where q i is the partial charge at atom i, r ic is the distance between atom i and the center of cube c, and N is the number of atoms in the molecule excluding a) the atoms at a distance < 1 Å from the center of the cube (the contribution is equal to its partial charge) and b) the atoms further than the user-defined cutoff distance. The coordinates of the center of the grid, the size of each side (in Angstrom) and the size of the cube are also defined by the user.