Since we omitted j 0 (2αrv) in the integral of Eq. 8, with ζ thus found, the residual in Eq. 7 will be less than ε. We evaluated α and ζ in the range of r C from 15 to 400 (Fig. ; the length unit can be considered as Ångströms). With smaller ε (higher accuracy), α and ζ become larger. However, even for ε = 10 -12 , ζ is less than 5, highlighting the practicality of the truncation. By using s ≡ v/ζ , Eq. 7 can be converted to erf(αr)