Birch-Murnaghan equation of state

[Ruoyan0103]

Thank you so much for sharing your code. I met some issues in the code. I think the equation used in file 'elastic.py' function 'BMEOS' is not Birch-Murnaghan equation.
It is Murnaghan equation: https://en.wikipedia.org/wiki/Murnaghan_equation_of_state
The correct equation is: https://en.wikipedia.org/wiki/Birch%E2%80%93Murnaghan_equation_of_state

[jochym]

You are right. I will make the correction in the text. Later, I will implement B-M version as well. (I will also accept PR if you have time ;). Tanks, @Ruoyan0103 !