Author Topic: Why the excess adsorption is negative, absolute adsorption is zero when doing MC  (Read 44 times)

YadiJiang

  • Newbie
  • *
  • Posts: 1
    • View Profile
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´╝Ü792K, 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

  • Full Member
  • ***
  • Posts: 101
    • View Profile
Negative excess adsorption indicates that you are in the liquid phase. That happens when you go above the vapor pressure.