  Ab initio multi-reference configuration interaction calculations are
performed for the Si(100) surface using a cluster approach. The convergence
with respect to the cluster size is checked and the final results are taken
from a $Si_{32} H_{28}$ cluster which models two dimers and six bulk layers. We
find for the ideal as well as for the p($1\times 2$) reconstruction a singlet
ground state consisting of several configurations. The energy gain due to
forming the symmetric dimer in the p($1\times 2$) structure is 1.75 eV, the
bond length of the dimer is 2.35 \AA which is very close to the bulk value. In
contradiction to the LDA results and in agreement with previous correlation
calculations we do not find an asymmetric p($1\times 2$) structure.
