2. An adaptive expansion technique
As we all know, the computational cost mainly depends on the M2L translation in the FMM. Thus one efficient way to improve the efficiency is reducing the operations of the M2L translations. In this section, we will reduce the operations of the M2L translations through decreasing the value of most expanded term in the M2L translations.
In this paper, we use adaptive values of for the series in all M2L translations. The value is determined by the distance between each pair of interacting cells and their sizes. The error estimation of the truncated series has been estimated in the original FMM paper [10]. According to Eq. (11) of Ref. [10], we can write the following equivalent equation for the case of the conventional boundary face method with FMM (CFMM)