x 0 e -t 2 dt is the error function and erfc(x) = 1-erf(x) is the complementary error function. As x increases, erfc(x) approaches 0 and erf(x) approaches to 1, thus they respectively take up the short-and long-range components of 1/r. α > 0 is the scale factor that determines the distance r c dividing the short-and long-range regions. For example, with a target accuracy ε, to use the long-range term for r ≥ r c , one requires erfc(αr c )