Dear Mohammad - I guess you are solving a complex 3D numerical problem - You can try simulating the same problem in 2D axisymmetric / 2D - May I know which type of problem you are solving?

Dear Hakeem,

Thank you for your interest. I am solving a 2D problem. I use an unstructured grid in which the matrix of coefficient of flow solver is a large matrix. I believe that the source of error is such a large matrix of coefficient (10000*10000 cells). In structured grid solvers the matrix is 5th diagonal and so the size of matrix is very small. In the case of unstructured grid this matrix is very large. I do not know how to deal with this problem.

I found that I should follow the siting/link/out and increase the memory page files of the Fortran in the compaq fortran version. In addition, if the size of cells is more than the required memory the spars system is a good option.