UNO‐ and ULO‐MRCC(Mk), AP‐UCC and AP‐UBD approaches to diradical systems

Abstract

Abstract In this article, we performed spin‐unrestricted Hartree Fock (UHF) based coupled‐cluster (CC) and Bruckner CC double calculations of methylene, fluorine, HHeH diradical, and ozone. Mukherjee‐type (Mk) Multireference coupled‐cluster (MRCC) calculations of these species were also conducted by the use of UHF natural orbital (UNO) and UHF localized natural orbitals (ULO) as reference orbitals to elucidate size‐consistent error. Comparing calculated results of spin‐unrestricted Hartree‐Fock based coupled‐cluster (UCC) and spin‐unrestricted Bruckner‐Double (UBD) with those of MkMRCC, it was found that spin‐contamination errors of UCC and UBD solutions still remained, though this type of errors for these solutions was considered to be negligible in general. The comparison also revealed scope and applicability of approximate spin‐projection (AP) scheme for UCC and UBD, indicating that the AP procedure corrected spin‐contamination errors to yield good agreement with MRCC in energy. We also performed pure‐ and hybrid‐ density functional theory (DFT) calculations of the same systems, and those calculations provided reasonable singlet‐triplet energy gaps, which were qualitatively good agreement with those of MRCC, AP‐UCC, and AP‐UBD calculations. Implications of the computational results are discussed in relation to applicability of hybrid DFT to large superexchange coupled systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010