Mixed-sector intermediate Hamiltonian Fock-space coupled cluster approach

Area of Science:

• Quantum Chemistry
• Computational Physics

Background:

• The Fock-space coupled cluster (FSCC) method is a powerful tool for electronic structure calculations.
• Existing formulations have limitations in the types of reference states and Fock-space sectors that can be used.

Purpose of the Study:

• To present an alternative formulation of the intermediate Hamiltonian Fock-space coupled cluster scheme.
• To extend the applicability of multireference coupled cluster methods to new types of reference states and Fock-space sectors.

Main Methods:

• Development of a new formulation for the intermediate Hamiltonian Fock-space coupled cluster method.
• Utilizing model spaces with determinants from different Fock-space sectors.
• Application to electron affinities, ionization potentials, and excitation energies.

Main Results:

• The new formulation enables the use of quasiclosed shells (e.g., p2, d4) as reference states.
• Achieved excellent agreement with experimental data (within hundredths of an electronvolt).
• Demonstrated significant improvement (5-10 times for p3 states) over previous FSCC results.

Conclusions:

• The presented formulation offers methodological and computational advantages over previous methods.
• It significantly expands the scope of accessible electronic states and systems for coupled cluster calculations.
• This advancement provides a more versatile and accurate approach for studying complex atomic and molecular systems.