Where Does Electron Correlation Lie? Some Answers from a Real Space Partition.

We discuss an intra- and interatomic partition of the electron correlation energy (Ecorr ) within the interacting quantum atoms (IQA) approach at the accurate coupled cluster level with single, double, and estimated triple excitations (CCSDT(T)). This division offers a privileged window into the spatial localization of this component of the molecular energy. We show that the total molecular Ecorr is dominated by the intra-atomic or intra-fragment terms and that interatomic contributions change smoothly from short- to long-range correlation (dispersion). The sign of these interatomic correlation terms differentiate between (i) mainly covalent or (ii) mainly ionic or nonbonded interactions. We propose that this feature may be used to define and examine intramolecular dispersion terms.