  Traditional mean-field theory is a simple generic approach for understanding
various phases. But that approach only applies to symmetry breaking states with
short-range entanglement. In this paper, we describe a generic approach for
studying 2D quantum phases with long-range entanglement (such as topological
phases). Our approach is a variational method that uses tensor product states
(also known as projected entangled pair states) as trial wave functions. We use
a 2D real space RG algorithm to evaluate expectation values in these wave
functions. We demonstrate our algorithm by studying several simple 2D quantum
spin models.
