New SCS- and SOS-MP2 Coefficients Fitted to Semi-Coulombic Systems

Spin-component scaled second-order Møller-Plesset perturbation theory (SCS-MP2) energy calculations, which independently scale the opposite- and same-spin components of the MP2 correlation energy, are known to consistently provide improved interaction energies in comparison to conventional MP2. This has led to the development of a number of SCS-MP2 derivatives that target particular classes of molecules, interactions or properties. In this study, SCS-MP2 scaling coefficients targeted to interaction energies of single ion pair semi-Coulombic ionic liquid (IL) systems are presented in view of circumventing the need for counterpoise correction to eliminate basis set superposition error (BSSE). A set of 174 IL ion pairs consisting of imidazolium ([C(1-4)mim](+)) and pyrrolidinium ([C(1-4)mpyr](+)) cations and routinely used anions such as Br(-), Cl(-), [BF4](-), [PF6](-), [DCA](-) (dicyanamide), [tos](-) (tosylate), [mes](-) (mesylate), and [NTf2](-) (bis(trifluoromethylsulfonyl)amide), each of which were arranged in multiple favorable conformations, were calculated at the MP2 level of theory with 17 popular basis sets ranging from double- to quadruple-ζ quality and at the CCSD(T)/CBS limit. For each basis set, the spin components of the IL set were scaled via least-squares multiple linear regression with respect to CCSD(T)/CBS benchmark interaction energies that were corrected for BSSE using the Boys and Bernardi approach. SCS-MP2 spin component coefficients of 1.05 and 0.68 are recommended for the opposite- and same-spin components, respectively, in conjunction with Dunning's cc-pVTZ basis set, which resulted in the most statistically reliable regression. Alternatively, a scaled opposite-spin MP2 (SOS-MP2) scaling factors of 1.64 is recommended for the opposite-spin component and should be used where the omission of the same-spin component results in a calculation speed-up. These two scaling schemes are termed SCS-IL-MP2 and SOS-IL-MP2, respectively. The SCS-IL-MP2 and SOS-IL-MP2 approaches show interaction energy errors on average less than 1.0 kJ mol(-1) with respect to CCSD(T)/CBS benchmark results and highlights the important consideration of basis set dependence when selecting spin-component coefficients. By calculating multiple conformations for each ion pair and scaling to reproduce BSSE corrected benchmark energies, it is suggested that improved energies may be obtained for larger IL clusters beyond ion pairs without performing costly counterpoise corrections.