One of the first approximations we perform in this study concerns the evaluation of the MM fields. Fast Multipole Methods (FMM) can reduce the scaling of a PE calculation from quadratic to linear with respect to the number of environment sites. The accuracy of FMM is controlled by two parameters: the multipole acceptance criteria θ, and the multipole expansion order (more details can be found in Ref. ). The effect of these parameters on the calculations will be shown in the results section.