A curious artifact in the potential energy surface for the atomic fluorine + hydrogen fluoride abstraction reaction

The reaction F+HF→HF+F has been studied using ab initio molecular orbital theory at UHF, MP2, MP3, MP4, CCD, CCSD, CID, CISD, QCISD, QCISD(T), and CASSCF levels of theory with the 3-21G, 6-31G ** , 6-311++G ** , and D95++(3df,2p) basis sets. The UHF, MP4, CIS, CISD, CCSD, QCISD, QCISD(T), and CASSCF levels of theory predict the transition state to be a highly bent C 2υ symmetry structure, whereas MP2, MP3, and CCD levels predict the C 2υ symmetry structure to be a shallow minimum bound by up to 0.4 kcal/mol