Comparison of the Brueckner and coupled-cluster approaches to electron correlation

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Abstract
The equilibrium structures, dipole moments, harmonic vibrational frequencies, and infrared intensities of NH3, FON, Be3, BeC2, and BeO2 have been determined using the coupled‐cluster and Brueckner electron correlation methods. The singles and doubles coupled‐cluster (CCSD) and the Brueckner doubles (BD) methods have been employed and the corresponding methods that include a perturbational estimate of connected triple excitations [i.e., CCSD(T) and BD(T)] have also been investigated. The T1 diagnostic [Int. J. Quantum Chem. Symp. 2 3, 199 (1989)] is found to provide a good indication of the magnitude of the difference between the results obtained with the coupled‐cluster and Brueckner methods. For NH3, the T1 diagnostic is small and so the differences between results obtained from coupled‐cluster and Brueckner theories are quite small. For the other four molecules the T1 diagnostic is larger, and so the differences between the coupled‐cluster and Brueckner methods become larger. However, it is found for all of the molecules considered in this study that inclusion of the contribution from connected triple excitations is more important than the differences between the Brueckner and coupled‐cluster correlation methods.