Application of the size-consistent self-consistent intermediate hamiltonian approach to different SDCI. The excited states of N2

The size-consistent self-consistent configuration interaction operator (SC)2CI has been applied to a hierarchy of SDCI approaches. It has been obtained for the ground state of N2 at the equilibrium distance. It has been applied to calculate excited roots of dressed SDCI built on single-reference, multi-reference (MR) and complete active space (CAS) reference spaces. The vertical excitation energies are compared to EOM-CCSD (15 states) and MR-CCSD (28 states) data. Both valence and Rydberg states can be calculated with similar accuracy, which compares well with that of the reference methods. A mean absolute error (MAE) of 0.15 eV relative to experiment can be obtained using small MR or CAS spaces for the SDCI to be dressed. The aug-cc-pVDZ basis set allowed a MAE lower than 0.1 eV for valence states but is inappropriate for Rydberg states.