To whom it may concern, I am writing to you to ask some technical problems that I am dealing with the use of PETSc.
The problem that I need to solve is a system of linear equations (Ax=b). The matrix A is a banded matrix (five-point matrix) resulting from the discretization of a second derivative in a 2D space. In other words, it is a pentadiagonal matrix, but the two outer bands are separated from the three central bands. This matrix is complex and is not hermitian (its actual shape is A= H - E - i*delta, where H is a hermitian five-point matrix and E and delta a real scalar). Its size is 1.8e7 x 1.8e7, thus the problem cannot be solved with direct methods but with iterative methods. For negative values of E, I have been able to solve the system using PETSc with a Krylov subspace method, with no problems. But for positive values, where the spectrum is quasi-degenerate, I cannot solve it. I have tried the following iterative methods: --> GMRES with the ILU preconditioner --> BICG --> BCGS and convergence was not reached in any of the cases. I have run out of ideas, so my question is: is it possible that you suggest me any method which I could use to deal with such a problem? Please forgive the intrussion if this question is not adequate in this email list. Thank you very much in advance, Rui Silva. ------------------- Rui Silva EMTCCM (European Master in Theoretical Chemistry and Computational Modelling) UAM, Departamento de Qu?mica, M?dulo 13 CAMPUS http://www.uam.es/departamentos/ciencias/quimica/spline/index.html -------------------
