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