# NEWTON_SOLVER FLOW¶

Specifies nonlinear solver parameters associated with the simulation. An example is as follows:

```
NEWTON_SOLVER FLOW
ATOL 1D-8
RTOL 1D-8
STOL 1D-30
END
```

An example showing all possible options is:

```
NEWTON_SOLVER FLOW
NO_INFINITY_NORM
NO_FORCE_ITERATION
NO_PRINT_CONVERGENCE
PRINT_DETAILED_CONVERGENCE
ATOL 1D-8
RTOL 1D-8
STOL 1D-30
ITOL 1D-6
ITOL_S 1D-6
ITOL_UPDATE 1D-6
MAXIT 10
MAXF 50
MATRIX_TYPE AIJ
PRECONDITIONER_MATRIX_TYPE AIJ
END
```

The variables in the keywords above have the following meanings:

## NEWTON_SOLVER Type¶

Can be FLOW or TRANSPORT. For TOIL_IMS, GAS_WATER, BLACK_OIL, TODD_LONGSTAFF and SOLVENT_TL, this must be FLOW.

## NO_INFINITY_NORM¶

If included, toggles off calculation of infinity norm on residual and update vectors. The default is to calculate infinity norm.

## NO_FORCE_ITERATION¶

If included toggles off the forcing of at least 1 linear iteration. The default is to force at least 1 linear iteration.

## NO_PRINT_CONVERGENCE¶

If included, prints convergence information.

## PRINT_DETAILED_CONVERGENCE¶

If included, printing of detailed convergence information.

## ATOL¶

Absolute tolerance. Absolute size of 2-norm of residual.

## RTOL¶

Relative tolerance. Relative decrease in size of 2-norm of residual.

## STOL¶

Relative update tolerance. Relative decrease in size of 2-norm of solution

Note: STOL needs to be treated with great caution, as it simply accepts the step when then solution updates fall below the specified criteria, and this may allow badly unconverged steps to be accepted. It is recommended that STOL be set very low (e.g. 1.0D-30, as in the examples above) so that this cannot happen

## ITOL¶

Infinity tolerance. Size of infinity norm of residual.

## ITOL_S¶

Infinity tolerance. Size of infinity norm of scaled residual.

## ITOL_UPDATE¶

Infinity tolerance. Size of infinity norm of update

## MAXIT¶

Maximum number of Newton iterations before reporting failed convergence.

## MAXF¶

Maximum number of function evaluations before reporting failed convergence.

## MATRIX_TYPE¶

Format of main solver matrix. PETSc Mat (i.e. AIJ, BAIJ, or HYPRESTRUCT).

## PRECONDITIONER_MATRIX_TYPE¶

Format of preconditioning matrix. Default is same as solver.