Applications of a Novel Spin-free Combinatoric Open-shell Coupled Cluster (COS-CC) Theory to Single-reference Doublets

Abstract

We describe in this paper a compact spin‐free coupled cluster (CC) theory for simple open‐shell configurations, like doublets and biradicals, which are not necessarily single determinants. A new cluster Ansatz for the wave‐operator is introduced, in which the cluster operators with direct valence spectator scatterings are replaced by closed‐shell‐like single and double excitation operators. The cluster operators with exchange valence spectator scatterings and the pure valence excitation operators are allowed to contract among themselves through the spectator orbitals. The novelty of the Ansatz is in the choice of a suitable automorphic factor accompanying each composite of non‐commuting operators, ensuring that each such composite appears only once. This leads to CC equations which terminate exactly at the quartic power of the cluster amplitudes, reminiscent of the closed‐shell CC theory. As a pilot example application, we compute the state energy of the ground state of the Fluorine radical with three dif...