in which 𝑭 ! (𝒓) is the local electric field vector, ε0 is the permittivity of the vacuum, 𝑞 ! is the atomic charge of atom 𝑖, 𝒓 -𝒓 ! is the distance between the point charge and the point of evaluation and 𝑹 / ! is the unit vector pointing from 𝒓 ! to 𝒓.