For a given Hamiltonian the exact time evolution of electron-transfer in a donor-bridge-acceptor system, DBA, can be calculated from its eigenvalues and eigenvectors for any choice of the initial and the final states. For complex systems the eigenvectors and eigenvalues are not calculable and approximation methods must used, e.g., perturbation theories often employing the Green’s function, The use of these approximations places rather stringent restrictions on the Hamiltonian parameters. We demonstrate this by computing the time-dependence of the probability of survival and electron transfer in the system D-a-b-A (the two-site Huckel bridge) for which analytical solutions exist and comparing them with those computed by approximation methods. Larger Huckel systems are also treated. The exact orbitals are related to the electron configurations which are often used to describe the electron transfer. We assume the existence of prepared states corresponding to electrons on the donor or holes on tile acceptor without stating how such states are prepared (i.e., optical pumping, thermal excitation etc.).