State-selective multi-reference coupled-cluster theory using multi-configuration self-consistent-field orbitals. A model study on H₈

The recently developed state-selective (SS) multi-reference (MR) coupled-cluster (CC) method is applied to a prototype molecular system composed of eight hydrogen atoms. To obtain a better description of the zeroth-order wavefunction in the quasidegenerate region, we employ orbitals resulting from the complex active space multi-configurational self-consistent-field (MC SCF) optimization. Comparison with the results obtained using the ground-state restricted Hartree-Fock SCF orbitals indicates invariant character of the SS CC theory with respect to the orbital choice, contrary to the behavior of the Hilbert-space state-universal MR CC method.