Strongly separated pairs of core electrons in computed ground states of small molecules

► Computations of small molecules' electron structure reveals separated electron pairs. ► Pairs are “separated” in the strong sense of the APSG ansatz. ► Presence of separated pair is established directly, by deriving density matrix of pair's quantum state. ► Density matrices are derived from highly-correlated (FCI) wavefunctions with hundreds of thousands of configurations. ► Pairs of core electron are most clearly revealed with cc-pCVDZ basis set.