Coupled-cluster Doubles Calculations Research Articles

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

AbstractIn 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

Approximate Brueckner orbitals and shakeup operators in electron propagator calculations: Applications to F? And OH?

Vertical electron detachment energies of F− and OH− have presented difficulties for perturbative electron propagator methods. A recently derived, nondiagonal, renormalized approximation and two additional improvements are studied here. These improvements are replacement of the Hartree–Fock reference determinant by a determinant of approximate Brueckner orbitals generated by a coupled-cluster doubles calculation and retention of correlation terms in the 2hp–2hp block of the superoperator Hamiltonian matrix. Agreement with experiment is significantly better with these methods. When Hartree–Fock orbitals are used, electron detachment energies are underestimated, but approximate Brueckner orbitals lead to overestimates. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 651–658, 1998