期刊
JOURNAL OF CHEMICAL PHYSICS
卷 148, 期 4, 页码 -出版社
AMER INST PHYSICS
DOI: 10.1063/1.5011033
关键词
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资金
- U.S. Department of Energy, Office of Basic Energy Sciences, Computational and Theoretical Chemistry Program [DE-FG02-09ER16053]
- Welch Foundation Chair [C-0036]
There have been assertions in the literature that the variational and unitary forms of coupled cluster theory lead to the same energy functional. Numerical evidence from previous authors was inconsistent with this claim, yet the small energy differences found between the two methods and the relatively large number of variational parameters precluded an unequivocal conclusion. Using the Lipkin Hamiltonian, we here present conclusive numerical evidence that the two theories yield different energies. The ambiguities arising from the size of the cluster parameter space are absent in the Lipkin model, particularly when truncating to double excitations. We show that in the symmetry adapted basis under strong correlation, the differences between the variational and unitary models are large, whereas they yield quite similar energies in the weakly correlated regime previously explored. We also provide a qualitative argument rationalizing why these two models cannot be the same. Additionally, we study a generalized non-unitary and non-hermitian variant that contains excitation, de-excitation, and mixed operators with different amplitudes and show that it works best when compared to the traditional, variational, unitary, and extended forms of coupled cluster doubles theories. Published by AIP Publishing.
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