Analysis of a failure of the CC2 coupled-cluster method for bond lengths of SnO and PbO

CC2 model is found to overestimate bond lengths of SnO and PbO by about 0.25 angstrom, while both second-order Moller-Plesset perturbation theory and coupled-cluster singles and doubles give reasonable results. Previously, analysis shows that the [[U, T-1], T-1] term in the doubles equation of CC2 is the origin of failure for CC2 and some truncated CC models have been suggested to achieve reasonable result for ozone, where CC2 is unable to obtain a stable structure. However, these remedies are unable to afford reasonable bond lengths of SnO and PbO. Based on a term-wise analysis, our results indicate that the [U, T-1] term results in failure of CC2. CC2 model by removing this term will provide results that agree well with those of MP2. Furthermore, the [[U, T-2], T-1] term absent in the CC2 while present in doubles equation of CCSD can balance this [U, T-1] term and CC2 model augmented with this term is able to afford reasonable results for PbO, SnO and ozone.