Here 𝑟 ⃗ is the vector from each atom to the S-C bond mid-point, 𝑑 8 is a unit vector pointing along the direction of the S-C bond from the center of the bond, 𝑞 is the charge of the atom in internal Amber units and 𝑘 is a conversion constant with a value of 332.4 kcal Å/ mol e 2 . The resulting units of 𝐸𝐹 strength are MV/cm.