What is the state of art technique in solving exact ground state of Heisenberg model, meaning minimization of the H terms (hamiltonian) provided infinite spin space?

http://en.wikipedia.org/wiki/Classical_Heisenberg_model

I read that there is a method called Bethe Ansatz. But as Ansatz suggests, is it a mathematical method or just some heuristics? Thank you. Are there some modern exposition of Bethe Ansatz or other methods to solve the exact ground state problem? Thank you.