HI Iiterber, I wonder if you are just using the basic preconditioner (Diagonal?). in 3D LowEnergyBlock is typically better or Block in 2D. However it is unusual that you are getting a nan in the error since that typically means something is not right about the mesh or configuration. So it is difficult to detect what the source of your issue could be. Cheers, Spencer. From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of İlteber Özdemir <rilteber@gmail.com> Date: Friday, 23 December 2022 at 14:31 To: nektar-users <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Exceeding Maximum Number of Iterations This email from rilteber@gmail.com originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list<https://spam.ic.ac.uk/SpamConsole/Senders.aspx> to disable email stamping for this address. Hello everyone, I get the following error in the first iteration (or sometimes after some iterations): CG iterations made = 5001 using tolerance of 1e-09 (error = -nan, rhs_mag = 13729.7) Fatal : Level 0 assertion violation Exceeded maximum number of iterations My question is a bit general: What are the possible causes of this error? What I understand is that the solvers are unable to converge to a solution (in finite iterations). The mesh is okay. I sometimes solve it by tweaking the initial conditions or increasing/decreasing the time step. But, this is a bit heuristic. As long-time users/developers, do you know other roots of this error? Does this type of error indicate to you a certain kind of problem? For example, if the solver diverges quickly or doesn't even complete one iteration I usually suspect the mesh (provided all the other schemes are set reasonably). However, I am able to get results with a very similar mesh and meshing technique, so I think it is okay in this case. Happy holidays, Best regards, Ilteber -- İlteber R. Özdemir