  Relationship among Wigner crystal, charge order and Mott insulator is studied
by the path-integral renormalization group method for two-dimensional lattices
with long-range Coulomb interaction. In contrast to Hartree-Fock results, the
solid stability drastically increases with lattice commensurability. The
transition to liquid occurs at the electron gas parameter $r_s \sim 2$ for the
filling $n=1/2$ showing large reduction from $r_s \sim 35$ in the continuum
limit. Correct account of quantum fluctuations are crucial to understand
charge-order stability generally observed only at simple fractional fillings
and nature of quantum liquids away from them.
