  We propose a new approach to implement the density matrix renormalization
group (DMRG) in two dimensions. With this approach the initial blocks of a L by
L lattice are built up directly from the matrix elements of a (L-1) by L-1)
lattice and the topological characteristics of two dimensional lattices is
preserved in the iteration of DMRG. By applying it to the spin-1/2 Heisenberg
model on both square and triangle lattices, we find that this approach is
significantly more efficient and accurate than other two-dimensional DMRG
methods currently in use.
