[gmx-users] Supercritical CO2

[V.V.Chaban]

This problem is natural. Even small change in (T,P) in the
supercritical range results in a huge change of density and, hence,
all other properties.

You need either to alter (T,P) or to adjust the force field model
(both change will be slight, but will bring large changes of density).

If you expect any phase separation in these setups, take care of the
Gibbs rule.




On Thu, Apr 30, 2015 at 6:38 PM, Alex <nedomacho at gmail.com> wrote:
> Only other suggestion I have is that 5 ns production runs are probably not
> long enough. Granted, we needed diffusion data, so I believe I had 50 to
> 100 ns long simulations, but even for the barostat to settle, it may be a
> good idea to have simulations significantly longer than 5 ns.
>
> Alex
>
> On Thu, Apr 30, 2015 at 3:14 PM, Alex <nedomacho at gmail.com> wrote:
>
>> 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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>>
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