Step 3 - Apply Charge Scaling
At this point we’ll use some prior knowledge of the EMIM-OAc system. Many ionic liquid forcefields to make the quite reasonable assumption of integer (i.e. +1 and –1) charges on the ions, however the polarisation of (or charge-transfer between) ionic liquid ions has been the subject of significant debate in the literature, and various simulation studies show that scaling the ion charges to mimic a “static” charge transfer between ions can yield improvements to calculated quantities such as the diffusion coefficient and ion-ion structure.
We’ll apply a charge scaling on the ions following the work in “Structure and dynamics of 1-ethyl-3-methylimidazolium acetate via molecular dynamics and neutron diffraction”. Therein, the authors scaled charges on the ions by a factor of 0.85 and showed improved agreement with experimental data (neutron, of course!):
Go to the
EMIM species tab
Species ⇨ Scale Charges…
Change the scale factor to
0.85
You’ll see the scaling reflected in the charges within the table on the species tab’s Forcefield, and the total Atomic Charge there should now be +0.85. We need to do the same for the anion, of course:
Go to the
Species ⇨ Scale Charges…
Apply the scale factor to
0.85
The final important step is to explicitly request that Dissolve use charges from the species atoms, as the default automatic choice is to use atom type charges wherever possible.
Go to the
Forcefield tab, section
In the Charge Source controls untick the Choose Automatically option and choose the Species Atoms option below
If we don’t do this then Dissolve will elect to use the charges on the atom types (which are still valid).