Coupled-cluster method Brillouin-Wigner

Finally, Pittner and coworkers " examined the PESs of singlet and triplet TMEs using two multireference coupled cluster methods (two-determinant CCSD and multireference Brillouin-Wigner CCSD) with the cc-pVDZ basis set. With both methods, the singlet is always below the triplet for all values of t. The smallest singlet-triplet gap, 1.3 kcal moE, occurs at an angle r = 45° and Ihis is reduced when a near-cc-pVTZ basis set is used. 

Demel, O. Pittner, J. Multireference Brillouin-Wigner coupled cluster method with singles, doubles, and triples efficient implementation and comparison with approximate approaches, 7. Chem. Phys. 2008,128,104108-104111. 

Multireference coupled cluster method based on the Brillouin-Wigner perturbation theory... 

MULTIREFERENCE BRILLOUIN-WIGNER COUPLED CLUSTER METHOD... 

Chapter 18 - Multireference coupled cluster method based on the Brillouin-Wigner perturbation theory. Pages 465-481, Petr Carsky, Jin Pittner and Ivan Hubac... 

Size consistency of the Brillouin-Wigner perturbation theory is studied using the Lippmann-Schwinger equation and an exponential ansatz for the wave function. Relation of this theory to the coupled cluster method is studied and a comparison through the effective Hamiltonian method is also provided. 

Multireference Brillouin-Wigner coupled clusters method with noniterative perturbative conrtected triples... 

Multi-reference Brillouin-Wigner coupled-cluster method with a general model space Molecular Physics 103,2239 (2005)... 

These two possible formulations of multi-reference Brillouin-Wigner coupled cluster theory are discussed further below. In Section 4.2.2.1, we present a multiroot formulation of Brillouin-Wigner theory. This formalism is employed in Section 4.2.2.2 to develop a multi-root, multi-reference Brillouin-Wigner coupled cluster theory, using a Hilbert space approach. In Section 4.2.2.3, we discuss the basic approximations employed in the multi-reference Brillouin-Wigner coupled cluster method. 

I. Hubac, Size Extensive Brillouin-Wigner Coupled-Cluster Theory. In A. Tsipis, V. S. Popov, D. R. Herschbach, and J. S. Avery (Eds.) New Methods in Quantum... 

A posteriori corrections can be developed for calculations performed by using the Brillouin-Wigner perturbation expansion. These a posteriori corrections can be obtained for the Brillouin-Wigner perturbation theory itself and, more importantly, for methods, such as limited configuration interaction or multi-reference coupled cluster theory, which can be formulated within the framework of a Brillouin-Wigner perturbation expansion. 

Method of moments for the continuous transition Petween the Brillouin-Wigner-type and Rayleigh-Schrodinger-type muitireference coupled cluster theories Molecular Physics 107, 1209 (2009)... 

The wave operator Cla, defined in equation (34), can be realized in various ways. The Brillouin-Wigner coupled cluster BWCC method is obtained by... 

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