Figure 5. Accuracy of the 2-charge practical point charge approximation (PPCA).

For a sample set of charge distributions relevant to biomolecular modeling. Accuracy of the point dipole and point quadrupole approximations are shown for comparison. Accuracy is calculated as the RMS error relative to the exact computation. (a) Error calculated at a distance of 10 Å where is the maximum extent of the charge distribution from the center of geometry. (b) Error calculated at a distance of 15 Å from the center of geometry. Error bars show the maximum and minimum absolute error. The upper values for the error bars that are cut off at the top are 0.14 and 0.006 in the left and right panels, respectively.