Multi-reference Fock space coupled-cluster method in the intermediate Hamiltonian formulation for potential energy...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Fock space (FS) coupled-cluster theory is used for ionization potentials.
• Multi-reference problems and intruder states pose challenges in CC calculations.

Purpose of the Study:

• To formulate and implement an effective intermediate Hamiltonian multi-reference coupled-cluster (CC) method for the doubly ionized (0,2) sector of Fock space (FS).
• To develop a robust computational scheme for solving FS-CC equations, overcoming the intruder state problem.
• To provide an efficient tool for calculating double ionization potentials and describing multi-reference CC problems.

Main Methods:

• Formulation of the effective and intermediate Hamiltonian multi-reference CC method for the (0,2) FS sector.
• Implementation of the method with singles and doubles.
• Introduction of an efficient factorization strategy for computational efficiency.

Main Results:

• The intermediate Hamiltonian approach effectively solves FS-CC equations without intruder states.
• The method accurately describes multi-reference problems, demonstrated for twisted ethylene and the F(2) potential energy curve.
• Calculations of double ionization potentials for H(2)O, CO, C(2)H(2), and C(2)H(4) were performed.

Conclusions:

• The developed method is a robust and efficient tool for studying double ionization potentials.
• It significantly advances the description of multi-reference problems within CC theory.
• The approach provides accurate results for challenging molecular systems.