CR-2 REVISITED

Cr-2 is studied at the coupled cluster singles and doubles level of theory, including a perturbational estimate of the connected triples (CCSD(T)). Both spin-restricted, RCCSD(T), and spin-unrestricted, UCCSD(T), approaches are used. The UCCSD(T) bond length is much longer than the experimental value, while the RCCSD(T) bond length is shorter. This difference is due to the inability of the spin restricted Hartree-Fock reference to describe the antiferromagnetic coupling at larger r values. In contrast with the results obtained using density functional theory in conjunction with older functionals, the results obtained with recently developed functionals are in agreement with the UCCSD(T) results.