Dear PETSc team, I would like to create two matrices. The first matrix is based on a compact stencil (COMPACT), and the second one has an extended stencil (WIDE) So the non-zero elements of the COMPACT matrix is contained in the WIDE matrix as well. But the WIDE matrix shall contain some additional elements. I try to create the COMPACT matrix first, then copy it to the WIDE matrix and then add some more terms to the WIDE matrix. We want to create a matrix that is based on a lower order approximation of our scheme which serves as basis matrix for preconditioning, where we want to have a more accurate jacobian which has a larger stencil, that is based on the lower approximation plus some additional non-zero elements. We want to avoid that we have to call the same routines twice to setup the more accurate matrix, but copy the elements from the lower order (compact) matrix. I try to use MatCopy to copy the COMPACT matrix into the WIDE matrix. Somehow I cannot figure out to do this, since there is always a problem wirh non conforming sizes [0]PETSC ERROR: Nonconforming object sizes [0]PETSC ERROR: Mat A,Mat B: global dim (4,8) (4,8) Here is a basic example that I try. Adding additional elements to the WIDE matrix is disabled right now, since MatCopy already gives a problem. I thought that in MatCopy(COMPACT, WIDE, DIFFERENT_NONZERO_PATTERN, ierr) the option DIFFERENT_NONZERO_PATTERN should enable to copy different matrix sizes into each other. Or is that not possible to do ?! Here the COMPACT matrix is a 4x4 matrix and the WIDE matrix 8x8, for a basic test. Thanks, Frank Bramkamp program petsc_matrix_example #include <petsc/finclude/petsc.h> use petsc implicit none PetscErrorCode :: ierr Mat :: COMPACT, WIDE PetscInt :: m, n, MM, NN, d_nz PetscInt :: i, j PetscScalar :: additional_value ! Initialize PETSc call PetscInitialize(PETSC_NULL_CHARACTER, ierr) ! Set dimensions for COMPACT matrix m = 4 ! Local rows n = 4 ! Local columns MM = 4 ! Global rows NN = 4 ! Global columns d_nz = 4 ! Estimated non-zeros per row (diagonal) o_nz = 0 ! Estimated non-zeros per row (off-diagonal) ! Create COMPACT matrix call MatCreateSeqAIJ(PETSC_COMM_SELF, MM, NN, PETSC_DECIDE, PETSC_NULL_INTEGER, COMPACT, ierr) ! Set some values in COMPACT matrix (example) do i = 0, M-1 do i = 0, NN-1 !do j = max(0, i-1), min(NN-1, i+1) do j = 0,NN-1 call MatSetValue(COMPACT, i, j, 1.0_PETSC_REAL_KIND, INSERT_VALUES, ierr) end do end do ! Assemble COMPACT matrix call MatAssemblyBegin(COMPACT, MAT_FINAL_ASSEMBLY, ierr) call MatAssemblyEnd(COMPACT, MAT_FINAL_ASSEMBLY, ierr) ! Set dimensions for WIDE matrix n = 8 ! Increase local columns NN = 8 ! Increase global columns m = 8 MM = 8 d_nz = 8 ! Create WIDE matrix call MatCreateSeqAIJ(PETSC_COMM_SELF, MM, NN, PETSC_DECIDE, PETSC_NULL_INTEGER, WIDE, ierr) ! Assemble WIDE matrix: MUST WE ASSMBLE THE MATRIX BEFORE MatCOPY ?! call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr) call MatAssemblyEnd(WIDE, MAT_FINAL_ASSEMBLY, ierr) ! Copy elements from COMPACT to WIDE call MatCopy(COMPACT, WIDE, DIFFERENT_NONZERO_PATTERN, ierr) !call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr) ! Add additional elements to WIDE matrix !!$ additional_value = 10.0 !!$ !!$ ! original !!$ !do i = 0, MM-1 !!$ ! do j = NN/2, NN-1 !!$ ! call MatSetValue(WIDE, i, j, additional_value, INSERT_VALUES, ierr) !!$ ! end do !!$ !end do !!$ !!$ do i = 0, NN-1 !!$ do j = max(0, i-1), min(NN-1, i+1) !!$ call MatSetValue(WIDE, i, j, 10.0_PETSC_REAL_KIND, ADD_VALUES, ierr) !!$ end do !!$ end do ! Assemble WIDE matrix again call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr) call MatAssemblyEnd(WIDE, MAT_FINAL_ASSEMBLY, ierr) ! View matrices (optional) call PetscPrintf(PETSC_COMM_WORLD, "COMPACT matrix:\n", ierr) call MatView(COMPACT, PETSC_VIEWER_STDOUT_WORLD, ierr) call PetscPrintf(PETSC_COMM_WORLD, "WIDE matrix:\n", ierr) call MatView(WIDE, PETSC_VIEWER_STDOUT_WORLD, ierr) ! Clean up call MatDestroy(COMPACT, ierr) call MatDestroy(WIDE, ierr) ! Finalize PETSc call PetscFinalize(ierr) end program petsc_matrix_example