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Messages - Jianbo

Dear Dubbeldam,

Thanks for your reply. It seems I misused the two effects, i.e. the boundary effect and the finite-size effect.

In the grand-canonical approach, the intermolecular force between the substrate of interest and the image substrate can be minimized by creating the free space in the z-direction. In other words, the finite-size effect can be handled.

However, for the Gibbs approach,  if the simulation box of the adsorbed phase is enlarged to avoid the finite-size effect, a fluid phase seems to appear in this simulation box.

So, is it practical to set the adsorbed phase box with a sufficiently large box volume?

Thanks in advance
Jianbo Wang
Dear RASPA users,

I want to simulate the adsorption of water on zeolites via Gibbs ensemble Monte Carlo (GEMC) simulation.

In a Grand Canonical Monte Carlo (GCMC) simulation, I can create a free space of several hundred Å to eliminate the effect.

However, it seems impractical to do the same in a GEMC simulation.

Thus, how can I eliminate the influence of image zeolites on water?

Thanks in advance
Jianbo Wang
Output files / Re: Vapor density by NVT-GEMC
August 22, 2020, 02:32:30 PM
Thanks for your kindly response!

I have checked the result:
The vapor pressure seems to fluactuate around the average pressure value which indicates the system are well equilibrated.
However, the average widom chemical potential of vapor phase differs a lot from the liquid phase which indicates the system are not equilibrated.

So, I am confused how can I tell the system is well equilbrated?
Output files / Vapor density by NVT-GEMC
August 03, 2020, 04:27:20 PM
Hello Dr. Dubbeldam,

I am doing NVT-Gibbs Ensemble Simulations to obtain SPC/E water vapor density.

I have no idea why negative pressre existed in the liquid box shows negative pressre existed. Is there anything wrong with my input file?

Besides, how can I tell the vapour-liquid equilibrium has been reached? I use averaged Widom chemical potential and averaged pressure to identify the equilibrium state, am I right?

Thanks in advance!

P. S. simulation file is as below:

SimulationType                MonteCarlo

NumberOfCycles                200000
NumberOfInitializationCycles  50000

RestartFile                   no
ContinueAfterCrash            no
WriteBinaryRestartFileEvery   1000

PrintEvery                    1000
PrintPropertiesEvery          1000

ChargeMethod                  Ewald
EwaldPrecision                1e-5
Forcefield                    SPC_E
CutOffVDW                     10
CutOffChargeCharge            10

Box 0
BoxLengths 22 22 22
BoxAngles 90 90 90
ExternalTemperature 300.0

Movies                        yes
WriteMoviesEvery              1000

ComputeEnergyHistogram        yes
ComputeNumberOfMoleculesHistogram      yes
ComputeMolecularPressure      yes

Box 1
BoxLengths 100 100 100
BoxAngles 90 90 90
ExternalTemperature 300.0

Movies                        yes
WriteMoviesEvery              1000

ComputeEnergyHistogram        yes
ComputeNumberOfMoleculesHistogram      yes
ComputeMolecularPressure      yes

GibbsVolumeChangeProbability 0.1

Component 0 MoleculeName             water_SPCE
            StartingBead             0
            MoleculeDefinition       SPC_E
            TranslationProbability   40
            RotationProbability      40
            ReinsertionProbability   3.3
            GibbsSwapProbability     10
            GibbsWidomProbability    3.3
            WidomProbability         3.3
            CreateNumberOfMolecules  320 0
So if I want to obtain water adsorption isotherm, I should do the followings:

1. calculate different fugacity values with corresponding relative humidity values
2. input these calculated increasing fugacity using command 'ExternalPressure'
3. set the value of 'FugacityCoefficient' to 1 rather 0

Is this right?

Thanks in advance.
Hello Prof. Dubbeldam,

1. I would like to use SPC/E water model in RASPA and I checked the TIP5P model mentioned in Example/Non-basics/MD_NPT_water.

However, the critical parameters given there of Tip5p water model doesn't match neither the experimental data of water vapour nor the simulation data[1].

Could you tell me how to determine these parameters for SPC/E model?

2. If I want to simulate water adsorption at different relative humidity, the vale of 'ExternalPressure' should be calculated form the correponding saturated vapour pressure of SPC/E model or the experiment data?

Kind regards,

[1] Vega, C., & Abascal, J. L. (2011). Simulating water with rigid non-polarizable models: a general perspective. Physical Chemistry Chemical Physics, 13(44), 19663-19688.