Dear Liucius and Dominik, thank you very much for your answers. Both of them were really useful. Sorry I am replying this late, but I wanted to do some tests before bringing my news. As Dominik said, my surface was wrong. So I corrected it, and tried again for graphene. However, and probably because of what Liucius said, the z2 was 0 again. In the convergence report, there was one check that didn't pass (MoveCheck). Anyway, I moved to other system: PbCF2 (Scientific Reports, http://www.nature.com/articles/srep26123). The surface I calculated was the same as for graphene, and I could get z2=1. So, thank you! I would ask you, if you don't mind, one more question. Is there a way to obtain the data in a text form to plot them myself? I am working on a cluster, and I can not plot through matplotlib. I added in the running file the flags print(result.wcc) and print(result.t), but the format of the output isn't very friendly. Thank you again for your help, Jimena