* With np>1, the multigrid algorithm hangs. Implementation decisions suggest that parallel was not intended: - Ghostswap is never called, and - stock Gauss-Seidel iterative scheme is used where red-black Gauss-Seidel is more appropriate. - The multigrid algorithm may not be parallelizable. * Serial code alternates between (-\infty, 0] and [0, +\infty) every timestep, with zeros along the border. This not the desired result.
