  We consider a variational problem for the two-dimensional (2D) Heisenberg and
XY models, using a trial state which is constructed as a 2D product of local
weights. Variational energy is calculated by use of the the corner transfer
matrix renormalization group (CTMRG) method, and its upper bound is surveyed.
The variational approach is a way of applying the density matrix
renormalization group method (DMRG) to infinite size 2D quantum systems.
