Relative errors between the gravitational potentials of a close binary computed by the original and modified convolution methods. The horizontal bar shows the errors in percent. Two mesh setups are used for this test problem: left column – two nested grids and right column – three nested grids. The grid interfaces are shown by the dashed lines for convenience. The binary position is schematically indicated by the red and yellow circles. The arrows show the gravitational field in the modified convolution method. Note that the field lines are continuous across the grid interfaces.