code: def _func(self, volume, params):
          
          e0, b0, b1, v0 = tuple(params)
          eta = (volume / v0) ** (1. / 3.)
          squiggle = -3.*np.log(eta)
          return e0 + b0 * v0 * squiggle ** 2 / 6. * (3. + squiggle * (b1 - 2))
docs: Pourier-Tarantola equation from PRB 70, 224107