[lammps-users] epsilon units and high pressures for liquid nitrogen

Hi all, if anyone has any ideas about what I am doing wrong then I would be most grateful for advice. Apologies in advance if any of this is stupidly trivial.
I am trying to simulate liquid nitrogen, using fix shake to constrain bond lengths for this diatomic LJ molecule. I am not confident about my use of epsilon however as I am having problems with my results. Taking epsilon = 5.037E-22 J (from a good source) I have been converting this into 'real' units to obtain 0.0725kcal/mol. I assume that I am supposed to be multiplying the original value by Avogadro's constant to obtain the energy/mol. When I run a NVE simulation for the liquid however, I get very high pressures (180atm when I am expecting 1atm) - which seems to suggest that my epsilon value is too low. Have I made a mistake here or is the problem more involved?

I know little about liquid nitrogen specifically, but my sense is that these results are pretty reasonable. The pressure of an incompressible liquid in a NVE simulation tends to fluctuate a lot, so seeing +- a few hundred atm is not so unusual. I would run it longer to make sure your pressure is converged, if that doesn't work a small decrease in density would likely get you closer to 1 atm, or you could use a barostat to enforce 1 atm pressure and see what the density turns out to be... likely not very different from what you start it out at.

Hope this helps,

Dear Henry,

I would first check the density with NPT simulations using these parameters and see if that makes sense. If you are just running NVE from the start, you can expect any pressure depending how many atoms ā€œNā€ are there in the system at constant ā€œVā€.