  We applied cluster density matrix embedding theory, with some modifications,
to a spin lattice system. The reduced density matrix of the impurity cluster is
embedded in the bath states, which are a set of block-product states. The
ground state of the impurity model is formulated using a variational wave
function. We tested this theory in a two-dimensional (2-D) spin-1/2 J1-J2 model
for a square lattice. The ground-state energy (GSE) and the location of the
phase boundaries agree well with the most accurate previous results obtained
using the quantum Monte Carlo and coupled cluster methods. Moreover, this
cluster density matrix embedding theory is cost-effective and convenient for
calculating the von Neumann entropy, which is related to the quantum phase
transition.
