TUTORIAL 8 : Exercise 1¶

Ideal gas in a planar pore¶

In this exercise we will simulate ideal gas partitioning in a slit confinement, in either “non-wetting” or “wetting” surface conditions.

Navigate to directory tutorial_8/slit-ideal

Check the FIELD file:

slit-ideal > more FIELD


The initial FIELD file for this tutorial should look as shown below.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Ideal gas in a slit CUTOFF 1.0 UNITS K NCONFIGS 1 ATOMS 1 LJ core 1.0 0.0 MOLTYPES 1 IG MAXATOM 256 FINISH VDW 1 LJ core LJ core hs 0.0 0.0 external 1 LJ core hs 0.0 1.0 1.0 CLOSE 

The important points to note are:

the cutoff = 1.0 (very small)

the energy unit is K (reduced units)

the particle-particle hard sphere interaction is zero (ideal gas!)

the external particle-wall hard square interaction is in the form of a ‘repulsive shoulder’:

width = 1.0 Angstrom (from a wall) and strength = 1.0 kT.

Check the CONTROL file:

slit-ideal > more CONTROL


The initial CONTROL file for this tutorial is

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 NVT simulation of ideal gas in the slit geometry use ortho finish seeds 12 34 56 78 # Seed RNG seeds explicitly to the default slit walls soft 2 #slit mfa 0 noewald all temperature 1.0 # Corresponds to T*=1.0 pressure 1.0 # katm (*10^9) steps 1000000 #0 # Number of moves to perform in simulation equilibration 0 # Equilibration period: statistics are gathered after this period print 100000 # Print statistics every 'print' moves stack 1000 # Size of blocks for block averaging to obtain statistics sample zdensity 120 10 sample coordinates 100000 archiveformat dlpoly2 #+dcd revconformat dlpoly2 # REVCON file is in DL_POLY CONFIG format acceptatmmoveupdate 100000000 maxatmdist 10.0 move atom 1 100 # Move atoms 100% of the time LJ core check 1000000 start 

Run the simulation and, when done, plot the density profile along z (ZDENSITY.000):

[tutorial_8]\$ gnuplot
gnuplot> plot [x=-6:6] [y=0.:3.0] 'ZDENSY.000' u 1:2 w l t "Density(z)"


• Is it “wetting” or “non-wetting” case?
• Why are there two levels in density?
• Can you calculate the exact density levels?
• How does the external potential look like?
• How to reconstruct the external potential (approximately) from the density data?
• What will happen if you increase the cell dimentions in X and Y in CONFIG?

If we increase the ‘steps’ number in CONTROL by a factor of 10 (uncomment the extra zero), we get a much better precision, as below.

Change the sign for the external interaction strength (in FIELD), and rerun the simulation.

• How do you think the density will look this time?

Hint: $$\rho(z) = \rho(0)\exp(-\beta U(z))$$

Next exercise:¶

TUTORIAL 8 : Exercise 2 - Water adsoprtion in a slit