Exact calculation of the current in the tunnel Hamiltonian method

A method is proposed for going over from the nonequilibrium formulation to the thermal Green's function technique in the tunnel model. The method is used to obtain an exact solution for the current and the radius of convergence of the series of perturbation theory with respect to the transparency. The divergence of the series is found to be related to the rearrangement of the spectrum in sufficiently transparent tunnel junctions.