This work focuses on the comparison of charge-scaled and polarizable simulations of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM+OTf–). Therefore, we performed several completely independent molecular dynamics simulations of 1000 ion pairs at 300 K with CHARMM68 (link) in a cubic box with a box length of 67.195Å under periodic boundary conditions for a simulation period of at least 35 ns with a time step of 0.5 fs on the basis of the classical force field of Pádua et al.29 ,69 ,70 The partial charges are changed to the values reported by Hanke et al. in Ref. 31 for an improved reproduction of the experimental viscosity.17 (link),66 (link),67 (link)The completely independent non-polarizable simulations were performed with charge-scaling factors Seff of 1.00, 0.90, 0.85, 0.80 and 0.74 applied to all partial charges of the cations and anions. The polarizable simulation used the original partial charges. 31 The induced dipoles were modeled by the so-called “Drude oscillators” with an uniform Drude charge qδ = -1.0 e and a Drude mass of mδ = 0.1 amu which was subtracted from the mass of the corresponding atom.43 (link),48 (link) The atomic polarizabilities αiβ were taken from Ref. 71. Drude particles are thermostated at 1 K to ensure the proximity to self-consistency. A more detailed description of the computational setup was given in Ref. 45 and 49.
Besides the force field all simulations were treated in the same way: Only bonds including a hydrogen were kept fixed by the SHAKE algorithm.72 Non-bonded and image lists were updated heuristically using a 16 Å neighbour list distance. Lennard-Jones energies and forces were smoothly switched off between 11 and 12 Å. The electrostatic forces were computed by the Particle-Mesh-Ewald technique.73 ,74 The “cutoff” for the real-space part interactions was 12 Å and the damping constant for the reciprocal-space interactions was 0.410 Å–1. The grid spacing equaled 1.05 Å and a sixth-order spline interpolation of the charge to the grid was used.
Besides the force field all simulations were treated in the same way: Only bonds including a hydrogen were kept fixed by the SHAKE algorithm.72 Non-bonded and image lists were updated heuristically using a 16 Å neighbour list distance. Lennard-Jones energies and forces were smoothly switched off between 11 and 12 Å. The electrostatic forces were computed by the Particle-Mesh-Ewald technique.73 ,74 The “cutoff” for the real-space part interactions was 12 Å and the damping constant for the reciprocal-space interactions was 0.410 Å–1. The grid spacing equaled 1.05 Å and a sixth-order spline interpolation of the charge to the grid was used.
