  We review the variational principle in the density matrix renormalization
group (DMRG) method, which maximizes an approximate partition function within a
restricted degrees of freedom; at zero temperature, DMRG mini- mizes the ground
state energy. The variational principle is applied to two-dimensional (2D)
classical lattice models, where the density matrix is expressed as a product of
corner transfer matrices. (CTMs) DMRG related fields and future directions of
DMRG are briefly discussed.
