Excited states with pair coupled cluster doubles tailored coupled cluster theory

Moneesha Ravi; Ajith Perera; Young Choon Park; Rodney J Bartlett

doi:10.1063/5.0161368

Excited states with pair coupled cluster doubles tailored coupled cluster theory

J Chem Phys. 2023 Sep 7;159(9):094101. doi: 10.1063/5.0161368.

Authors

Moneesha Ravi¹, Ajith Perera¹, Young Choon Park², Rodney J Bartlett¹

Affiliations

¹ Quantum Theory Project, University of Florida, Gainesville, Florida 32611-8435, USA.
² Korea Institute of Fusion Energy, 37 Dongjangsan-ro, Gunsan, Jeollabuk-do 54004, Republic of Korea.

PMID: 37655762
DOI: 10.1063/5.0161368

Abstract

It is known that some non-dynamic effects of electron correlation can be included in coupled cluster theory using a tailoring technique that separates the effects of non-dynamic and dynamic correlations. Recently, the simple pCCD (pair coupled cluster doubles) wavefunction was shown to provide good results for some non-dynamic correlation problems, such as bond-breaking, in a spin-adapted way with no active space selection. In this paper, we report a study of excited states using "tailored coupled cluster singles and doubles," to attempt to use pCCD as a kernel for more complete coupled-cluster singles and doubles (CCSD) results for excited states. Several excited states are explored from those primarily due to single excitations to those dominated by doubly excited states and from singlet-triplet splittings for some diradical states. For the first two situations, tailored pCCD-TCCSD offers no improvement over equation of motion-CCSD. However, when we explore the singlet-triplet gap of diradical molecules that are manifestly multi-reference, a pCCD kernel provides improved results, particularly with generalized valence bond orbitals.