Computational details
All DFT calculations were performed using the real-space grid-based
projector-augmented wave DFT code (GPAW 1.2).28–31 In
all calculations, we used the projector-augmented wave (PAW) method
within the Perdew–Burke–Ernzerhof (PBE) generalised gradient
approximation,32,33 a grid spacing of 0.16 Å, and the
residual minimisation method – direct inversion in iterative subspace
(RMM-DIIS).34,35 Compared to the most accurate
functionals, PBE shows slightly lower performance in respect to the
dissociation energy for the chosen set of IL ion
pairs.36–38 However, PBE is sufficiently fast and
accurate for the ΔKS type calculations that give the total energy
difference between the ground state and the first core-excited
state.39,40 To obtain the 1s Kohn–Sham orbital
eigenvalues of selected atoms, we generated all-electron setups using
gpaw-setup tool with the parameter –core=””. All other calculations
were run with a default frozen core for GPAW setup 0.9.2.
The gas-phase relaxed structures of ion pairs were taken from Refs
10,37, where they were optimised with B2PLYP double hybrid functional
and triple-zeta basis set. Forty different IL ion pairs, all depicted in
Figure 1, were formed by combining eight anions
(B(CN)4−, TFSI−,
FSI−, PF6−,
BF4−, Cl−,
Br−, I−) with five cations
(EMIm+, BMIm+,
BMPyr+, BPy+,
TEPA+). We introduced 8 Å of vacuum between an ion
pair and the side of the calculation box. Convergence problems occurred
for several carbon and nitrogen atoms in BPyTFSI and BPyrTFSI, as well
as for few carbon atoms in BPyB(CN)4, TEPAI, TEPABr,
BMPyrI, BMPyrBr. To tackle this issue, we changed the convergence
criterion for energy from 10−8 to
10−4 and density criterion from default
10−4 to 10−2. In the Supplementary
information, Table S1 shows how the variation of the named criteria
influences the energy values starting from the third decimal point,
which can be considered insignificant for the ΔKS type calculations. The
Supplementary information also includes all calculated ΔKS BE values.
To automate calculations, we used the NaRIBaS framework – a collection
of scripts that allows generating input files for all the required
systems by defining all the variables one wishes to
combine.41 To conduct charge localisation analysis of
atoms, we used density derived electrostatic and chemical (DDEC6)
charges.42–44