Electrical transport across the styrene-silicon junction exhibits a negative differential resistance (NDR) at a bias position of -2.5 V. However, the Non-equilibrium Green's Function (NEGF) transport model using LDA orbital energies fails to predict the NDR position correctly and has stimulated contrasting views on the mechanism of NDR. In this presentation, we demonstrate that NEGF based on accurate HOMO and LUMO energies from DFT predicts the styrene-silicon NDR bias position correctly (-2.4V). These results support the model of the NDR mechanism based on biased induced alignment of the molecular energy levels with the semiconductor band edge. Consequently, NEGF correctly describes electronic conduction across molecule-silicon junctions if both the HOMO energy and HOMO-LUMO gap are accurate. We find that the electrostatic field is responsible for the experimentally observed desorption effect and the polarity dependence of the threshold desorption field. We also predict NDR bias positions for 1,3-butadiene on Si(100)-2x1 for both the [2+2] and [4+2] adsorbed surface states.