Abstract: It is considerable interest to notice that a stable structure SSOis in Cs symmetry, but its melastable structure SOSin C_2v symmetry. Using the contracted split-type Gaussian base sets with polarization functions, SOSwas discovered for the first time at HFSCFlevel. There have been no detailed studies, of the SSOsurface, although it was examined by a microwave spectrum as early as 1959. In this paper an analytical potential function based on the many-body expansion has been derived for both the SSOand the SOSsurfaces,which are adequate accuracy in the whole region for the dynamical research.