Stabilization at wrong pressure


I’m simulating a box composed by 20 molecules of CO2. I first equilibrate it with NVE+temp/rescale for 1million time step (1fs) and then unfix the temp/rescale for the production steps. Temperature is 300 K as fixed. The averaged value of pressure is 1.85 atm but I calculated the number of molecules and the box dimensions in order to have 1 atmosphere, according to NIST database. I also tried to increase the number of molecules and the volume proportionally, but the average pressure is still 1.85. Do you think that this is an acceptable error or maybe something is wrong? Which parameters would you look at, if you would have a similar problem?

Kind regards,


I think 20 molecules is far too small to compute a pressure and volume (density) to match
with expt. Also what units and force field are you using?