In statistical mechanics, the n -vector model or O(n) model is a simple system of interacting spins on a crystalline lattice. It was developed by H. Eugene Stanley as a generalization of the Ising model, XY model and Heisenberg model. In the n -vector model, n -component unit-length classical spins s i are placed on the vertices of a d -dimensional lattice. The Hamiltonian of the n -vector model is given by: