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 “SiteRDF” module in the list and rename it to
RDF-CA
Change the DistanceRange maximum to
14.0(slightly less than our half-cell length)
Set SiteA to
RingNNon the cation
Set SiteB to
COMon the anion
The
SiteRDF module can also calculate simple coordination numbers over specific distance ranges, however typically we don’t know what this range is until we’ve seen the actual RDF data, so we’ll leave that calculation turned off for now.
Now to add on two more
SiteRDF modules for the other correlations of interest:
Drag a
Double-click the new
RDF-CC
Change the DistanceRange maximum to
14.0
Set SiteA and SiteB to
RingNNon the cation
Drag another
Double-click the second new
RDF-AA
Change the DistanceRange 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:
Find the Coordination Numbers options in the
Set RangeA to span
0.0to7.0
Click RangeAEnabled to request the calculation
Start the simulation up once more:
Ctrl-R
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.