When Dynamics Matters: Structure-Matching Alone is Insufficient in the Development of Realistic Force Fields for Nonaqueous Electrolytes

Molecular dynamics are promising methods to compute numerous properties of nonaqueous electrolytes, but quantitative predictions depend critically on the prescribed force fields. Here, we show that several quantum-mechanically refined force fields for the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) - acetonitrile electrolyte yield structures that are in good agreement with the experimental neutron pair distribution function (PDF), while dynamics is dramatically different and inconsistent with NMR measurements. Such glaring discrepancies indicate that inadequate representation of long-range interactions leads to excessive frustration in the free energy landscape. Better agreement is achieved by proportionally scaling down the atomic charges of the ions. This simplification enabled the simulation of concentration dependences of ionic diffusion for 0.2-2 M LiTFSI solutions without sacrificing fit quality of the PDFs. We argue that not only structures but also dynamics constitute important checkpoints on the road to computationally design functional electrolytes.