Hi Ed. As far as I can see, R20B, and R20C is not used in ns_mmq_2site and 3site? def func_ns_mmq_2site(self, params): """Target function for the combined SQ, ZQ, DQ and MQ CPMG numeric solution. @param params: The vector of parameter values. @type params: numpy rank-1 float array @return: The chi-squared value. @rtype: float """ # Scaling. if self.scaling_flag: params = dot(params, self.scaling_matrix) # Unpack the parameter values. R20 = params[:self.end_index[0]] dw = params[self.end_index[0]:self.end_index[1]] dwH = params[self.end_index[1]:self.end_index[2]] pA = params[self.end_index[2]] kex = params[self.end_index[2]+1] But they are parsed to the lib function. # Back calculate the R2eff values for each experiment type. self.r2eff_ns_mmq[ei](M0=self.M0, R20A=r20, R20B=r20, pA=pA, dw=aliased_dw, dwH=aliased_dwH, kex=kex, inv_tcpmg=self.inv_relax_times[ei], tcp=self.tau_cpmg[ei], back_calc=self.back_calc[ei], num_points=self.num_disp_points[ei], power=self.power[ei]) Is this by purpose?