[gmx-users] Supercritical CO2

[Jarrett Lee Wise]

Hello all-

I am trying to run simulations containing supercritical CO2 and oil asphaltenes.  I have tested various force fields (TraPPE flex, TraPPe rigid, Charmm, EPM2, Cygan, and Zhang), but all of my results give either extremely low densities (~200 kg/m^3) or extremely high densities (~1600 kg/m^3) when the density should be around 630 kg/m^3.


I equilibrated a system of 5000 CO2 molecules with Nose-Hoover and Berendsen for 5 ns, then did a production run using Nose-Hoover and Parrinello-Rahman for another 5 ns using 1 fs time step.

I have found that tau_p plays an important factor.  A relaxed tau_p gives low density while a vigorous tau_p gives a high density and crashes using Parrinello-Rahman pressure coupling.


Has anyone else encountered similar problems or have any advice?

Here is a copy of my MDP file.  Any advice is greatly appreciated.

Thanks


Jarrett Wise
PhD Student
Petroleum Engineering
University of Wyoming


title           = CO2 Density Run
integrator      = md
nsteps          = 5000000
dt              = 0.001
nstxout         = 0
nstvout         = 0
nstfout         = 0
nstxtcout       = 20000
nstenergy       = 100
nstlog          = 1500

constraint_algorithm = lincs
constraints     = H-Bonds
lincs_iter      = 1
lincs_order     = 4

cutoff-scheme   = Verlet
ns_type         = grid
nstlist         = 100
rlist           = 1.4
rcoulomb        = 1.4
rvdw            = 1.4
nstcalcenergy  = 10
comm_mode = linear

coulombtype     = PME
pme_order       = 4
fourierspacing  = 0.16

tcoupl          = Nose-Hoover
tc-grps         = System
tau_t           = 5.0
ref_t           = 320


pcoupl          =  Parrinello-Rahman
pcoupltype      = isotropic
tau_p           = 3.0
ref_p           = 520.0
compressibility = 1.45e-4

pbc             = xyz
continuation    = yes
gen_vel         = no
gen_temp       = 320