be able to reduce this large number and use as few parameters as possible. However, it shall be noted that, among these parameters, many are only material dependent (i.e., they do not depend on frequency or temperature). For instance, for lead-free relaxor, $E_{b}$ is a constant; for lead-based relaxor, $E_{b}$, $\chi_{1}$, $\chi_{2}$, $\theta$ are close to constants (see Tab. \[tab:Fitting-parameters-of-PZN\]). For a given sample, these parameters may only need to be calibrated once. In this way, the number of parameters will be significantly reduced.

Second, in this work we have focused on the temperature dependence of susceptibility. The dependence on frequency needs further investigation. For instance, $T_{C}\left(\nu\right)$ and $\gamma\left(\nu\right)$ in Eq. (\[eq:PZN-1\]) need to be specified explicitly to address this issue. We note that results shown in Tab. \[tab:Fitting-parameters-of-PZN\] will provide clues to $T_{C}$ and $\gamma$’s dependence on $\nu$, and eventually help finding analytical expressions for $\chi\left(T,\nu\right)$. In addition, we generally ignored the long-range correlation of dipoles, which is another limitation to this approach. While such long-range correlation makes relaxor physics so rich, it will bring back Bose-Einstein statistics and make the current formulation more complicated. To what extent the Bose-Einstein and the Maxwell-Boltzmann distribution shall be used for ferroelectric 