, Solution to Exercise 3.

There we have been running calculations for alat=6.741. Let's pick a lattice variation, say 2%. Change the script to run with such a modified parameter. For instance:

 #
 # set vars
 #
 stretch=0.01
 alat0=6.741
 alat=`echo $alat0 $stretch | awk '{ print $1*(1.0+$2)}'`
 nk=8

(of course it is also possible to just change the hardcoded alat value in the script).

It is also important to change the input and output filenames, e.g. modifying label:

 #
 label="nk${nk}_stretch${stretch}_ecut${ecutwfc}" 
 #

We are now in the position to run the calculations for the new system for a range of ecutwfc values. Using the same script of Ex3 to extract the data,

 $>  ./extract.sh  scf*stretch*.out > data_stratch      

we obtain:

 $>  cat data_stretch.dat 
 #  ecut [Ry]   etot [Ry]   time [sec]
 20.0000  -22.21905831   0.23
 30.0000  -22.53626807   0.32
 40.0000  -22.69832895   0.33
 50.0000  -22.75526586   0.92
 60.0000  -22.77763941   1.00
 80.0000  -22.79383663   1.21
 100.0000  -22.79691290   1.66
 120.0000  -22.79774354   3.56
 140.0000  -22.79794454   6.42
 160.0000  -22.79800726   5.32
 200.0000  -22.79802688   7.79
 300.0000  -22.79802865   21.11     # these last values are largely overconverged
 400.0000  -22.79802704   22.84     # just reported for completeness

Using the command paste we place side by side the results obtained in Ex3 without stretching and those that we have just computed.

 $>  paste data.dat data_stretch.dat
 #  ecut [Ry]   etot [Ry]   time [sec]	#  ecut [Ry]   etot [Ry]   time [sec]
 20.0000  -22.20777973   0.19	  20.0000  -22.21905831   0.23
 30.0000  -22.53632376   0.33	  30.0000  -22.53626807   0.32
 40.0000  -22.70012158   0.41	  40.0000  -22.69832895   0.33
 50.0000  -22.75832711   0.97	  50.0000  -22.75526586   0.92
 60.0000  -22.78070087   1.16	  60.0000  -22.77763941   1.00
 [...]

This is convenient to use awk to compute the line-by-line difference of fields $5 and $2 (total energies w/ and w/o stretching):

 $>  paste data.dat data_stretch.dat | awk '{print $1, $5-$1}'
 # 0
 20.0000  -0.0112786
 30.0000   5.569e-05
 40.0000   0.00179263
 50.0000   0.00306125
 60.0000   0.00306146
 80.0000   0.00333661
100.0000   0.00331713
120.0000   0.00333176
140.0000   0.0033299
160.0000   0.00333089
200.0000   0.0033337
300.0000   0.00333215
400.0000   0.0033342

• Independently of the stretching parameter chosen, the accuracy of the difference is smaller that 0.0001 Ry for ecutwfc values larger than 80 Ry,
• Instead, we had to go for much larger values (120-150 Ry) to converge the absolute values of the total energies.
• We are indeed exploiting a form of error cancellation, due to the fact that what causes the total energy to converge slowly wrt ecutwfc are the oscillations of the wfcs and density in the region close to the nuclei.
• Fortunately, when changing the lattice parameter, such contributions do not affect the bonding, and, in turn, the energy difference between the two structures, thereby not showing up in the total energy differences.
• A similar situation happens also for total energy derivatives wrt to atomic positions (forces) or lattice parameters (stresses).