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Issue 14, 2012
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Performance of dispersion-corrected density functional theory for the interactions in ionic liquids

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Abstract

Potential energy curves for the dissociation of cation–anion associates representing the building units of ionic liquids have been computed with dispersion corrected DFT methods. Non-local van der Waals density functionals (DFT-NL) for the first time as well as an atom pair-wise correction method (DFT-D3) have been tested. Reference data have been computed at the extrapolated MP2/CBS and estimated CCSD(T)/CBS levels of theory. The investigated systems are combined from two cations (1-butyl-3-methylimidazolium and tributyl(methyl)posphonium) and three anions (chloride, dicyanamide, acetate). We find substantial stabilization from London dispersion energy near equilibrium of 5–7 kcal mol−1 (about 5–6% of the interaction energy). Equilibrium distances are shortened by 0.03–0.09 Å and fundamental (inter-fragment) vibrational frequencies (which are in the range 140–180 cm−1) are increased by typically 10 cm−1 when dispersion corrections are made. The dispersion-corrected hybrid functional potentials are in general in excellent agreement with the corresponding CCSD(T) reference data (typical deviations of about 1–2%). The DFT-D3 method performs unexpectedly well presumably because of cancellation of errors between the dispersion coefficients of the cations and anions. Due to self-interaction error, semi-local density functionals exhibit severe SCF convergence problems, and provide artificial charge-transfer and inaccurate interaction energies for larger inter-fragment distances. Although these problems may be alleviated in condensed phase simulations by effective Coulomb screening, only dispersion-corrected hybrid functionals with larger amounts of Fock-exchange can in general be recommended for such ionic systems.

Graphical abstract: Performance of dispersion-corrected density functional theory for the interactions in ionic liquids

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Publication details

The article was received on 22 Dec 2011, accepted on 07 Feb 2012 and first published on 29 Feb 2012


Article type: Paper
DOI: 10.1039/C2CP24096C
Citation: Phys. Chem. Chem. Phys., 2012,14, 4875-4883
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    Performance of dispersion-corrected density functional theory for the interactions in ionic liquids

    S. Grimme, W. Hujo and B. Kirchner, Phys. Chem. Chem. Phys., 2012, 14, 4875
    DOI: 10.1039/C2CP24096C

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