I am Liang Huang from University of California, Berkeley.
Our group is trying to validate the density of flexible CO2 model in bulk phase at high pressure with the hybrid GCMC +NVT method. The problem is that with flexile CO2 model at 300 atm and 338 K, the density we got from GCMC-NVT (~0.33 g/cm3) is far smaller than the NIST data (~0.81 g/cm3). The obtained temperature is around 338 K, but the averaged pressure is around 434 atm, much larger than 300 atm.
We have tested the flexible CO2 model with NPT MD simulation, which can give us consistent density with the NIST data. So, we believe the problem is in the GCMC part.
We are performing MPI parallel run with Kspace and tail yes (full energy option on). The LAMMPS version is 23 Oct 2017. We used pressure and fugacity coefficient, the fugacity coefficient is from the Peng -Robison equation.
We have looked through the email list and tried some possible solutions.
As suggested, we have added the intra_energy option. But there were no big differences using the intra_energy or not (it may because that we get a very small intra_energy for CO2). We got this value (-0.00556 Kcal/mol) for one CO2 molecule at zero step by performing NVE simulation with consistent parameters as GCMC+NVT. Hopefully, the way we got it is correct.
(2) Initial equilibrium
We have performed minimization and initial NVT equilibrium before the hybrid GCMC+NVT method. But we got no improvement on the CO2 density.
(3) Maniputing N, X, M paraments
We have set M to be 0, and tested both 100, 10, 0 and 10000 2500 0. The density results for both cases are still much smaller than targeted value.
(4)Full energy option no
We have made another attempt to close the full energy option by deleting Kspace and tail correction. Again, it didn’t work.
We have used the delete-bonds command (delete_bonds co2 multi remove) to close the intramolecular interaction in the template molecule. It didn’t work too.
We have also tested the results of pure GCMC. Although it was said that pure GCMC in LAMMPS is not efficient. The density is increased to be 0.65 g/cm3, but still smaller than the target one (0.81 g/cm3).
We have tried several force fields. Even the commercial PCFF+ of Medea and COMPASS of Material Studio cannot give a good density with the hybrid GCMC+NVT method.
Now, we are totally lost. I have attached our input file, CO2 molecule file and Log file. Could you give us some advices on what we miss in our simulation? Any of your comment and suggestion will be highly appreciated.
co2exam.in (3.7 KB)
co2mol_gcmcnvt.dat (429 Bytes)
log.lammps (12.3 KB)