## C vacancy in Diamond in the -2 charge state. 

The lattice correction term, E_lat, is given by 
(Eqn. 3 in the CoFFEE paper):
E^{lat} = E^{iso,m}- E^{per,m}

To obtain the isolated model energy, the model periodic energy 
is computed for three supercell sizes, say 4x4x4, 5x5x5 and 
6x6x6 for this case. These energies are fitted with a polynomial, 
of the form Eqn 13 in CoFFEE paper, 
p(\Omega) = f_1 + f_2/(\Omega^(1/3)) + f_3/(\Omega),
and extrapolated to infinity to obtain E_m,iso. 
See also Fig 4 (c) for the fit. 

Note: Extrapolation is dependent on the Sigma used in the calculation!

To run this example, perform the following steps:
1. Go to the folder 4x4x4/ and run the following command:
path_to_coffee_folder/coffee.py in > out
2. Go to the folder 5x5x5/ and run the following command:
path_to_coffee_folder/coffee.py in > out
3. Go to the folder 6x6x6/ and run the following command:
path_to_coffee_folder/coffee.py in > out

The model total energy is printed in the out files.
The following command will show this energy:
grep ! out

For this example, the model energies should be:
4x4x4: 1.0744 eV
5x5x5: 1.2549 eV
6x6x6: 1.3800 eV

Use plot_fit.py to compute the polynomial fit and extrapolation
to obtain E^{iso,m} for these values.
E^{iso,m} is computed to be 2.04 eV

The corrections, for these super cells then are:
(E^{iso,m}- E^{per,m})
4x4x4: 0.97
5x5x5: 0.785
6x6x6: 0.66
