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  1. ; ; (, Pure and Applied Chemistry)
    Abstract While limited coupled cluster theory isformallynonvariational, it is not broadly appreciated whether this is a major issuein practice. We carried out a detailed comparison withde factofull CI energies for a relatively large and diverse set of molecules near equilibrium geometries. Fully iterative limited CC methods such as CCSDT, CCSDTQ, and CCSDTQ5 do represent practical upper bounds to the FCI energy, as do CCSDT-3, CCSDTQ-3, and CCSD(cT). While quasiperturbative approaches such as CCSD(T) and especially CCSDT(Q) may significantly over-correlate molecules if there is significant static correlation, this is much less of an issue with Lambda approaches such as CCSDT(Q)Λ. 
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    Free, publicly-accessible full text available August 27, 2026
  2. ; ; ; ; ; ; ; (, The Journal of Physical Chemistry A)