Padua & Canongia Lopes (2019)

Provides

Overview

The complete ionic liquids forcefield of Padua and Canongia Lopes, as found at https://github.com/agiliopadua/ilff (commit 1df07bd), covering a variety of normal and fluorinated cations and anions.

Implementation Notes

Atom Types

• Renamed trifluoroacetate oxygen to O2F in order to distinguish it from normal acetate O2 as it has differing Lennard-Jones parameters.
• For alkylsulphates (RSO4-) the bridging oxygen is wrongly labelled as OS4 in the structures pdf (should be OC4).
• Alkane-style carbons are labelled CT2 (two hydrogens) and CT3 (three hydrogens).
• Alkylimidazolium hydrogen on ring nitrogen HNA’s original equivalent type was HA, but this is mapped to HNA since its Lennard-Jones parameters are zero (while those for other HA are not).
• Alkylimidazolium carbon next to ring nitrogen with H attached (CW) has different Lennard-Jones parameters to dialkylimidazolium CW, so this is mapped to CWH here.
• Lennard-Jones parameters for CS3 and CS4 are mapped to CT, but have slightly different epsilon values in the original forcefield (0.276 vs 0.27614) - this is assumed to be rounding, and the value listed to 5 d.p. is used.
• Similarly, HS3 and HS4 have a Lennard-Jones epsilon of 0.126, but the mapped type HC has an epsilon of 0.12552. Again this is assumed to be a rounding error, and the 5 d.p. value is used.
• Tosylate hydrogen HTS is mapped to HC but is listed as having Lennard-Jones sigma of 2.42, which is more consistent with an aromatic hydrogen (e.g. HA). The expected value of 2.5 for an aliphatic hydrogen (i.e. HC) is used.
• OPLS-AA alcohol carbon (CTO) and hydrogen (H10) not implemented - see OPLSAA2005/Alcohols for the full description.
• Phosphonium P renamed from P3 to P4 for consistency with N4 and https://github.com/agiliopadua/ilff/blob/master/ilff_structures.pdf.
• Polarisable C1A not implemented.

Intramolecular Parameters

• Bond constraints are implemented as harmonic bonds with the specified force parameters.
• Trifluoroacetate bond parameters are missing in the il.ff file, so are taken from examples/old.il.ff.
• Benzylimidazolium AMBER wildcard torsions CA-CT-NA-CR and NA-CT-CA-CA are implemented as NoForm torsions, and the missing CA-CT-NA-CW has been added (e.g. found in benzc1im).
• Alkoxy torsion NA-CT-CT-OH taken from old.il.ff.