关于考虑周期性镜像的静电势能计算公式问题

GauCPGro

在计算静电势能时，在比较多地方看到的都是如图这个公式。但我有一点疑惑的地方是，比如假设盒子有3个粒子，并考虑一个周期镜像与盒子的相互作用，求和号中n=0时有6对相互作用（排除i=j的情况），n=1时有9对相互作用，加起来15对，除以2后是7.5对？感觉是不是n=0时才需要除以2，n≠0时不需要除以2？

-HMDS-

Sorry, my work computer can not type Chinese.

The answer should be no. Because you take one box from the periodic system to calculate the system potential energy, not the box potential energy. If the equation is as you saying, it double considers the energy contribution between two different boxes. For example, you take box 1 and consider the contribution from box 2, the total potential is U, this U is shared by box 1 and 2, or you will have U on box 1 and U on box 2, the total energy of this two boxes, if you take this two together as one box, the energy is 2U, not U.