# Example 01: Calculation of free energy#

In this example, a simple calculation of the Helmholtz free energy $$F(NVT)$$ is illustrated. We use a Fe BCC structure at 100K.

## Solid free energy#

The input file for calculation of solid BCC Fe at 100 K is provided in the folder. A detailed description of the input file is available here. The calculation can be started from the terminal using:

calphy -i input.yaml


This should give a message:

Total number of 1 calculations found


calphy is running in the background executing the calculation. A new folder called fe-BCC-100-0, and files called fe-BCC-100-0.sub.err and fe-BCC-100-0.sub.out will also be created.

After the calculation is over, the results are available in the report.yaml file. The file is shown below:

average:
spring_constant: '3.32'
vol/atom: 11.994539429692749
input:
concentration: '1'
element: Fe
lattice: bcc
pressure: 0.0
temperature: 100
results:
error: 0.0
free_energy: -4.263568357143783
pv: 0.0
reference_system: 0.015029873513789175
work: -4.278598230657573


The calculated free energy is $$-4.2636$$ eV/atom, the value reported in the publication is $$-4.2631147(1)$$ eV/atom. The calculated value can be further improved by increasing the system size, and by increasing the switching time.