Reg H2 adsorption in zeolites

[vgd_127]

Dear Prof. Dr. Dubbledam,
 
I am presently trying to simulate H2 adsorption on LTA with the exact force field parameters provided in the literature.

When I run the simulation, the simulation is finished without any warnings but the amount adsorbed for different pressures does not vary much. I varied the pressure from 1 Pa to 10^5 Pa but still, the amount adsorbed was in the range of 7.3 mol/kg FW-10 mol/kg FW indicating only a slight increase in the uptake in spite of increasing the pressure by several orders of magnitudes.

My experimental work suggests that the amount adsorbed should vary in the range of 0.002-5.24 mol/kg for the same pressure range. I varied the pressure between 1 and 10^5 Pa. I did not block pockets as hydrogen has the tendency to pass through the sodalite cages of zeolite as well.  Hence,  I assume that I am missing out on something in my input file.

SimulationType                   MonteCarlo
NumberOfCycles                   25000
NumberOfInitializationCycles     10000
PrintEvery                       1000

Forcefield                       Local
ChargeMethod             Ewald
EwaldPrecision              1e-06

Framework 0
FrameworkName LTA4A
RemoveAtomNumberCodeFromLabel yes
ModifyOxgensConnectedToAluminium yes
UnitCells 1 1 1
HeliumVoidFraction 0.5
ExternalTemperature 77.4
ExternalPressure 1
Movies yes
WriteMoviesEvery 1000

Component 0 MoleculeName                   sodium
            MoleculeDefinition             Local
            TranslationProbability         1.0
            RandomTranslationProbability   1.0
            ExtraFrameworkMolecule         yes
            CreateNumberOfMolecules        96

Component 1 MoleculeName                   H2
            MoleculeDefinition             Local
            IdealGasRosenbluthWeight       1.0
            BlockPockets                   no
            BlockPocketsFilename           LTA
            TranslationProbability         0.5
            ReinsertionProbability         0.5
            SwapProbability                1.0
            ExtraFrameworkMolecule         no
            CreateNumberOfMolecules        0

Can you please advise me on how to mitigate this issue?  I would be very grateful to you if you can spare me a few minutes and help me navigate through this issue.

[David Dubbeldam]

At low temperatures, the isotherm are usually suddenly increasing a lot and sharply peaked. The pressure where that happens is very sensitive to forcefields and simulations setup etc. I would compute the isotherm from 1e-8 to 1e5 Pa for example to have a better idea of the full shape of the isotherm. One of two orders off in pressure where it shoots up is not too bad.
Also note, that this type of system is sensitive to the correct positioning of the cations, which is another topic in itself. This is especially important at low temperature. Equilibration of these types of systems is long and difficult.

[vgd_127]

Thank you very much Prof. Dr. Dubbeldam.

Do you mean that I have to do a Runtype : "minimization" separately for the framework + cations and then use that for the simulations? Should I keep the framework flexible or rigid in this case?

SimulationType          Minimization
NumberOfCycles          1
RestartFile             no
PrintEvery              1

MaximumNumberOfMinimizationSteps 1000
RMSGradientTolerance 1e-6
MaxGradientTolerance 1e-6

Ensemble NVT

Forcefield                  local
ChargeMethod                Ewald
EwaldPrecision              1e-10
InternalFrameworkLennardJonesInteractions yes

Framework 0
FrameworkName LTA4A
UnitCells 1 1 1
ExternalTemperature 298.0
Movies yes
WriteMoviesEvery 1

Component 0 MoleculeName                   sodium
            MoleculeDefinition             Local
            TranslationProbability         1.0
            RandomTranslationProbability   1.0
            ExtraFrameworkMolecule         yes
            CreateNumberOfMolecules        96


I ask this because I assumed that while performing GCMC calculations, just like for adsorbate, the cations were inserted in the most probable sites while keeping the energy minimum.