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.

GRAPHICAL ABSTRACT

Keywords:

• Spin-free open-shell coupled cluster
• size-extensive
• Jeziorski-Monkhorst like normal ordered cluster Ansatz
• energy differences of spectroscopic interest

Acknowledgements

DC thanks Professor Ankan Paul for providing laboratory facilities at IACS. DM thanks Lan Cheng and Matthias Hanauer for initiating a discussion regarding avoidance of sufficiency conditions in an earlier formulation of a cumulant-based state-specific CC theory using multireference normal ordering, which led to many new insights and ideas. DM also thanks the TCG-CREST for providing computational support from the CDAC, Pune.

DM congratulates John Stanton, a friend of many years, for the happy occasion of his reaching sixty. He also acknowledges with gratitude many fruitful discussions he had with John on various topics, not just limited to science. DM wishes John many more years of creative pursuits and also for providing creative leadership for the future generations.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

DC acknowledges the University Grants Commission (UGC) for it's financial support [Grant No.- 20/12/2015(ii)EU-V]. DM acknowledges the research funding from the CQuERE, TCG-CREST.