Reappraisal of the normal ordered Jeziorski-Monkhorst ansatz in the UGA-OSCC theory for a study of IP, EA and EE

Abstract

The Unitary Group Adapted Open Shell Coupled Cluster (UGA-OSCC) theory, which is the single reference CSF limit of the UGA-State Universal Multireference CC (UGA-SUMRCC) formulation by Mukherjee et al. [Sen, Shee, and Mukherjee, J. Chem. Phys. 137, 074104 (2012)], was proposed and successfully applied via a normal ordered Jeziorski-Monkhorst (J-M)-like CC ansatz for the wave operator. This generates a spin-adapted, size extensive energy. The computed energy differences of spectroscopic interest such as IP, EA and EE were consequently size intensive. Here we revisit the formulation by suggesting a rigorous version which completely bypasses the use of a sufficiency condition posited in our earlier formulation via an exact factorisation of the wave operator in the Bloch equation. This brings in certain connected composites involving contractions among the cluster operators in the resultant equations. This formulation is flexible enough to accommodate truncation of these composites while remaining rigorously size-extensive for each such approximant. We test the effect of these new terms in the Bloch equation for this formulation by evaluating IP, EA and EE for a test set of molecules, which shifts the respective results closer in the order of 0.1 to 0.001 eV towards the benchmark EOM-CCSDt or FCI or experimental values.