# MatMFFDComputeJacobian#

Tells the matrix-free Jacobian object the new location at which Jacobian matrix vector products will be computed at, i.e. J(x) * a. The x is obtained from the SNES object (using SNESGetSolution()).

## Synopsis#

#include "petscsnes.h"
#include "petscdm.h"
PetscErrorCode MatMFFDComputeJacobian(SNES snes, Vec x, Mat jac, Mat B, void *dummy)


Collective

## Input Parameters#

• snes - the nonlinear solver context

• x - the point at which the Jacobian vector products will be performed

• jac - the matrix-free Jacobian object of MatType MATMFFD, likely obtained with MatCreateSNESMF()

• B - either the same as jac or another matrix type (ignored)

• flag - not relevant for matrix-free form

• dummy - the user context (ignored)

## Option Database Key#

• -snes_mf - use the matrix created with MatSNESMFCreate() to setup the Jacobian for each new solution in the Newton process

## Notes#

If MatMFFDSetBase() is ever called on jac then this routine will NO longer get the x from the SNES object and MatMFFDSetBase() must from that point on be used to change the base vector x.

This can be passed into SNESSetJacobian() as the Jacobian evaluation function argument when using a completely matrix-free solver, that is the B matrix is also the same matrix operator. This is used when you select -snes_mf but rarely used directly by users. (All this routine does is call MatAssemblyBegin/End() on the Mat jac.)

MatMFFDGetH(), MatCreateSNESMF(), MatMFFDSetBase(), MatCreateMFFD(), MATMFFD, MatMFFDSetHHistory(), MatMFFDSetFunctionError(), MatCreateMFFD(), SNESSetJacobian()