3. Equation of states tutorial

3.1. Introduction

In this tutorial, you will see how to compute the lattice constant of diamond using total energy calculations. We will use a Birch–Murnaghan equation of state fit to extract the equilibrium volume. The Birch–Murnaghan equation of state reads

\[E(V) = E_0 + \frac{9V_0B_0}{16} \left\{ \left[\left(\frac{V_0}{V}\right)^\frac{2}{3}-1\right]^3B_0^\prime + \left[\left(\frac{V_0}{V}\right)^\frac{2}{3}-1\right]^2 \left[6-4\left(\frac{V_0}{V}\right)^\frac{2}{3}\right]\right\}.\]

3.2. Write an input file template

We will perform 7 total energy calculations around the experimental value of 3.57 A. We first write an input file template like (scf_base.input). Note the lines

info.savepath = './results/scf_fxfacx'
info.outfile = 'fscf_fxfacx.out'
aa = 3.57 * xfacx / 100

There is a tag xfacx that will be replaced by numbers, and hence we call this a template. For example, setting the tag to 100 will make the lattice constant 3.57 * 100 / 100 = 3.57. The first two lines set the name of the result file and standard output log file respectively. The third line sets the lattice constant. As for the rest of the file, you should define the structure and set the resolution and k-sampling to sensible values. If the accuracy parameters are not stringent enough, the fit will not be of good quality.

3.3. Perform the calculations

As mentioned above, we will perform 7 total energy calculations around the experimental value of 3.57 A. To generate the 7 corresponding input files, one may run the script genall.sh. The important thing here is {94..106..2} which means that the tag will be replace by 94, 96, 98, 100, 102, 104, 106. Depending on the material, this range may be too large or too narrow. You should adjust it so that the energy varies by at least a few meV. Our template uses the pseudopotential file C_SZDP.mat which should be at the root of the tutorials directory. One can perform the calculations one by one, or run them in parallel using the script runall.sh. Before using runall.sh, modify the script considering that N is the number of simultaenous calculations and NCORE is the total number of cores you want to use.

3.4. Determine the equilibrium volume

The last step is to extract the equilibrium volume from the equation of states. This is done with the script read_eos.m. It will list the files that match the pattern results/scf_f*.mat, there should be 7 in total. Then it will call the function eosfit to do the fit. Finally, it will print

v_eq = 5.647781 ang^3/atom

From that we can deduce the lattice constant for the conventional diamond lattice. Multiply by 8 and take the cubic root, and get 3.56 A. You may also plot the EOS as follows

figure(); hold on
xlabel('volume (A^3/atom)')
ylabel('energy (eV/atom)')

which gives

Diamond equation of states

Fig. 3.4.1 Diamond equation of states