Exact Results for a Spin-${\bf 1}$ lattice

We consider a lattice of spin-1 particles with a general pairwise interaction $[ {\rm cos} \gamma ({\bf S}_{l} \cdot {\bf S}_{l+1}) \ + {\rm sin} \gamma ({\bf S}_{l} \cdot {\bf S}_{l+1})^2 \ ]$. We show that, for a large class of lattices with even number of sites, the ground state for the region $- {3 \pi \over 4} < \gamma < - {\pi \over 2}$ belongs to total spin $S_{\rm tot} = 0$, whereas the state of minimum excited energy but with finite $S_{\rm tot}$ belongs to $S_{\rm tot} = 2$. These results are constrasted with the generalized Marshall theorems, applicable to a bipartite lattice and $- {\pi \over 2} < \gamma \le 0$.