Relativistic Fock-space multireference coupled cluster theory with full iterative triples for the one-, two-, and...

Area of Science:

• Quantum Chemistry
• Relativistic Calculations
• Atomic and Molecular Physics

Background:

• Coupled cluster methods are essential for accurate electronic structure calculations.
• Relativistic effects become significant for heavy elements and high ionization states.
• Fock-space methods simplify calculations for ionization potentials and electron affinities.

Purpose of the Study:

• To present and implement a relativistic Fock-space coupled cluster method with full iterative inclusion of connected triple excitations (FS-CCSDT).
• To benchmark the newly developed FS-CCSDT method in various atomic and molecular systems.
• To assess the accuracy of FS-CCSD and FS-CCSDT models in different Fock space sectors.

Main Methods:

• Development and implementation of the relativistic Fock-space coupled cluster method (FS-CCSDT).
• Inclusion of connected triple excitations in the Fock space framework.
• Benchmarking calculations on atomic ions (Ar, I) and molecular ions (Ar2+, HI+, HI2+).

Main Results:

• FS-CCSDT calculations show high accuracy for ionization potentials and adiabatic excitation energies (0.005-0.02 eV uncertainty).
• The relativistic FS-CCSDT model is 3-5 times more accurate than FS-CCSD in low Fock space sectors.
• Models for the 3h0p sector provide acceptable accuracy for excitation energies in complex ions.

Conclusions:

• The relativistic FS-CCSDT method provides a significant advancement in computational accuracy for electronic structure problems.
• The developed method is reliable for calculating properties of highly charged atomic and molecular systems.
• FS-CCSDT offers a robust and accurate tool for theoretical chemistry and physics research.