Nature of the SDW state in FeAs-based Compounds

We show that the significant underestimation (about 10\%) of Fe-As bond length in FeAs-based compounds by LDA is due to the strong correlation effect. By properly taking into acount the on-site correlation, we are able to reproduce experimental values (to about 1\%) using self-consistent LDA+Gutzwiller method. Also we will show that the strong on-site orbital fluctuation will dramatically reduce the anti-ferromagnetic long range order in the parent compound. All these results are in good agreement with experiments.