Excited states with pair coupled cluster doubles tailored coupled cluster theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled cluster theory is a powerful method for describing electron correlation.
• Tailoring techniques can incorporate non-dynamic electron correlation effects into coupled cluster methods.
• Pair Coupled Cluster Doubles (pCCD) has shown promise for non-dynamic correlation problems like bond-breaking.

Purpose of the Study:

• To investigate the utility of pCCD as a kernel for tailored coupled cluster singles and doubles (TCCSD) for excited states.
• To evaluate TCCSD performance for various types of excited states, including single and double excitations.
• To assess the accuracy of pCCD-kernel TCCSD for singlet-triplet splittings in diradical systems.

Main Methods:

• Application of tailored coupled cluster singles and doubles (TCCSD) using pCCD as a kernel.
• Exploration of excited states, ranging from single to doubly excited.
• Calculation of singlet-triplet gaps for diradical molecules, utilizing generalized valence bond orbitals.

Main Results:

• TCCSD with a pCCD kernel showed no improvement over equation of motion-CCSD for single and doubly excited states.
• Significant improvements were observed for the singlet-triplet gap in diradical molecules.
• The pCCD kernel demonstrated particular efficacy when combined with generalized valence bond orbitals for multi-reference diradical states.

Conclusions:

• The pCCD kernel in TCCSD is not universally beneficial for all excited state calculations.
• This approach shows promise for accurately describing the electronic structure of diradical systems.
• Future work may focus on refining pCCD-based methods for challenging multi-reference problems.