Thanks again for your replies.<br><br>I just modified AHFinderDirect to work with our Cactus thorns, and I'm getting the dreaded "one or more points on the trial horizon surface point is/are outside the grid (or too close to the grid boundary)" error message. Our Cartoon-based code requires a grid with the following properties:<br><br>key: 0) xyz(arrayindex_x,arrayindex_y,arrayindex_z) = coordinate_value<br>1) x(0,0,0) = -dX/2; x(1,0,0) = +dX/2; x(2,0,0) = +3 dX/2; etc.<br>2) y(0,0,0) = -dY; y(0,1,0) = 0; and y(0,2,0) = +dY<br>3) z(0,0,0) = -dZ/2; z(0,0,1) = +dZ/2; z(0,0,2) = +3 dZ/2; etc.<br><br>I.e., we need 1 symmetry ghostzone in the x, y, and z directions. <br><br>AHFinder (the AH finder in the CactusEinstein thorn) appears to work with this grid without any interpolation errors, but only (a) after I fixed a bug in that code having to do with the case where bitant+cartoon is assumed, and (b) when the interpolation order in LocalInterp is set to 1 ("uniform
cartesian" interpolation). [When the interpolation order is set to 2, LocalInterp has issues with a point where theta = pi/2 (i.e., z=0).]<br><br>To get AHFinderDirect to work with our grid, it appears as though I'll need to write my own Hermite+Lagrange interpolation routines or somehow modify the chosen stencil inside AEILocalInterp so that AEILocalInterp does not attempt to go off our grid. Aside from reworking all our Cactus thorns to accept arbitrary values for ghost_size_x, ghost_size_y, and ghost_size_z, does anyone have a better idea?<br><br>-Zach<br><p> 
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