After the vector is assigned, the optimal position along the vector and the charge of the off-center point is determined. This is carried out using a grid search of parameters to find the values which best recreate the QM ESP. Assigning a symmetry-derived search direction reduces the number of variables that need to be optimized from four (the x, y, z coordinates and the charge) to two (the distance along the vector and the charge). This simplification is particularly important when multiple off-center point charges are added, as described in the following section. The atom-centered point charge is assigned a value such that the net charge of the atom is unchanged. The method is summarized with a flowchart in Figure .