The ab initio theoretical approach to calculations of molecular electron transfer rate and molecular junction conductance is presented. The implemented approach is founded on Landauer formalism coupled to electron propagator theory (Green-Keldysh functions formalism) of solving the Dyson equation for molecular junction. The electron transfer rate calculation algorithm implementing quantum chemistry computational software output is proposed. The conductance through a single molecular orbital is found to be equal to a quantum of conductance, 2e(2)/h. The existence of the maximal value of electron transfer rate through a single molecular orbital is proposed, (pi(2)/4h)Gamma(L)Gamma F-R, whose order-of-magnitude estimate is 10(11)-10(13) s(-1). (C) 2014 Elsevier B.V. All rights reserved.