A new formalism for the calculation of the conductance of a molecule attached to two macroscopic metal contacts is proposed. Starting from an effective one-electron picture, the contacts are accounted for by certain potentials that are added to the molecular Hamiltonian. By choosing appropriate boundary conditions for the molecular eigenvalue problem, stationary states can be obtained describing the current transport through the molecule. The approach presented here is not restricted to the zero-voltage and zero-current limit. A simple and transparent formula for the conductance is derived that resembles Fermi's golden rule. The relation of the proposed method to nonequilibrium transport theory is discussed. (C) 2003 American Institute of Physics.