We study the vicinal surface with adsorption below the roughening temperature, using a solid-on-solid model coupled with the Ising model. We calculate the step tension gamma and the step-interaction coefficient B using the density matrix algorithm. We find a temperature T-s where B vanishes and that the surface free energy has the form f(p) - f(0) = gammap + (const) p(5) +..., where p is the surface gradient.