Accurate prediction of the structure and vibrational spectra of ionic liquid clusters with the generalized energy-based fragmentation approach: critical role of ion-pair-based fragmentation

Abstract

A generalized energy-based fragmentation (GEBF) approach has been developed to facilitate ab initio calculations of the ground-state energies, structures and vibrational spectra of general ionic liquid (IL) clusters. For the selected IL clusters, the accuracy of the GEBF approach with two different fragmentation schemes (ion-pair-based fragmentation and ion-based fragmentation) is evaluated with the conventional quantum chemistry calculations. Our results demonstrate that for the selected IL clusters, the GEBF approach with the ion-pair-based fragmentation scheme can provide much more accurate descriptions than that with the ion-based fragmentation scheme. The main reason for these results is that the non-integer charge behavior of each ion (cation or anion) in IL systems may induce significant errors for the GEBF approach with the ion-based fragmentation scheme, in which every ion is assumed to have an integer charge. However, this problem can be avoided by the ion-pair-based fragmentation scheme, in which each ion pair is assumed to be electrically neutral. Our illustrative results show that the GEBF approach with a dynamic ion-pair-based fragmentation scheme, in which ion pair fragments are updated for every structure, can provide satisfactory descriptions on the ground-state energies, optimized structures, and vibrational spectra of general IL clusters. The performance of the GEBF approach is found to be almost independent of the basis sets or theoretical methods, and the computational cost of the GEBF approach scales linearly with the system size at density functional theory (DFT) and second-order Møller–Plesset perturbation theory (MP2) levels. Due to its excellent parallel efficiency, the GEBF approach is expected to be a cost-effective tool for investigating the structure, vibrational spectra, as well as other properties of large IL clusters.