Hi, Mark. I tested my system today with NVE and time step =1fs and 0.5fs. The drift still exists. I didn't test the system by double precision GROMACS yet. Thank you. _____________________________________________ Xiaobin Cao Dept of Geology & Geophysics E235, Howe-Russell Geosciences Complex, Louisiana State University, Baton Rouge, LA 70803, U.S.A.