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Comparison of the quadratic configuration interaction and coupled cluster approaches to electron correlation including the effect of triple excitationsThe recently proposed quadratic configuration interaction (QCI) method is compared with the more rigorous coupled cluster (CC) approach for a variety of chemical systems. Some of these systems are well represented by a single-determinant reference function and others are not. The finite order singles and doubles correlation energy, the perturbational triples correlation energy, and a recently devised diagnostic for estimating the importance of multireference effects are considered. The spectroscopic constants of CuH, the equilibrium structure of cis-(NO)2 and the binding energies of Be3, Be4, Mg3, and Mg4 were calculated using both approaches. The diagnostic for estimating multireference character clearly demonstrates that the QCI method becomes less satisfactory than the CC approach as non-dynamical correlation becomes more important, in agreement with a perturbational analysis of the two methods and the numerical estimates of the triple excitation energies they yield. The results for CuH show that the differences between the two methods become more apparent as the chemical systems under investigation becomes more multireference in nature and the QCI results consequently become less reliable. Nonetheless, when the system of interest is dominated by a single reference determinant both QCI and CC give very similar results.
Document ID
Document Type
Contractor Report (CR)
Taylor, Peter R. (Eloret Corp. Sunnyvale, CA., United States)
Lee, Timothy J. (NASA Ames Research Center Moffett Field, CA., United States)
Rendell, Alistair P. (NASA Ames Research Center Moffett Field, CA., United States)
Date Acquired
September 6, 2013
Publication Date
January 1, 1990
Subject Category
Report/Patent Number
NAS 1.26:187855
Funding Number(s)
Distribution Limits
Work of the US Gov. Public Use Permitted.

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