Density matrix embedding theory of excited states for spin systems.

We use cluster density matrix embedding theory (CDMET) to calculate the excited states for quantum spin systems. The bath states are a set of block-product states and optimized by the variational method. By considering the symmetry in the form of penalty function, the degeneracy of excited eigenstates can be reduced. We prove the accuracy of our method by obtaining different excited states of square Heisenberg model. The ground state is nondegenerate state with gapless excitation in the ordered phase or degenerate state with an excitation energy gap in disordered phase. This is consistent with some previous results and supports the Lieb–Schultz–Mattis theorem. Moreover, we find some interesting behaviors of excited states for different coupling coefficient and different target energy under different symmetries. [ABSTRACT FROM AUTHOR]