Dear Khaled, The problem is that first you are suppose to use cmath.exp and not exp cmath.sqrt and not sqrt in most of the case this should go trough since those will be handle by fortran anyway but not in this context. However, you have a four fermion interaction with a fermion flow violation (with identical particle) which is one case that we can not handle. In order to generate events for your theory you need to introduce fake scalar to split the 4 point interaction in a succession of three point interaction. Cheers, Olivier On 24 Jun 2015, at 17:11, Khaled Teilab