Coupled Cluster for High-Spin Open-Shell Systems - A Mixed Orbital Approach
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- Institutt for kjemi 
The description of high-spin open-shell systems with coupled cluster (CC) theory presents problems not encountered in the closed-shell case. Spin-contamination and additional computational cost compared to closed-shell calculations are the major challenges, and these have so far only been remedied at the cost of a more complicated CC theory and implementation. In this work a new CC singles and doubles (CCSD) scheme for doublet systems was developed, where an alternative closed-shell mixed orbital reference was used instead of an open-shell Hartree-Fock reference. The alternative reference makes possible the use of the closed-shell CCSD formalism. The objective is easy implementation and costs approaching that of the comparable closed-shell calculation. The CCSD wave function is spin-contaminated but satisfies ⟨R|S^2|CCSD⟩ = s(s + 1), where |R⟩ is the mixed orbital reference and S^2 is the operator for the total spin of the open-shell system. The new mixed orbital CCSD (MXO-CCSD) method was implemented by making only a few adaptations to the closed-shell CCSD code of the DALTON software. Energy calculations were performed for the two smallest atomic doublets with electron correlation, Lithium and Boron, and the peroxy radicals O2^−, HO2 and CH3O2. MXO-CCSD results for these doublet systems are comparable to other CCSD methods for open-shell systems. For the atoms more than 89% of the correlation energy is recovered. The initial steps towards the development of MXO-CCSD for triplet systems were also taken.