EFP parameters for each cation and each anion in the data set were computed at the HF/6-311++G(3df,2p) level, using the GAMESS program. We use the same (rigid) monomer geometries and thus the same EFP parameters for each of the eight intermolecular separations. The EFP energy calculations discussed below were performed at the HF+D(EFP)/6-311++G(d,p) level using a locally-modified version of the Q-Chem program, which includes an interface to the libEFP library.