Nitrogen second-order correlation energy

The results of the distributed basis set study of the second-order correlation energy for the nitrogen molecule(83) is compared with the second-order correlation energies obtained by employing other recently reported basis sets for this system in Figure 1. 

Figure 1. The shape of the potential curve for nitrogen in a correlation-consistent polarized double-zeta basis set is presented for the variational 2-RDM method as well as (a) single-reference coupled cluster, (b) multireference second-order perturbation theory (MRPT) and single-double configuration interaction (MRCl), and full configuration interaction (FCl) wavefunction methods. The symbol 2-RDM indicates that the potential curve was shifted by the difference between the 2-RDM and CCSD(T) energies at equilibrium.

Table I presents the results of EOM calculations of the three lowest IPs of nitrogen. Comparison of the first two columns of Table I demonstrates that there is a difference of 0.2 to 0.3 eV in the IPs when the EOM A matrix is symmetrized as by Simons, 21-order method I, and when the symmetrized form of the EOM equations, (21), 2j-order method II, is employed. The lack of symmetry in <0 (0,[//,0 ]) 0) in a 2 -order calculation arises from the inclusion of certain second-order A and terms, which contain the products of electron-electron interaction matrix elements with first-order double excitation correlation coefficients, and the neglect of other second-order A and A - terms, which involve second-order single excitation correlation coefficients multiplied by linear combinations of orbital energies. The discrepancies between the EOM 2 -order methods I and II are a measure of the importance of the terms due to single excitations in the ground-state wave function. In Section III.C, we consider the third-order terms not included in this primitive 2 -order EOM theory. The calculations imply although these terms are small, they are certainly not negligible. ...

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