Structure and Energetics of Li+–(BF4–)n, Li+–(FSI–)n, and Li+–(TFSI–)n: Ab Initio and Polarizable Force Field Approaches

The Li+–BF4– and BF4––BF4– interactions are studied using second order perturbation theory (MP2) and coupled cluster singles and doubles approach, including the effect of connected triples, CCSD­(T). The MP2 and CCSD­(T) results are in excellent agreement. Using only the MP2 approach, the interactions of Li+ with bis­(trifluoromethane)­sulfonimide anion (TFSI) and Li+ with bis­(fluorosulfonyl)­imide anion (FSI) are studied. The results of these high level calculations are compared with density functional theory (DFT) calculations for a variety of functionals and with the APPLE&P force field. The B3LYP approach well reproduces the accurate calculations using both a small and large basis set. The M06 and M06L functionals in the larger basis set are in good agreement with the high level calculations. While the APPLE&P force field does not outperform the best functionals, the APPLE&P results agree better with the accurate results than do some of the functionals tested.