Employing broken symmetry effects from unrestricted coupled cluster wave function to determine dynamic and non-dynamic electron correlation during triple bond breaking in the N2 molecule

The cluster structure of the singlet full symmetric component of the unrestricted Hartree-Fock (UHF)-based CCSD wave function describing the triple bond breaking in the nitrogen molecule has been subjected to a detailed analysis for the possibility of determining the dynamic and near-degeneracy electron correlation. The results obtained show that the spin and symmetry contaminations are not responsible only for the appearance of the artificial hump in the potential energy curve (PEC) generated by the UHF-based CCSD calculations. A theoretical analysis of this issue indicates that the UHF-based CCSD wave function and its singlet full symmetric component have a multi-reference structure. This form of the wave function allows to explain the mechanism for creating cluster contributions in the projected UHF-based CCSD wave function, which also provides the opportunity to explain the cause of the hump in the nitrogen PEC generated by the UCCSDecCCSD method.