Dear Abbout, I apologize for not being clear in my question. My system is infinite (that is there are leads attached to the system) and I mounted virtual leads at points where I wanted kwant to evaluate green's function. Sorry about that.
The formula that I am using is from the supplementary material of the following paper: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.047702 It is possible that I am misinterpreting the formula as given in the paper as am very new to the subject. I want to evaluate critical current in the Josephson Junction by maximizing current-phase dependency, for that I need total current in the system as a function of phase difference phi, I am not sure how to do that with local current operator in kwant. Thank you for clarifying the output of green's function I am now sure that I was reading the output wrong which must have been the primary reason for getting zero. I again apologize if I am not clear about what I am saying.
