I really thank you for your reply and guidance. According to your comment, shear thinning is observed for most of Newtonian fluid in molecular simulation. Is can be related to the inability of the fluids to relax and adopt with shearing, at high shear rate? Generally what is its reason?
About the shearing time, I tried with 100, 200 and 500 ps separately and obtained the same results. Do you think that my results may have problem? Or were these devoted time enough?