How ‘ <i>de facto</i> variational’ are fully iterative, approximate iterative, and quasiperturbative coupled cluster methods near equilibrium geometries?

Abstract While limited coupled cluster theory is formally nonvariational, it is not broadly appreciated whether this is a major issue in practice . We carried out a detailed comparison with de facto full 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 .