A theoretical study of the Cox-Symons model for normal muonium in Si is presented. The calculations are performed using polarized basis set ab-initio Hartree Fock calculations followed by corrections for electron correlation. It is shown that, if lattice relaxations are included, the antibonding site becomes a minimum of the potential energy surface (PES) for neutral interstitial hydrogen. The energy at this minimum is lower than that at the undistorted tetrahedral interstitial site. The spin density changes from being almost entirely on the muon (for Mu at the T site) to being almost entirely on a three-fold coordinated Si atom (for Mu in the AB configuration). The mechanism required to explain the isotropy and magnitude of the observed hyperfine tensor of Mu in c-Si is complicated. Large displacements of some host atoms are needed, and the system must be dynamic. However, this model is the first able to produce a minimum of the PES together with an isotropic hyperfine interaction and a delocalized spin density.