Formation Enthalpies of Ions: Routine Prediction Using Atom Equivalents

In view of identifying routine procedures to estimate formation enthalpies of ionic systems such as energetic salts or ionic liquids on the basis of density functional theory (DFT), various combinations of atom equivalent (AE) schemes, functionals, and basis sets are compared, using a specially designed training set to parametrize the models. After correction, none of the functionals considered proves significantly more reliable than B3LYP. A small but systematic improvement is noted as AE values are allowed to depend on the atomic environment. However, AE parameters fail to make up for basis set limitations, in constrast to previous observations for neutrals. Finally, a good trade-off between reliability and cost is obtained for ions using B3LYP/6-31++G** energies.