  The lattice gas model for adsorption of alkaline elements on W(112) surface
is studied by Monte Carlo simulations. The model includes dipole--dipole
interaction as well as long-range indirect interaction. The numerical results
show that truncation of the indirect interaction even at 200$\AA$ may change a
phase diagram, i.e., new phases containing domain walls might occur. It is
demonstrated that a defected phase can exist at high temperatures even if it is
not stable at T=0. The phase diagram for Li/W(112) is constructed and long
periodic chain structures (9*1), (6*1), (4*1), (3*1), and (2*1) are found to be
stable at low temperatures. Role of thermal fluctuation is discussed by
comparison of Monte Carlo results with mean field approximation results.
