A study of a single acetylene adsorbed onto the Si(100)-(2x 1) surface is presented. A MCSCF+MRMP2 wave function is used to study this system, which is shown to be significantly multiconfigurational over large sections of the potential energy surface. The lowest energy isomer is shown to have the acetylene adsorbed directly above a single dimer, di-sigma, where the silicon- silicon dimer bond remains intact. These results are compared with those from a previous DFT study.