code: def _get_fit_params(self, x, fit_key):
          
          q, chiAz, chiBz = x[0], x[3], x[6]
          eta = q/(1.+q)**2
          chi_wtAvg = (q*chiAz+chiBz)/(1.+q)
          chiHat = (chi_wtAvg - 38.*eta/113.*(chiAz + chiBz))/(1. - 76.*eta/113.)
          chi_a = (chiAz - chiBz)/2.
  
          fit_params = x
          fit_params[0] = np.log(q)
          fit_params[3] = chiHat
          fit_params[6] = chi_a
  
          return fit_params
docs: Transforms the input parameter to fit parameters for the 7dq2 model.
      That is, maps from
      x = [q, chiAx, chiAy, chiAz, chiBx, chiBy, chiBz]
      fit_params = [np.log(q), chiAx, chiAy, chiHat, chiBx, chiBy, chi_a]
  
      chiHat is defined in Eq.(3) of 1508.07253, but with chiAz and chiBz instead
      of chiA and chiB.
      chi_a = (chiAz - chiBz)/2.