Spin contamination in MP2 and CC2, a surprising issue

When calculating the spin multiplicity at either second-order Møller-Plesset (MP2) or iterative approximate coupled-cluster singles and doubles (CC2) levels of theory using same strategy for calculation expectation value as in regular CC together with usual definitions MP2 CC2 density matrices, artificial contamination occurs closed-shell molecules. Non-intuitively, open-shell systems, results based on this procedure even suggest stronger correlated level than Hartree–Fock reference, although treatment electron correlation should lower contamination. In Communication, reasons behind inconsistency are investigated a solution is proposed, which removes molecules leads to physically meaningful cases. Additionally, we show that significantly outperforms describing systems strongly spin-contaminated reference performance similar full substitutions (CCSD).