Step 9 - Ion-Ion RDFs
Here we’ll set up calculation of some basic ionic liquid properties in the form of the three unique ion-ion radial distribution functions - the cation-anion, cation-cation, and anion-anion flavours - and the cation-anion coordination number.
So, let’s create and prepare suitable layer:
Layer ⇨ Create ⇨ Analysis ⇨ RDF & Coordination Number
Go to the Analyse RDF/CN layer
Double-click the “CalculateRDF” module in the list and rename it to
RDF-CA
Change the Range maximum to
14.0(slightly less than our half-cell length)
Set SiteA to
RingNNon the cation
Set SiteB to
COMon the anion
We’ll also rename the CalculateCN module and disable it for now, since typically we don’t know over what range(s) we want to calculate the coordination numbers yet.
Double-click the CalculateCN module in the list and rename it to
CN-CA
In the Module Control controls above the module list, disable the module (click on slider button so it displays a red cross)
Now to add on two more CalculateRDF modules for the other correlations of interest:
Drag a CalculateRDF module from the palette up to the module list
Double-click the new CalculateRDF module and rename it to
RDF-CC
Change the Range maximum to
14.0
Set SiteA and SiteB to
RingNNon the cation
Drag another CalculateRDF module from the palette up to the module list
Double-click the second new CalculateRDF module and rename it to
RDF-AA
Change the Range maximum to
14.0
Set SiteA and SiteB to
COMon the anion
Now start up the simulation again:
Ctrl-F
Set the number of iterations to
250and hit
Because our system is quite small (100 ion pairs) the RDFs will take a few thousand iterations to become properly averaged, but 250 steps should be enough to get a feel for the results. If you look at the output from the RDF-CA module you should see that the first minimum occurs at around 7 Å, so we’ll set this value as the upper limit for our coordination number calculation:
Click the CalculateCN in the list to display its options
Set RangeA to span
0.0to7.0
In the Module Control controls above the module list, enable the module again
Start the simulation up once more:
Ctrl-R
On the output page of the CalculateCN module you’ll see both the calculated coordination number and the source RDF data from which it is calculated. The coordination number should average out at about 6.2 anions (within 7.0 Å of a central cation), compared to 6.8 (EPSR) or 6.9 (pure molecular dynamics) in the 2010 paper.