Thank you ,professor,
Here is the log script
At 308.15K and 197.38atm,the density obtained from NIST is 867.52 kg/m3,and the results from this work is around 370kg/m3. log script has been attached in attachment.
Another strange things is when the angle coeff is reduced ten times,as a result of 14.7 energy/radian^2，all outcome are in good agreement with NIST. but as for this reducement, I don’t have any theoretical basis but it’s just an attempt
Actually the right parameter is 147.7(I have checked it many times compared with the original paper).
"Please also keep in mind, that an MD simulation like LAMMPS is performing is usually employing settings and parameters for condensed systems. " This excellent reply gave me some inspiration,because the density is quite similar at supercritical conditions(high temperature and pressure)
thanks for more advice.
i have no time to review or correct your simulation.
At 308.15K and 197.38atm,the density obtained from NIST is 867.52 kg/m3,
expecting simulations to match data this way is not understanding what force field parameters are determined and how they can be applied.
what you need to compare to is the data from the publication where you are taking the parameters from, and the requirement there is that those parameters have to be validated to reproduce the properties in question for the desired conditions. classical force fields require making compromises and simplifications and thus the parameter sets will only work as expected for a limited range of conditions. as parameter set generated for a condensed phase will not like to be accurate for gas phase simulations.
as mentioned before, for most gas phase simulations i would not expect that those have been parameterized to reproduce densities for a given external pressure, but are tested to produce a reasonable pressure for a given volume and temperature. and even then there may be significant errors on that properties depending on what the parameters are optimized for and what is the reference state (or states) that were used.
so if you want to validate your parameters you have to reproduce what was done in the paper that you are taking the parameters from. if that is not possible, you have to figure out why. for everything else you are on your own.
Thank you for your very useful advice.it works well when i set the volume/density to the desired value( in other words, equilibirm in the NVT and get the samples in the NVE ensemble).But I still have a question, when i set the volume and temperature i desired, i got the right self diffusion coefficient(300atm,308K,for example) which agree well with the experiment(the same pressure 300atm). however,the pressure simulated in box fluctuated in the range of 300 atm to 500 atm and the desired values which i used to calculated was 300 atm.What’s wrong with this? i look forward to your insights.thank you, professor.
i already gave my opinion on what to expect classical force fields in my previous responses. i have nothing to add to that.
pressure fluctuations have been discussed many, many times on this forum, and i have nothing to add to that, too.
ultimately, you are now asking me to do your thinking for you, and that is your job, not mine.