Dear PETSc team, I try to use: call MatMatMult(A,B,MAT_INITIAL_MATRIX,PETSC_DEFAULT_DOUBLE_PRECISION,C,ierr);CHKERRQ(ierr) Where both A and B are rectangular, but A is sparse and B is dense. Both are double complex and distributed. The product PETSc gives me contains some errors in some part of the matrix. I output A, B and C then computed product in matlab. Attached you see figure plotted as: imagesc(log10(abs(C-Cm))) Where Cm -- product computed in matlab. The pattern and amplitude vary depending on the number of cores I use. This picture is obtained for 48 cores (I've tried 12, 64 cores as well). Where should I look for possible explanation? -- Regards, Alexander