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We present high-level benchmark calculations of interaction energies of 236 ion pair structures of ionic liquids constituting a new IL-2013 set. 33 different approaches using various basis sets are validated against these benchmark data. Overall, traditional functionals like B3LYP, without an explicit dispersion correction, should be avoided when investigating ionic liquids. We can recommend the third version of Grimme's empirical dispersion correction (DFT-D3) and the LC-BOP functional, as well as most functionals of the Minnesota family of the M0X type. Our results highlight the importance of diffuse basis set functions for the accurate prediction of the IL energetics using any DFT functional. The best combination of reasonable accuracy and reasonable cost was found to be the M06-L functional in combination with the 6-31++G** basis set, producing a remarkable mean absolute deviation of only 4.2 kJ mol(-1) and a maximum deviation of -12.5 kJ mol(-1). Second-order Møller-Plesset perturbation theory (MP2) in combination with counterpoise-corrected triple-ζ basis sets can also be recommended for reliable calculations of energetics of ionic liquids.

Fully ab initio large-scale calculations of archetypical ionic liquids consisting of up to eight ion pairs are presented for the first time. These are used to validate the computationally efficient Fragment Molecular Orbital approach applied to these semi-Coulombic systems, paving the way towards accurate prediction of their transport properties., (This journal is © The Royal Society of Chemistry 2012)

In this work, we examine the performance of a range of local and hybrid DFT functionals (including BLYP, PBE, PW91, B3P86, B3LYP, and TPSS), new generation DFT functionals (including KMLYP, BMK, M05, and M05-2X), and DFT functionals with the explicit empirical correction for dispersion interactions (including BLYP-D, PBE-D, and B3P86-D) in calculating ion-pair binding energies of pyrrolidinium-based ionic liquids, [C(n)mpyr][X] (n = Me, Et, n-Pr and n-Bu and X = Cl, BF(4), PF(6), CH(3)SO(3) (mesylate), CH(3)PhSO(3) (tosylate), N(CN)(2), and NTf(2)). Calculated IPBEs were compared to the results of the selected benchmark method, MP2/6-311+G(3df,2p). Modified MP2 methods such as SCS-MP2 and SOS-MP2 were also considered in the study. Errors of the DFT-based and ab initio-based methods in calculations of IPBEs of ionic liquids, trends in relative IPBEs, and basis-set superposition errors are discussed in depth. DFT functionals that can potentially be used to study binding energies of ionic liquids are identified.

Serious (up to 87 kJ mol(-1)) systematic DFT errors in a series of isodesmic reactions are found to be due to the DFT exchange component, and can be largely corrected by substitution of the DFT exchange energy with the Fock exchange energy.

The performance of a variety of DFT functionals (BLYP, PBE, B3LYP, B3P86, KMLYP, B1B95, MPWPW91, MPW1B95, BB1K, MPW1K, MPWB1K, and BMK), together with the ab initio methods RHF, RMP2, and G3(MP2)-RAD, and with ONIOM methods based on combinations of these procedures, is examined for calculating the enthalpies of a range of radical reactions. The systems studied include the bond dissociation energies (BDEs) of R-X (R = CH3, CH2F, CH2OH, CH2CN, CH2Ph, CH(CH3)Ph, C(CH3)2Ph; X = H, CH3, OCH3, OH, F), RCH(Ph)-X (R = CH3, CH3CH2, CH(CH3)2, C(CH3)3, CH2F, CH2OH, CH2CN; X = H, F), R-TEMPO (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3, CH2CH2CH3, CH2F, CH2OH, CH2CN, CH(CN)CH3, CH(Cl)CH3; TEMPO = 2,2,6,6,-tetramethylpiperidin-1-yloxyl) and HM1M2-X (M1, M2 = CH2CH(CH3), CH2CH(COOCH3), CH2C(CH3)(COOCH3); X = Cl, Br), the beta-scission energies of RXCH2* and RCH2CHPh* (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3; X = O, S, CH2), and the enthalpies of several radical addition, ring-opening, and hydrogen- and chlorine-transfer reactions. All of the DFT methods examined failed to provide an accurate description of the energetics of the radical reactions when compared with benchmark G3(MP2)-RAD values, with all methods tested showing unpredictable deviations of up to 40 kJ mol-1 or more in some cases. RMP2 also shows large deviations from G3(MP2)-RAD in the absolute values of the enthalpies of some types of reaction and, although it fares somewhat better than the DFT methods in modeling the relative values, it fails for substituents capable of strongly interacting with the unpaired electron. However, it is possible to obtain cost-effective accurate calculations for radical reactions using ONIOM-based procedures in which a high-level method, such as G3(MP2)-RAD, is only used to model the core reaction (which should contain all substituents alpha to the reaction center), and the full system is modeled using a lower-cost procedure such as RMP2.

Abstract: The accuracy and reliability of the CIS(D) quantum chemical method and a spin-component scaled variant (SCS-CIS(D)) are tested for calculating 0–0 excitation energies of organic molecules. The ground and excited state geometries and the vibrational zero-point corrections are taken from (TD)DFT-B3LYP calculations. In total 32 valence excited states of different character are studied: π→π∗ states of polycyclic aromatic compounds/polyenes and n→π∗ states of carbonyl, thiocarbonyl and aza(azo)-aromatic compounds. This set is augmented by two systems of special interest, i.e., indole and the TICT state of dimethylaminbenzonitrile (DMABN). Both methods predict excitation energies that are on average higher than experiment by about 0.2 eV. The errors are found to be quite systematic (with a standard deviation of about 0.15 eV) and especially SCS-CIS(D) provides a more balanced treatment of π→π∗ vs. n→π∗ states. For the test suite of states, both methods clearly outperform the (TD)DFT-B3LYP approach. Opposed to previous conclusions about the performance of CIS(D), these methods can be recommended as reliable and efficient tools for computational studies of excited state problems in organic chemistry. In order to obtain conclusive results, however, the use of optimized excited state geometries and comparison with observables (0–0 excitation energies) are necessary. [Copyright &y& Elsevier]