 Classical O(n) lattice model

The classical O(n) lattice model assigns a unit vector ${\sigma }_x\in S^{n-1}$ to each lattice site and gives ferromagnetic nearest-neighbour configurations weight proportional to $\exp (\beta {\sum }_{xy}{\sigma }_x\cdot {\sigma }_y+h{\sum }_x{\sigma }_x\cdot e)$.