The maximum of 𝑃𝑃𝑃𝑃 is reached when 𝑐𝑐 = 0.08 and 𝑐𝑐 = 0.92, corresponding to the electron and hole doping. This suggests that the optimal doping ratio in Holstein model when 𝜏𝜏 ≪ 𝜆𝜆 is that HOMOs are 8% filled by holes or LUMOs are 8% filled by electrons. Besides, 𝑃𝑃𝑃𝑃 increases with increasing 𝜏𝜏 monotonically. These all agree with Fig. (e).