Gravitational potential and acceleration on nested grids in the OiCG method. Left pair of columns: relative error in the calculation of the gravitational potential (first column) and gravitational acceleration (second column) for an oblate ellipsoid with the semi-major axis ratio 0.5:1.0:1.0. The horizontal bars present the relative errors in percent (log units). The top row illustrates the errors obtained with no mesh refinement (m = 1). The middle and bottom rows show the errors for the case of four nested grids (m = 4). In particular, the middle row corresponds to a linear interpolation of the boundary potentials, whereas the bottom row presents the errors when a simple extrapolation is used. Right pair of columns: relative error in the gravitational potential (first column) and gravitational acceleration (second column) for a binary star, the spatial location of which is outlined by two circles. The left component is twice as massive as the right one.