A full coupled-cluster singles and doubles model: The inclusion of disconnected triples

Abstract

The coupled‐cluster singles and doubles model (CCSD) is derived algebraically, presenting the full set of equations for a general reference function explicitly in spin–orbital form. The computational implementation of the CCSD model, which involves cubic and quartic terms, is discussed and results are reported and compared with full CI calculations for H₂O and BeH₂. We demonstrate that the CCSD exponential ansatz sums higher‐order correlation effects efficiently even for BeH₂, near its transition state geometry where quasidegeneracy efforts are quite large, recovering 98% of the full CI correlation energy. For H₂O, CCSD plus the fourth‐order triple excitation correction agrees with the full CI energy to 0.5 kcal/mol. Comparisons with low‐order models provide estimates of the effect of the higher‐order terms T₁T₂, T²₁T₂, T³₁, and T⁴₁ on the correlation energy.