Kucharski-Bartlett method

For A = 0, this equation reduces to the Kucharski-Bartlett method [125], leading, if a complete model space is assumed, to a result which is fully extensive. Note, however, that the neglect of coupling terms on the right hand side leaves equations, which for A = 0 become uncoupled (i.e. the amplitudes of each reference configuration are independent), but only connected and thus extensive... 

Bartlett, R.J. Kucharski, S.A. Noga, J. Watts, J.D. Trucks, G.W. In Many-Body Methods in Quantum Chemistry Kaldor, U., Ed. Lecture Notes in Chemistry, Vol. 52 Springer Berlin, 1989 p. 124. 

R. J. Bartlett, S. A. Kucharski, and J. Noga, Alternative coupled-cluster anstaze. 2. The unitary coupled-cluster method. Chem. Phys. Lett. 155, 133 (1989). 

Kucharski SA, Bartlett RJ (1992) The coupled-cluster single, double, triple, and quadruple excitation method. J Chem Phys 97 4282-4288. 

Piecuch P, Kucharski SA, Bartlett RJ (1999) Coupled-cluster methods with internal and semiinternal triply and quadruply excited clusters CCSDt and CCSDtq approaches. J Chem Phys 110 6103-6122. 

S. A. Kucharski and R.. Bartlett, /. Chem. Phys., 97, 4282 (1992). The Coupled-Cluster Single, Double, Triple, and Quadruple Excitation Method. 

Although the past 20 years have witnessed a great progress in the Hilbert space multi-reference coupled cluster methods (see, for example, the work of Mukherjee and Pal [99],Paldus [101], Jeziorski and Paldus [102], Jankowski et al. [103],Paldus et al. [104], Paldus et al. [105], Meissner et al. [106], Kucharski and Bartlett [107], Balkovd et al. [108], Baikova and Bartlett [109], Balkovd et al. [110], Baikova et al. [Ill], Berkovic and Kaldor [112]) only a few applications of this approach have been reported, mostly oriented to the simple model systems exploiting a lowdimensional model space. Among the reasons for this paucity of applications are the choice of an appropriate model space, convergence difficulties arising from intruder state problems and from multiple solutions of non-linear coupled cluster equations. 

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