[en] By employing the mean field and variational Monte Carlo methods, we investigate the iron-based superconductors (FeSCs) based on a realistic five-orbital Hubbard model in which an off-site Coulomb interaction is explicitly included. Our results demonstrate that plays an important role in stabilizing the nematic state in both Fe-pnictides and FeSe. Below a critical , the model is shown to lie in the striped antiferromagnetic ground state, and an increasing of leads to an energy degeneracy between different magnetic configurations. This finding provides a natural explanation for the magnetism in Fe-pnictides with relatively small , and more importantly, unveils the microscopic mechanism behind the absence of magnetic order in FeSe with larger . Simultaneously, the common anisotropy of and orbital occupations in different magnetic configurations accounts for the similar orbital ordering observed in Fe-pnictides and FeSe. In addition, the unusual smallness of the Fermi surfaces in FeSe can be obtained when the renormalization effect of on the band structure is taken into account. (paper)