Abstract
An open-shell restricted Hartree—Fock singles and doubles coupled-cluster method including perturbative triple excitations, CCSD (T), that does not include any spin contamination for the correlation energies, is computationally formulated and implemented. The new open-shell CCSD (T) program is applied in conjunction with large atomic natural-orbital basis sets to calculate the energy difference between the linear and bent isomers of C + 3. With a [5s4p3d2f1g] basis set, the CCSD (T) method predicts the bent (cyclic) 2B 2 state to lie 6.8 kcal/mol lower than the linear 2Σ + u species. The importance of correlation effects included at the CCSD (T) level of theory is analyzed in connection to the existence of symmetry-broken Hartree—Fock solutions for this problem.
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