Percentage difference between force F from GUBAS using second and fourth order potentials, relative to force from surface integration method. The xy-plane shows the surface of the primary viewed from the top, along the z-axis. Each point in the plane corresponds to a surface connection point on the primary, that is, a new position of the secondary. The top and bottom panels are for second and fourth order approximations, respectively, and the panels show results for three different models of the primary, see Table 1. The pair-wise comparisons between second and fourth order potentials share the same color scaling.