Cohesiveness and Nondiffusive Rotational Jump Dynamics of Protic Ionic Liquid from Dispersion-Corrected FPMD Simulations.

We have investigated the liquid phase of an ionic liquid (IL), methylammonium formate (MAF), through the first principles molecular dynamics simulations using van der Waals (vdW) corrected exchange and correlation functionals of the density functional theory. The simulations were carried out to obtain a comparative study of various properties of the MAF using two different generalized gradient approximation functionals (Becke-Lee-Yang-Parr (BLYP) and Perdew-Burke-Ernzerhof (PBE)) along with three types of dispersion corrections (D2, D3, and dispersion-corrected atom-centered one-electron potentials), and two values of the plane-wave cutoff (300 and 600 Ry). We have evaluated the effects of various electronic parameters in describing the hydrogen-bonded structure and dynamical properties of MAF by performing 10 sets of molecular dynamics simulations. Thermodynamic properties are found to be sensitive to the details of electronic structure calculations. Our results of PBE functionals with the semiempirical vdW method provide the best agreement with experimental density. The overall density predictions match the cohesive energy trends, and the calculations incorporating dispersion forces exhibit enhanced intermolecular interactions within the hydrogen-bonded IL framework. All of the vdW-corrected BLYP functionals, mainly the dispersion-corrected atom-centered one-electron potential (DCACP) method, illustrate a well-defined structure of liquid MAF. To look into the dynamical perspective of the hydrogen-bond descriptions, we elucidate two possible mechanistic pathways of the hydrogen-bond jump events between the counterions. The hydrogen-bond breaking and forming mechanism along with the collision dynamics can be best described by incorporating dispersion interactions alongside the exchange and correlation functionals within the Kohn-Sham scheme. The rattling dynamics of ions are observed for dispersion-corrected functionals. Hence, an accurate representation of the delicately balanced interactive forces within ionic liquids is a necessary step toward a better description of its thermophysical and structural properties along with the associated ionic dynamics.