iRASPA Community

RASPA => General => Topic started by: YadiJiang on April 28, 2020, 04:34:13 AM

Title: Why the excess adsorption is negative, absolute adsorption is zero when doing MC
Post by: YadiJiang on April 28, 2020, 04:34:13 AM
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
# general rule tailcorrections
# number of defined interactions
# 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

Title: Re: Why the excess adsorption is negative, absolute adsorption is zero when doing MC
Post by: David Dubbeldam on May 14, 2020, 08:10:12 PM
Negative excess adsorption indicates that you are in the liquid phase. That happens when you go above the vapor pressure.