Performance of the RI and RIJCOSX approximations in the topological analysis of the electron density

The performance of the Møller–Plesset (MP2) method in its resolution of the identity (RI-MP2), and the chain of spheres exchange (RIJCOSX-MP2) variants within the Quantum Theory of Atoms in Molecules (QTAIM) wavefunction analyses is examined. We have obtained QTAIM descriptors at bond critical points for a series of small molecules and water clusters of different sizes. We also considered integrated properties, like QTAIM atomic charges or localization and delocalization indices. The performance of RI methods with respect to the plain MP2 benchmark results is excellent, with mean deviations for all the properties considered below 0.15%. However, in systems where electron delocalization plays a more important role, we found differences up to 5% (e.g. C ₆H ₆) from a suitable reference. The account of RIJCOSX-HF results shows that the RIJCOSX approximation works better when electron correlation is included. Finally, a topological analysis of the electron density on the endofullerene complex H ₂O@C ₇₀ is reported. Overall, herein we suggest the possibility to carry out the QTAIM topological analysis using correlated wavefunctions in large molecules and molecular clusters, thereby extending the applicability of this important methodology.