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)Cl-2 has been applied to a hierarchy of SDCI approaches. It has been obtained for the ground state of N-2 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. (C) 1998 Elsevier Science B.V. All rights reserved.