Hello Sam, Thanks for your compliment about Kwant. The conductance calculation in your script needs about 1.4 GiB of RAM on my machine (Debian GNU/Linux, amd64 architecture). So while 500x1000 is large system, it should run with 2 GiB of RAM. See Fig. 7 of the Kwant paper for memory usage of the MUMPS solver. Are you running the Kwant packages for Windows that were prepared by Christopher Gohlke? Unfortunately, none of us is an expert of the Windows system (or even uses it). The simplest stopgap solution that I can recommend you is to use some variant of Linux for your calculations. Independently of that, we are interested to see Kwant running well under Windows. Could you please try to debug the issue a bit more? I find it curious that the failure mode seems to depend on how much RAM the machine has while you do not run out of memory. “Exception code: 0xc00000fd” seems to mean stack overflow (not the website but a real one!). The default stack size in windows is 1024 kiB and as far as I can see it can be increased only upon compilation. While the default stack is 8192 kiB on Linux, I verified that your script runs even with a reduced stack of 256 kiB. So MUMPS does not seem to need a particularly large stack even for huge matrices. If you are using Christoph Gohlke’s packages, it would be great if you could report the problem directly to him (I’m interested to be put in CC). He might now how to fix it. Best Christoph