Click the Parameters button on the Structure Analysis tab of the Job Preferences dialog to view the Iterative Solver (Sparse M) Parameters dialog.
Two reordering methods are available: nested dissection method (NDM) and minimal degrees algorithm (MDA). Application of both methods consists in reduction of the non-zero fill-ins in the equation matrix during factorization. Such reordering methods give rise to the so-called sparse matrices which do not have band structure. Factorization methods for such matrices are called sparse methods.
Two types of sparse direct solvers are provided (see also Solvers available in the Robot program). Sparse solvers always apply NDM. SparseM solvers may use both NDM and MDA. It is impossible to predict a-priori which reordering method is better. At present, the application of SparseM solver with MDA is recommended for the majority of building structures. For solids, the NDM method will usually be more appropriate. Nonetheless, these are the recommendations of the approximate character.