[gmx-users] Supercritical CO2

[Alex]

Unfortunately, I had a pretty terrible experience trying to simulate fuels,
fuel mixtures, and CO2 dissolved in fuels with Gromacs (using OPLS-AA) at
high pressures and/or temperatures. It is my understanding that at the
moment this is isn't the area where these forcefields are seriously tested.

I have no results for other forcefields, but for OPLS-AA it was
unpublishable and came out as an internal note at the institution. If
curious, see
http://nvlpubs.nist.gov/nistpubs/TechnicalNotes/NIST.TN.1805.pdf
In fact, if you get any results that correspond to experimental data, I'll
really appreciate it if you let me know. :)

Alex

On Thu, Apr 30, 2015 at 2:04 PM, Jarrett Lee Wise <jwise6 at uwyo.edu> wrote:

> 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
>
>
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