Triple Electron Attachments with a New Intermediate-Hamiltonian Fock-Space Coupled-Cluster Method.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Physics

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Fock-space coupled-cluster (FSCC) methods are tailored for ionization potentials and electron affinities.
• Developing efficient and accurate FSCC schemes for open-shell systems remains a challenge.

Purpose of the Study:

• To implement and evaluate a novel intermediate-Hamiltonian Fock-space coupled-cluster (IHFSCC) scheme for the (3,0) sector.
• To assess the accuracy of the (3,0) IHFSCC method for calculating triple ionization potentials and excitation energies.
• To investigate the impact of orbital types and the number of active virtual orbitals on the results.

Main Methods:

• Implementation of a new intermediate-Hamiltonian Fock-space coupled-cluster (IHFSCC) approach.
• The method considers three-body contributions in the cluster operator S^(3,0).
• Calculations were performed for atoms and molecules with p^3 ground configurations and adiabatic excitation energies.

Main Results:

• The (3,0) IHFSCC method provides reasonable results for triple ionization potentials and excitation energies, especially with a minimal reference space.
• Using canonicalized restricted open-shell Hartree-Fock (ROHF) orbitals yields results that agree well with reference values and are less sensitive to the number of active orbitals.
• The method demonstrates applicability to open-shell systems with three unpaired electrons.

Conclusions:

• The developed (3,0) IHFSCC scheme is a viable and accurate method for studying electronic properties of open-shell systems.
• Canonicalized ROHF orbitals are recommended for achieving reliable and robust IHFSCC calculations.
• The method offers a good balance between accuracy and computational cost.