Why the excess adsorption is negative, absolute adsorption is zero when doing MC

[YadiJiang]

Dear Dubbeldam,

I'm using RASPA to simulate the alkane adsorption in Montmorillonite. But in the output , the absolute adsorption is zero, the excess loading is negative, I don't know why and how to get the correct amount of adsorption.

Kind regards,
Yadi Jiang

*************
This is the component's critical parameter：792K, 0.98MPa, 0.8.
(In the simulation, temperature is 358K, pressure is 30MPa,40MPa.)

*************
This is the simulation.input file:

SimulationType                MonteCarlo
NumberOfCycles                100000
NumberOfInitializationCycles  50000
PrintEvery                    1000
RestartFile                   no

ContinueAfterCrash no
WriteBinaryRestartFileEvery 5000

ChargeMethod                    Ewald
Forcefield                      clayff  //Custom force field
CutOffVDW                       12.5
RemoveAtomNumberCodeFromLabel   no

Framework 0
FrameworkName 10-MMT
UseChargesFromCIFFile no
UnitCells 3 3 3
HeliumVoidFraction  1
ExternalTemperature 358
ExternalPressure 3e7 4e7
Movies yes
WriteMoviesEvery  5000
ComputeDensityProfile3DVTKGrid           yes
WriteDensityProfile3DVTKGridEvery        1000
DensityProfile3DVTKGridPoints            150 150 150
AverageDensityOverUniteCellsVTK          yes
DensityAverageTypeVTK                    FullBox

Component 0 MoleculeName              C22
            MoleculeDefinition        TraPPE
            TranslationProbability    1.0
            RotationProbability       1.0
            ReinsertionProbability    1.0
            IdentityChangeProbability 0.0
            SwapProbability           1.0
            CreateNumberOfMolecules   0


***************
The force field parameter:

# general rule for shifted vs truncated
shifted
# general rule tailcorrections
no
# number of defined interactions
4
# type interaction, parameters.    IMPORTANT: define shortest matches first, so that more specific ones overwrites these
O_             lennard-jones    78.263        3.5532
Al_            lennard-Jones    0.00067       4.7943
Si_            lennard-Jones    0.000926      3.7064
Ca_            lennard-Jones    50.362        3.2237

# general mixing rule for Lennard-Jones
Lorentz-Berthelot

[David Dubbeldam]

Negative excess adsorption indicates that you are in the liquid phase. That happens when you go above the vapor pressure.

[sara abbasi]

Dear Dr. Dubbeldum
I am using RASPA to calculate propane/propene selectivity of pure silica zeolite LTA using the following code. In output absolute loading for both propane and propene is positive, But excess loading is negative.  Then I tried this simulation in 1 bar pressure, But excess loading is negative. Thanks a lot for guiding me.
SimulationType                MonteCarlo
NumberOfCycles                100000
NumberOfInitializationCycles  10000
PrintEvery                    1000
RestartFile                   no

ContinueAfterCrash            no
WriteBinaryRestartFileEvery   500

Forcefield                    local
UseChargesFromCIFFile         yes

Framework 0
FrameworkName Framework_0_final_1_1_1
UnitCells 3 3 3
HeliumVoidFraction 0.460673
ExternalTemperature 308.37
ExternalPressure 1970000
Movies yes
WriteMoviesEvery 1000

Component 0 MoleculeName               propene
            MoleculeDefinition         local
            IdealGasRosenbluthWeight   1.0
            MoleFraction               0.91
            TranslationProbability     0.5
            RotationProbability        0.5
            ReinsertionProbability     0.5
            IdentityChangeProbability  1.0
            NumberOfIdentityChanges    2
            IdentityChangesList        0 1
            SwapProbability            1.0
            CreateNumberOfMolecules    0
           
Component 1 MoleculeName               propane
            MoleculeDefinition         local
            IdealGasRosenbluthWeight   1.0
            MoleFraction               0.09
            TranslationProbability     0.5
            RotationProbability        0.5
            ReinsertionProbability     0.5
            IdentityChangeProbability  1.0
            NumberOfIdentityChanges    2
              IdentityChangesList      0 1
            SwapProbability            1.0
            CreateNumberOfMolecules    0