Step 4 - Make the Ionic Liquid
Time to create a configuration to represent the ionic liquid.
Generate the Configuration
Configuration ⇨ Create ⇨ Simple random mix
Highlight both species and click
Double-click on the configuration tab’s title and change its name to
Bulk
Now we just need to set the correct density for the box:.
Select the Parameters node in the generator
Set the value of the "rho" variable to
0.09985which is the density of the ionic liquid (1.0854 g cm-3) in atoms Å-3
Click to recreate the configuration with our adjusted parameters
The default population of each molecule is 100 which is quite small for an ionic liquid, but in the interests of time we’ll leave it as that. You’ll notice that the cubic box length for 100 ion pairs at this density comes out at 29.6398 Å, so we need to adjust our pair potential range which is currently set to 15 Å and exceeds the half-cell width:
Go to the Forcefield tab, Atom Types / Pair Potentials section
In the Pair Potentials controls, change the Range to
12.0
The final thing to do is change the temperature (our experimental data was measured at 323 K) and set a size factor for our box - since we have rings in our cation we need to apply a size factor during the initial equilibration to help prevent occurrences of interlocking rings.
Go to the Bulk configuration tab
Set the Temperature to
323.0
In the Size Factor Scaling controls set the Requested Size Factor to
10.0
Initial box of 1-ethyl-3-methylimidazolium acetate
Equilibrate
Let’s now add a suitable evolution layer for the system and set things running:
Layer ⇨ Create ⇨ Evolution ⇨ Standard Molecular (MC/MD)
Now we can equilibrate our system:
Ctrl-F
Set the number of iterations to
1000and hit
You might want to track the change in energy of the system in the tab of the Energy module.
After one thousand iterations the size factor of the configuration should have returned to 1.0 (check this on the configuration tab!) and the intermolecular energy of the system should have reached a stable negative value of about –100 kJ mol-1 (for 100 ion pairs). The charged nature of the species in the present system means that reaching equilibrium can take a lot longer than for a system containing only small, neutral molecules - bear this in mind when running such systems. Also note that the total energy of the configuration remains large and positive due to the intramolecular energy contributions.
Equilibrated box of 1-ethyl-3-methylimidazolium acetate