Step 2 - Set up a Starting Forcefield

Of course we need a forcefield to describe our ionic liquid, and we’ll focus on the widely-used parameters of Padua and Canongia-Lopes. In Dissolve this forcefield is implemented in two distinct parts, one covering cations and one covering anions.

EMIM Cation

Go to the   EMIM   species tab

Species ⇨ Add Forcefield Terms…

From the available forcefields choose PCL2019/Cations and click Next

We will use the default Determine atom types for all atoms option to add atom types for every atom in the species, so click Next

There will be no conflicts with existing atom types as there are no atom types already defined, so click Next

For the intramolecular terms we want to assign them and reduce to master terms (the default settings) so click Next to proceed

There will be no conflicts with existing master terms, so click Finish to exit the wizard

Now we have a full description of our cation - if you look on the Forcefield section on the species tab you can see that Dissolve has calculated the total charge on the molecule from both atom types and species atoms. Both of these should be +1.

You’ll notice that there are warning icons next to the total species atom and atom type charges. These are there only to indicate that the total charge is non-zero - we know that our species are ions, so we ignore these warnings.

OAc Anion

Now for the anion:

Go to the   Acetate   species tab

Species ⇨ Add Forcefield Terms…

From the available forcefields choose PCL2019/Anions and click Next

We will use the default Determine atom types for all atoms option to add atom types for every atom in the species, so click Next

At this point you will see that there is a conflict between one of the atom types Dissolve wants to apply to the OAc species (HC) and those that we’ve just created on the cation. While from a forcefield perspective the atom type HC is transferable between the cation and the anion, when we’re thinking about refining pair potentials against experimental data it is probably wise to separate it into two types so that we can allow both to vary independently in the refinement process. So:

Select the HC entry in the atom types list

Click the Suffix and type in A. Click Ok to change the name from HC to HCA to resolve the naming conflict.

Click Next to continue

For the intramolecular terms we want to assign them and reduce to master terms (the default settings) so click Next to proceed

Click Finish to exit the wizard

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