26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Development of novel relativistic multireference coupled cluster approaches for complex quasi-degenerate electronic structures.

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Invited talk AIP | Atomic and Molecular Physics (ATMOP)

Description

The relativistic Fock-space coupled cluster (FS-RCC) method is among the most powerful and elegant multireference tools for the ab initio treatment of open-shell heavy element systems. Yet in its standard valence-universal form, it is largely confined to low Fock-space sectors, and its reach into genuinely multivalence, quasi-degenerate structures is limited by intruder states, and by the missing amplitude relaxation imposed by the subsystem embedding condition. This contribution presents a coordinated program of developments designed to overcome these limitations and extend benchmark-quality relativistic coupled-cluster theory across the entire periodic table.
Four complementary directions are discussed. First, the extension of FS-RCC to high Fock-space sectors, giving a balanced description of systems with several valence particles or holes, combined with an intermediate Hamiltonian that tames the intruder-state problem. Second, a mixed-sector coupled cluster formulation that treats, on an equal and size-extensive footing, several sectors sharing the same total electron number, removing asymmetries among near-degenerate states of differing valence character. Third, the adaptation of the internally contracted multireference coupled cluster (ic-MRCC) method to the relativistic four-component domain; retaining the strengths of the Fock-space ansatz while relaxing its most restrictive constraints, it offers a robust route to complicated quasi-degenerate states of d-, f-, and superheavy-element compounds. Fourth, a double Fock-space coupled cluster construction, in which a doubled set of Fock-space subsectors extended for bosonic degrees of freedom enables a simultaneous and flexible treatment of QED effects, pushing electronic structure calculations toward the Lorentz-invariance limit.
The formal connections among these approaches, their relative merits, and efficient implementation strategies — including compact virtual natural orbitals and tensor-decomposition techniques for cost control — are analyzed. Perspectives on combining these developments with high-order excitations, QED corrections, and analytical property evaluation are outlined, aiming to achieve a unified, systematically improvable relativistic multireference coupled cluster framework for benchmark electronic structure calculations.