Hello,
I am trying to simulate molecule adsorption in MOFs using Gibbs ensemble MC algorithm. I was wondering how do you calculate the initial number of molecules in the box?
Kind regards,
Ignas
The number of molecules in Gibbs is an input parameter. Usually you want a sufficient number of molecules to reduce finite size effects (like 256 or 512) in liquid/vapor equilibrium Gibbs.
In case of adsorption, you need more molecules that the saturation loading in the framework plus then enough molecules in the vapor phase.
Dear Dr. Dubbeldam
I prepare the attached code to calculate vapor liquid equilibrium and the density of vapor and liquid phases for propane using TraPPE-UA force field parameters. But the vapor density is not accurate. I changed the number of molecules so many times and ran the code, but I did not get the right answer. Would you please guide me? Thank you for your kindness and support.
SimulationType MonteCarlo
NumberOfCycles 25000
NumberOfInitializationCycles 5000
PrintEvery 1000
RestartFile no
Forcefield TraPPE-UA
CutOff 14
Box 0
BoxLengths 30 30 30
BoxAngles 90 90 90
ExternalTemperature 344.0
Box 1
BoxLengths 30 30 30
BoxAngles 90 90 90
ExternalTemperature 344.0
GibbsVolumeChangeProbability 0.1
Component 0 MoleculeName propane
StartingBead 1
MoleculeDefinition TraPPE-UA
TranslationProbability 0.5
RotationProbability 0.5
ReinsertionProbability 0.5
GibbsSwapProbability 0.5
CreateNumberOfMolecules 185 15
What do you mean wiith "not accurate"? Compared to what? experiment or the result of the TraPPE force field?
Force TraPPE, make sure you use an unshifted potential and tailcorrections.
Dear Dr. Dubbeldum
Thanks a lot for your response. I compared my results with experimental data. The results for liquid density are accurate at all temperatures with the error of 1%, but the results for vapor density are not accurate with the error of 20%. When I compared my results with the results of a paper ,published in 1998, I understood that they could not get the accurate results for vapor density using TraPPE-UA.
What is the reason that vapor density is not achieved? Is not it because of the length of boxes?
SimulationType MonteCarlo
NumberOfCycles 100000
NumberOfInitializationCycles 10000
RestartFile no
ContinueAfterCrash no
WriteBinaryRestartFileEvery 1000
PrintEvery 1000
PrintPropertiesEvery 1000
ChargeMethod Ewald
EwaldPrecision 1e-5
Forcefield TraPPE-UA
CutOffVDW 14
CutOffChargeCharge 14
Box 0
BoxLengths 30 30 30
BoxAngles 90 90 90
ExternalTemperature 200
Movies yes
WriteMoviesEvery 1000
ComputeEnergyHistogram yes
ComputeNumberOfMoleculesHistogram yes
ComputeMolecularPressure yes
Box 1
BoxLengths 30 30 30
BoxAngles 90 90 90
ExternalTemperature 200
Movies yes
WriteMoviesEvery 1000
ComputeEnergyHistogram yes
ComputeNumberOfMoleculesHistogram yes
ComputeMolecularPressure yes
GibbsVolumeChangeProbability 0.05
Component 0 MoleculeName propane
MoleculeDefinition TraPPE-UA
TranslationProbability 0.5
RotationProbability 0.5
ReinsertionProbability 0.5
GibbsSwapProbability 0.5
CreateNumberOfMolecules 100 100
The length of the boxes is important to reduce finite-size effects.
But more fundamentally, it is quite hard with a simple set of LJ parameters to get both the liquid and the vapor branch accurate. This is btw one the main reason to fit parameters on VLE-data. But the united-atom approach has its limits. More complicated models like full-atom models have more parameters and are harder to optimize.
So... having a good liquid phase and 20% off for the vapor phase is not that bad.