hello everyone, I am trying to determine the self-diffusion coefficient of water but end up with values far above the experimental value of D = (2.42 ± 0.08) × 10^−9 m^2 /s. Your advice is highly appreciated. I have followed the following steps;

1. Used amorphous cell module to construct a water box with 1200 molecules.

2. geometry optimized using forcite, COMPASS III forcefield, smart algorithm (also tried steepest descent), 2000 iterations, summation method for non-bond interaction; VDW-atom-based, electrostatic-Ewalds.

3. Relaxed the cell; annealing-5cycles (also tried 10 cycles), NVE. Followed by dynamics run under NVT ensemble, time step-1fs, steps-50000 (50ps). Production run with NVE, for 50ps.

4. forcite>analysis>mean square displacement.

am not sure if periodic boundary conditions are automatically applied but the molecules ended up out of the box during the dynamics. The diffusion coefficient was ranging from 4.366x10^-9m^2/s and above in all the trials I attempted. 

I then decided to constrain the oxygen atoms of the water molecules at the walls of the box and repeated the runs. Diffusion coefficient dropped to 1.055x10^-9 m^2/s.

Ps; I am a beginner with materials studio. Thank you, in advance for your help.