code: def _func(self, volume, params):
          
          e0, b0, b1, v0 = tuple(params)
          eta = (v0 / volume) ** (1. / 3.)
          return (e0 +
                  9. * b0 * v0 / 16. * (eta ** 2 - 1)**2 *
                  (6 + b1 * (eta ** 2 - 1.) - 4. * eta ** 2))
docs: BirchMurnaghan equation from PRB 70, 224107