Notably, the form of the pair density gives rise to the exchange hole in the case of parallel spins (δ σ i ,σ j = 1) and leads to a depletion of probability of finding a second, same-spin electron near a reference electron. However, for multi-determinental wave functions, the pair density behaves quite differently. Instead of leading to a Fermi (exchange) hole, with a depletion of parallelspin electron density around a reference point, multi-reference systems can instead have a Fermi "heap" around the reference point, 351 with an accumulation of parallel-spin electron density. This physics is not captured by Kohn-Sham DFAs, which model a localized exchange hole.