  We develop a theoretical approach to quantum transport through single
conjugated organic molecules that accurately accounts for all molecular excited
and ground states via a semi-empirical many-body molecular Hamiltonian. We then
calculate the linear and non-linear transport properties of a benzenedithiol
molecule covalently bonded to two gold electrodes at room temperature,
eliminating all free parameters by comparing the calculated thermopower and
linear-conductance with experiment. The resulting `molecular diamond' structure
exhibits experimentally observed many-body effects inaccessible to mean-field
approaches.
