Molecular Modelling of Ionic Liquids: Situations When Charge Scaling Seems Insufficient

Abstract

Charge scaling as an effective solution to the experiment–computation disagreement in molecular modelling of ionic liquids (ILs) could bring the computational results close to the experimental reference for various thermodynamic properties. According to the large-scale benchmark calculations of mass density, solvation, and water-ILs transfer-free energies in our series of papers, the charge-scaling factor of 0.8 serves as a near-optimal option generally applicable to most ILs, although a system-dependent parameter adjustment could be attempted for further improved performance. However, there are situations in which such a charge-scaling treatment would fail. Namely, charge scaling cannot really affect the simulation outcome, or minimally perturbs the results that are still far from the experimental value. In such situations, the vdW radius as an additional adjustable parameter is commonly tuned to minimize the experiment–calculation deviation. In the current work, considering two ILs from the quinuclidinium family, we investigate the impacts of this vdW-scaling treatment on the mass density and the solvation/partition thermodynamics in a fashion similar to our previous charge-scaling works, i.e., scanning the vdW-scaling factor and computing physical properties under these parameter sets. It is observed that the mass density exhibits a linear response to the vdW-scaling factor with slopes close to −1.8 g/mL. By further investigating a set of physiochemically relevant temperatures between 288 K and 348 K, we confirm the robustness of the vdW-scaling treatment in the estimation of bulk properties. The best vdW-scaling parameter for mass density would worsen the computation of solvation/partition thermodynamics, and a marginal decrease in the vdW-scaling factor is considered as an intermediate option balancing the reproductions of bulk properties and solvation thermodynamics. These observations could be understood in a way similar to the charge-scaling situation. i.e., overfitting some properties (e.g., mass density) would degrade the accuracy of the other properties (e.g., solvation free energies). Following this principle, the general guideline for applying this vdW-tuning protocol is by using values between the density-derived choice and the solvation/partition-derived solution. The charge and current vdW scaling treatments cover commonly encountered ILs, completing the protocol for accurate modelling of ILs with fixed-charge force fields.

Host Dynamics under General-Purpose Force Fields

“…A popular example is ionic solvents, where numerous highly charged species are packed together. In these situations, additional alterations are required to accurately consider these remaining influential effects [ 34 , 35 , 36 ].…”

Screening Power of End-Point Free-Energy Calculations in Cucurbituril Host–Guest Systems

“…The chemical structures of all molecules are either grabbed from the PubChem database or constructed by modifications of existing molecules of similar chemical structures (i.e., template modeling). According to our previous validation of popular charge schemes, the restrained electrostatic potential (RESP) scheme serves as a robust and accurate option for many macrocyclic hosts including cucurbiturils ,, and many other organic and inorganic molecules. − We thus optimize each molecule B3LYP − /6-31G* and then derive RESP charges based on the HF − /6-31G* electrostatic potential. The other parameters of fixed-charge force fields (e.g., bond stretching, torsion, and vdW) are grabbed from the transferable GAFF2 parameter set for all guest molecules, while for the macrocyclic hosts, we apply either GAFF or GAFF2 in our modeling due to the observed differences in host dynamics reported in our recent works. , …”

Improvement of Gd(III) Solvent Extraction by 4-Benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one: Non-Aqueous Systems

“…In particular, in the field of metal extraction, separation, and recycling by liquid-liquid processes, hydrophobic ILs of the imidazolium type [C n C m im + ][Tf 2 N − ] have already proved to be tremendously efficient media for a large variety of metals in the periodic table including lanthanoids (cations are denoted as [C n C m im + ] for n-alkyl-m-alkylimidazolium, whereas they are denoted as the anion [(CF 3 SO 2 ) 2 N − ] (bis(trifluoromethylsulfonyl) for imide, [Tf 2 N − ]) [10][11][12][13][14][15]. However, in one way or another, the prediction of diluent effects is a challenging task in solvent extraction chemistry [16][17][18] or separation methods such as adsorption, biosorption, precipitation, reverse osmoses, ion exchange, [19][20][21][22][23][24][25][26], etc.…”