### Tutorial

Quantum ESPRESSO has a package called `ev.x`

that can be used to calculate the bulk modulus of a material. `ev.x`

basically offers calculation of 4 equation of states, namely: These equation of states give the equilibrium volume (lattice constant in case of cubic) as well as the bulk modulus and it’s first derivative.

The procedure is pretty simple. You first need to calculate the energy of the system at different volumes of unit cell. So you would basically need to run several SCF calculations at different values of lattice parameters (thereby different volumes) and note down the corresponding energies in a `text`

file. For cubic systems you can have the lattice parameter in the first column and the Energy in the second column, while for non-cubic systems, you’d note down the volumes in the first column.

A typical file would look like this:

Let’s say this text file is called: `volvsE.txt`

Now, we can use this to get the Murnaghan [1] equation of state which is very popular among the scientific community.

Murnaghan EOS is given by:

Now you just need to run `ev.x`

from the command-line (terminal) and supply inputs to the following prompts:

[email protected]:~$ /home/manas/qe-6.1/bin/ev.x

Lattice parameter or Volume are in (au, Ang) > Angstrom

Assuming Angstrom

Enter type of bravais lattice (fcc, bcc, sc, noncubic) > sc

Enter type of equation of state :

1=birch1, 2=birch2, 3=keane, 4=murnaghan > 4

Input file > volvsE.txt

Output file > eos

Once, the calculation is done you should have file (`eos.txt`

) that looks like this:

This contains information of the equilibrium volume, bulk modulus and it’s derivative and the minimum energy.

You can use the above generated data to create a neat plot of the equation of state (as you’d see in many publications), using gnuplot. The following shell script, plots the calculated data-points from file `volvsE.txt`

as well as the fitted equation of state curve. [NOTE: This shell script is for cubic systems only. You’d need to modify the code to implement it for non-cubic systems]

Create a file called script.sh and add the following code to it:

#!/bin/bash a="$1" k="$2" dk="$3" emin="$4" V=$(echo "$a*$a*$a"|bc) echo " set terminal png size 1000,500 set output 'EOSplot.png' set xlabel 'Volume (Ang^3)' set ylabel 'Energy (Ry)' set title 'Equation of state' #set key box linestyle 1 f(x)= "$emin"+(x*10**(-30)*"$k"*10**9/"$dk"*(("$V"/x)**"$dk"/("$dk"-1)+1)-"$k"*10**9*"$V"*10**(-30)/("$dk"-1))/(1.6*10**(-19)*13.6) plot f(x) w l title 'Murnaghan EOS fit', 'volvsE.txt' u 1:2 w points pointtype 7 pointsize 2 title 'Calculated data-points' set terminal postscript enhanced color solid 22 set output 'EOSplot.eps' set xlabel 'Volume (Ang^3)' set ylabel 'Energy (Ry)' set title 'Equaiton of state' set autoscale plot f(x) w l title 'Murnaghan EOS fit', 'volvsE.txt' u 1:2 w points pointtype 7 pointsize 2 title 'Calculated data-points' set term x11" >EOSplotScript.p

Now save the file and open the terminal and run the command:

`chmod u+x script.sh`

Now simply run the script by providing the four arguments, in the following order: lattice parameter, bulk modulus, derivative of BM, and minimum energy.

Example: `./script.sh 5.5 81 3.9 -588.656 `

The above command will create a gnuplot script file called: `EOSplotScript.p`

This can then be run using gnuplot using the following command:

`gnuplot ./EOSplotScript.p`

When you run this command, it will generate a PNG image as well as an EPS file.

The output looks like this:

### References

[1] F.D. Murnaghan, Proceedings of the National Academy of Sciences **30**, 244 (1944).

can you help to calculate the bulk modulus of an alloy?

Hi there

Please check out this video

https://youtu.be/G375GDFqpN4

And let me know if you still have a problem

I am working on DFT. Can you help me to know the adsorption study using quantum espresso and let me know how to fix the layers of the slab.

Hello Dear Manas

Thanks for this instructive video.

The main thing in your calculation which was not mentioned by you is that for using EOS we have to change the volume in the crystal and put the “relax” mode calculation, not scf calculation.

Because the energy obtained in a specific volume is not true. So, the we need to put “relax” mode for each volume.

Best Regards

Vahid

Hello Dear Manas

Thanks for this instructive video.

The main thing in your calculation which was not mentioned by you is that for using EOS we have to change the volume in the crystal and put the “relax” mode calculation, not scf.

Because the energy obtanied in a specific volume is not true, since the atoms have not been relaxed.

So, for each volume we need to put “relax” mode in our calculations.

Best regards

Vahid

Sorry, but I don;t think I agree with that.

Actually, this method is hardly used to calculate the crystal parameters nowadays. The only use is to calculate the bulk modulus. The best way to obtain the crstal parameters and atomic coordinates owuld be to run a vc-relax calculation as you must already know.

But then running a relax calculation for every volume won;t give very meaningful results. You see, when the volume is changed the atomic positions (in crystal/fractional coordinates) are still expected to be the same, for the above formula to work correctly. As only then can the energies for different volumes be compared. You see this way the only two things that are changing are volume and energies. But if I start using relax for every volume, then it could be that the atoms relax to be in very strange positions for very large or very smals volumes, which may affect the results. Usually, if you expect an atom to be at 0.33 fractional coordinate in x-axis, then just use that for all the volumes.