Mulliken Center for Theoretical Chemistry, Institut für Physikalsche und Theoretische Chemie, Universität Bonn, Beringstr. 4, D-53115 Bonn, Germany.
Phys Chem Chem Phys. 2012 Apr 14;14(14):4875-83. doi: 10.1039/c2cp24096c. Epub 2012 Feb 29.
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.
已使用具有色散校正的 DFT 方法计算了代表离子液体构建单元的阳离子-阴离子配合物离解的势能曲线。首次测试了非局部范德华密度泛函（DFT-NL）和原子对修正方法（DFT-D3）。参考数据是在 MP2/CBS 外推和估计 CCSD(T)/CBS 理论水平上计算的。所研究的系统由两种阳离子（1-丁基-3-甲基咪唑𬭩和三丁基（甲基）膦𬭩）和三种阴离子（氯化物、双氰胺、乙酸盐）组成。我们发现，在平衡时，伦敦分散能提供了相当大的稳定性，约为 5-7 kcal mol(-1)（约为相互作用能的 5-6%）。平衡距离缩短了 0.03-0.09 Å，基本（片段间）振动频率（在 140-180 cm(-1)范围内）增加了通常 10 cm(-1)，当进行色散校正时。色散校正的杂化泛函势通常与相应的 CCSD(T)参考数据非常吻合（典型偏差约为 1-2%）。DFT-D3 方法的表现出人意料地好，大概是因为阳离子和阴离子的色散系数之间的误差相互抵消了。由于自相互作用误差，半局部密度泛函在 SCF 收敛方面存在严重问题，并提供了人为的电荷转移和不准确的相互作用能，因为片段间的距离较大。尽管这些问题在凝聚相模拟中可以通过有效库仑屏蔽得到缓解，但对于这种离子体系，通常只能推荐使用具有较大 Fock 交换量的色散校正杂化泛函。
