Ground state of antiferromagnetic Heisenberg rings studied by the projector method

Abstract
A very powerful method for the numerical study of the ground state of quantum systems is proposed using the quantum transfer matrix as a projection operator onto the ground state. We calculate the ground-state correlation functions of antiferromagnetic Heisenberg rings of S=(1/2) and 1 with spin sizes up to 20 and 12, respectively. The size dependence of the correlations is reexamined. For S=1 the ground-state energy is found to show a quite different size dependence according to whether the ring has even or odd spins. The difference is interpreted by considering a dimer suggested by Hubbard.