Dear friends

Hello
I am studying aggregation of SiO2 nanoparticles using discover module of MS. In the simulation boxes there are Na2SO4 and H2O molecules too. I am following a paper which they studied aggregation of NZVI(nanoscale zero valen iron). After counstructing simulation boxes (amorphous cell/counstruction), geometry optimization (forcite module) and MD simulation steps (Discover minimizer/Discover dynamics), the SiO2 nanoparticles were not closer (aggregation process) while for NZVI they presented a figure which 2NZVI nanoparticles connected to each other. 
Since I am exactely following their inputs and procedures, Do you think what is the problem? why SiO2 nanoparticles do not move? after geometry optimization their distance was 1nm while after MD simulation it was around 0.9nm and they did not contact each other. please help me, Please consider the attached figures for more information

Best wishes
Reza Gholizadeh