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The dual exponential coupled cluster (CC) theory proposed by Tribedi etal.[J. Chem. Theory Comput. 2020, 16, 10, 6317-6328] performs significantlybetter than the coupled cluster theory with singles and doubles excitations(CCSD) due to the implicit inclusion of high-rank excitations. The high-rankexcitations are included through the action of a set of vacuum annihilatingscattering operators that act non-trivially on certain correlated wavefunctionsand are determined via a set of local denominators involving the energydifference between certain excited states. This often leads the theory to beprone to intruders. In this manuscript, we show that restricting the correlatedwavefunction, on which the scattering operators act upon, to be spanned by onlythe singlet paired determinants can avoid the catastrophic breakdown. For thefirst time, we present two nonequivalent approaches to arrive at the workingequations, viz. the projective approach with sufficiency conditions and theamplitude form with many-body expansion. While the effect of the tripleexcitation is quite small around molecular equilibrium geometry, this schemeleads to a better qualitative description of the energetics in the regions ofstrong correlation. With a number of pilot numerical applications, we havedemonstrated the performance of the dual-exponential scheme with both theproposed solution strategies while restricting the excitation subspaces coupledto the corresponding lowest spin channels.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Fixing the Catastrophic Break-down of Single Reference Coupled Cluster  Theory for Strongly Correlated Systems: Two Paradigms towards the Implicit  Inclusion of High Rank Correlation with Low-Spin Channels