code: def murnaghan( vol, e0, b0, bp, v0 ):
      
      energy = e0 + b0 * vol / bp * (((v0 / vol)**bp) / (bp - 1) + 1) - v0 * b0 / (bp - 1.0)
      return energy
docs: Calculate the energy as a function of volume, using the Murnaghan equation of state
      [Murnaghan, Proc. Nat. Acad. Sci. 30, 244 (1944)]
      https://en.wikipedia.org/wiki/Murnaghan_equation_of_state
      cf. Fu and Ho, Phys. Rev. B 28, 5480 (1983).
  
      Args:
          vol (float): this volume.
          e0 (float):  energy at the minimum-energy volume, E0.
          b0 (float):  bulk modulus at the minimum-energy volume, B0.
          bp (float):  pressure-derivative of the bulk modulus at the minimum-energy volume, B0'.
          v0 (float):  volume at the minimum-energy volume, V0.
          
      Returns:
          (float): The energy at this volume.