# Preconditioners for MLFMM

A few preconditioners are available for the multilevel fast multipole method (MLFMM).

Note:

- It is recommended to use the default settings. Changing the preconditioner is recommended only for advanced users.
- The number in brackets corresponds to the value of the I
_{3}field at the CG card in the model .pre file.

On the Solve/Run tab, in the
Solution Settings group, click the Solver Settings icon. On the Solver Settings dialog, click the
Advanced tab.

- Multilevel FEM-MLFMM LU/diagonal decomposition (2010)
- Preconditioner for a hybrid MLFMM / FEM solution that uses a multilevel sparse LU
decomposition of the combined and partitioned system.
When using the parallel Solver, the factorisation type, which slightly impacts runtime and memory, can be specified.

- Sparse approximate inverse (SPAI) (8192)
- Preconditioner for an MLFMM solution. This preconditioner uses less memory than the default in most cases, but runtime could be longer.
- Sparse LU (8193)
- Preconditioner for an MLFMM solution that uses a
sparse LU decomposition of the MLFMM near field
matrix.
When using the parallel Solver, the factorisation type, which slightly impacts runtime and memory, can be specified.

Note: The SPAI (8192) and Sparse LU (8193) are the recommended preconditioners for MLFMM without FEM.