Vapor density by NVT-GEMC

[Jianbo]

Hello Dr. Dubbeldam,

I am doing NVT-Gibbs Ensemble Simulations to obtain SPC/E water vapor density.

I have no idea why negative pressre existed in the liquid box shows negative pressre existed. Is there anything wrong with my input file?

Besides, how can I tell the vapour-liquid equilibrium has been reached? I use averaged Widom chemical potential and averaged pressure to identify the equilibrium state, am I right?

Thanks in advance!

P. S. simulation file is as below:

SimulationType                MonteCarlo

NumberOfCycles                200000
NumberOfInitializationCycles  50000

RestartFile                   no
ContinueAfterCrash            no
WriteBinaryRestartFileEvery   1000

PrintEvery                    1000
PrintPropertiesEvery          1000

ChargeMethod                  Ewald
EwaldPrecision                1e-5
Forcefield                    SPC_E
CutOffVDW                     10
CutOffChargeCharge            10

Box 0
BoxLengths 22 22 22
BoxAngles 90 90 90
ExternalTemperature 300.0

Movies                        yes
WriteMoviesEvery              1000

ComputeEnergyHistogram        yes
ComputeNumberOfMoleculesHistogram      yes
ComputeMolecularPressure      yes

Box 1
BoxLengths 100 100 100
BoxAngles 90 90 90
ExternalTemperature 300.0

Movies                        yes
WriteMoviesEvery              1000

ComputeEnergyHistogram        yes
ComputeNumberOfMoleculesHistogram      yes
ComputeMolecularPressure      yes

GibbsVolumeChangeProbability 0.1

Component 0 MoleculeName             water_SPCE
            StartingBead             0
            MoleculeDefinition       SPC_E
            TranslationProbability   40
            RotationProbability      40
            ReinsertionProbability   3.3
            GibbsSwapProbability     10
            GibbsWidomProbability    3.3
            WidomProbability         3.3
            CreateNumberOfMolecules  320 0

[dubbelda]

The pressure of the liquid-phase contains a large error (because of the strong molecular interactions). Usually people take the gas-phase value after the simulation is equilibrated.

[Jianbo]

Thanks for your kindly response!

I have checked the result:
The vapor pressure seems to fluactuate around the average pressure value which indicates the system are well equilibrated.
However, the average widom chemical potential of vapor phase differs a lot from the liquid phase which indicates the system are not equilibrated.

So, I am confused how can I tell the system is well equilbrated?

[David Dubbeldam]

Using the Widom-insertion in dense system is known to fail. That is why people take the value from the vapor phase assuming you have run long enough to be equilibrated.